# USB DAC Design questions



## Clutz

Hello all,

 I've decided for my next project I'm going to try and design and build a USB DAC. While there are already lots of interesting options available out there, designing my own really appeals to me. 

 I'd like to keep the parts budget between $150 to $200 (i.e. in the same range of a PPA, which will be used with this DAC). My goal is to have a USB DAC that is higher end than the PCM270x USB DACs but less complicated than DaKi][ers DAC. 

 Since I plan on using this primarily as a desktop DAC to use with my PC, I don't have any particular desire to include a SPDIF input. This will allow me to spend the budget elsewhere on the DAC on things I care more about, and it will simplify the signal path (i.e. USB -> PCM2704 to convert to I2S -> DAC chip). That said, I would appreciate any input on the cost/benefit of including an SPDIF in. My personal opinion is that given my price restrictions, I'd simply be giving up too much to include it. 

 My second question is, what are the benefits of going with two one channel DACs versus a single two channel DAC? The most obvious one that comes to mind is reduced stereo cross talk, but I'm not sure how much of an issue that is (currently the DAC chips I'm contemplating are the Cirrus Logic CS4351, the Texas Instruments PCM1738, PCM1792, PCM1798. All of the texas instraments DACs seem to have differential outputs, while the CS4351 has a single ended output - but has a 2Vrms output. 

 I haven't picked exactly which DAC yet, but so far I think I am leaning towards a design using a PCM2705/6 -> PCM1794. I think I would use either the OPA627 or possibly the AD843,although these will be socketed, so it doesn't REALLy matter. Another possibility would be the 6171, but if I decided to do that,or wanted that possibility, I'd have to design that into the schematic from the start. 

 I'm also interested if people think it would be worthwhile using an external digital filter from the DAC or if that money would be better spent elsewhere.

 I'm not yet sure how I'll do the power supply, I'll need a 3v3 and 5v psu for the DAC itself, but I'm not sure about the power supply for the op-amps. 

 Anyway, I'd appreciate your input. I hope to learn a lot from this.

 Cheers,
 Clutz


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## 00940

For I2S you need the pcm2707, not the pcm2704.

 The pcm1798 seems a good candidate (less expensive that the pcm1794 but still very good).

 Since you have a balanced output, I would use the fully differential OPA1632 as I/V stage, not classical opamps. You will need an usual opamp afterwards as filter and unbalanced converter.

 I would try to get the best power supply possible rather than an external digital filter. Two small transformers with dual secondaries would not be very expensive. One transformer for the digital +5v and +3.3v and one for the +/-10V for the opamps.

 For the opamps, a bare minimum would be +/-5V. Check the LT1964 and LT1763 for the opamps, the reg101 for the rest.

 Pcb are rather expensive. Factor the cost in.


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## Clutz

Quote:


  Originally Posted by *00940* 
_For I2S you need the pcm2707, not the pcm2704._

 

 Oops. 
	

	
	
		
		

		
			





 Thanks!

  Quote:


 The pcm1798 seems a good candidate (less expensive that the pcm1794 but still very good). 
 

It does seem like a good candidate- but it's only about $7 less, and that into the cost of the rest of the project, I think would disappear.... though, perhaps the extra specs of the 1798 would be missed on such a design?

  Quote:


 Since you have a balanced output, I would use the fully differential OPA1632 as I/V stage, not classical opamps. You will need an usual opamp afterwards as filter and unbalanced converter. 
 

What are the benefits of using a fully differential amp for the I/V stage instead of classical op-amps? The benefit I can see from using classical op-amps, is I can start out with some relatively cheap 134s and then upgrade to better op-amps later? (I'm not against using a filly differential opamp, I'm just curious). 

  Quote:


 I would try to get the best power supply possible rather than an external digital filter. Two small transformers with dual secondaries would not be very expensive. One transformer for the digital +5v and +3.3v and one for the +/-10V for the opamps. 
 

That's what I was thinking of doing. One transformer with dual 5 volts out for one, and the other with something like +-12 to 15? 


 [QUPTE]Pcb are rather expensive. Factor the cost in.[/QUOTE]

 Alright. I was thinking about doing the boards with ExpressPCB. Do you think that this will result in a reasonable compromise between DaKiller's and the recent PCM270x DACs with my budget?

 Cheers,
 Clutz


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## 00940

Quote:


  Originally Posted by *Clutz* 
_What are the benefits of using a fully differential amp for the I/V stage instead of classical op-amps? The benefit I can see from using classical op-amps, is I can start out with some relatively cheap 134s and then upgrade to better op-amps later? (I'm not against using a filly differential opamp, I'm just curious)._

 

Cost of two opa134 at digikey : 4.5$
 Cost of one opa1632 at digikey : 3.94$

 The specs of the opa1632 are in line with the opa627 or better (especially settling time which is WAY better). It is what is used in the Bel Canto DAC2 btw (ths4130, same stuff as opa1632).

 Using one chip instead of two saves also on decoupling caps, size of the pcb and so on. Since the structure is balanced, crosstalk is not a concern.

 some additional stuff : avoid the pcm1792. It must be controlled by a microcontroller, while the pcm1794/98 (they are pin compatible btw) are easy to control in hardware mode.


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## Clutz

Quote:


  Originally Posted by *00940* 
_Cost of two opa134 at digikey : 4.5$
 Cost of one opa1632 at digikey : 3.94$

 The specs of the opa1632 are in line with the opa627 or better (especially settling time which is WAY better). It is what is used in the Bel Canto DAC2 btw (ths4130, same stuff as opa1632).

 Using one chip instead of two saves also on decoupling caps, size of the pcb and so on. Since the structure is balanced, crosstalk is not a concern.

 some additional stuff : avoid the pcm1792. It must be controlled by a microcontroller, while the pcm1794/98 (they are pin compatible btw) are easy to control in hardware mode._

 

Well, consider me convinced. 
	

	
	
		
		

		
		
	


	




 I'll use two OPA1632s and either a pair of 627s or AD843s. 

 So I guess now that I have that worked out, I should start drawing the schematic in Eagle.


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## 00940

old stuff that should help : http://www6.head-fi.org/forums/showt...light=perreaux


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## Clutz

Hey 00940,

 In the link you provided, it seems as though you were attempting to do something very similar. Why are you taking the 16bit 44.1kHz at 256fs from the pcm2707 and putting it into the AD1896 to convert it to 24bit 96kHz at 256fs to pass that through to the DAC? Why convert the 44.1kHz 16bit (CD audio) to 96kHz at 24bits...? If we can only get 16 bits from the I2S format from the USB Receiver? If anything, wouldn't we then want to resample up to 176.4? 

 Thanks
 Clutz


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## Garbz

Whatever you do keep the output stage differential. It also gives you the option of using it as a balanced source later. If you don't stick with the differential OPAMPS, then use 4 OPAMPS for I/V and filter, one for each channel.


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## Clutz

Yes, I'll definitely use the differential op amps suggested. 

 Now my next task is to figure out how to use Eagle.


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## MegaMeee

silly newbie question here but what is a differential output and how can it be used to make a single ended or balanced source?


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## 00940

Quote:


  Originally Posted by *Clutz* 
_Hey 00940,

 In the link you provided, it seems as though you were attempting to do something very similar. Why are you taking the 16bit 44.1kHz at 256fs from the pcm2707 and putting it into the AD1896 to convert it to 24bit 96kHz at 256fs to pass that through to the DAC? Why convert the 44.1kHz 16bit (CD audio) to 96kHz at 24bits...? If we can only get 16 bits from the I2S format from the USB Receiver? If anything, wouldn't we then want to resample up to 176.4? 

 Thanks
 Clutz_

 

For the 24bit part, I cannot honnestly remember. I suppose I got caught by the marketing dreams of upsampling ?

 For the 96khz, the reason is that the THD figure are better than at higher frequencies, the consumption reduced, the heat reduced and then there is Wodgy's post 5. Since I used an ASRC, the exact output frequency didn't really matter, it was convenient to get a clock for 96khz.

 If you want to keep it cheap and simple, scrap the ad1896. What you could do is to make provisions to cut the I2S lines at one point to add a small card with an ad1896 and a good clock later on. Through hole resistors on the I2S lines would be perfect for that.


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## Francis_Vaughan

You really have little choice but to use an ASRC. It is either that or a PLL. The ASRC gets you the ability to suck more than one sample rate - and since this is a USB device we can pretty much assume that people will need such flexibility. 

 The 1794 can only let the system clock get 6 cycles away from the LR clock - if it drifts further the chip resynchronises. You have no way of keeping the system clock this close without either a local clock (and thus you need an ASRC to manage the vagaries of the USB delivered sample rate) or you need a PLL - and thus you adapt to the vagaries of the USB sample rate. 

 00940's design is spot on. It is exactly what I would look at doing. You could also look at the CS8421 ASRC. Building a PLL is difficult at best.

 I would be flexible about the I/V conversion. There are many options. 

 Also, I will keep emphasising, you need to make this design as a whole. Great care and attention to signal and ground return integrity throughout will be the make or break of it. Making the ASRC an option, expecially a duaghter card will wreck this. The integrity of the clock as it reaches the ASRC is exactly as critical as the clock that reaches the DAC. Any contamination on either will ruin the result. Careful layout is key.


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## ble0t

Hrmmpff...I totally forgot about 00940's design from last spring. It seems Clutz is looking for something similar. Do we as a group want to take another stab at this? I for one am quite interested


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## guzzler

I'd definately be interested in trying to get it off the ground again.

 Another DAC worth considering is the 1738, as used in Cyrus CD players. Fantastic sound, and fairly cheap as well (think it's ~£7)

 There is the other option of using the 179*3* which has differential voltage outputs. Aos used that in his Ally and Piccollo DACs. It's not got such a good reputation as the 1794/8, but just thought I'd mention it.


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## Clutz

I should be on my way to work, so I'm going to try to be quick.

  Quote:


  Originally Posted by *Francis_Vaughan* 
_You really have little choice but to use an ASRC. It is either that or a PLL. The ASRC gets you the ability to suck more than one sample rate - and since this is a USB device we can pretty much assume that people will need such flexibility._

 

People will need this flexibility because people will probably want to be able to play different audio formats through their computer (i.e. other than CD Audio)? (At the risk of offending anyone, I'm just asking to understand, I'm not trying to find a passive-aggressive way to argue). 

  Quote:


 The 1794 can only let the system clock get 6 cycles away from the LR clock - if it drifts further the chip resynchronises. You have no way of keeping the system clock this close without either a local clock (and thus you need an ASRC to manage the vagaries of the USB delivered sample rate) or you need a PLL - and thus you adapt to the vagaries of the USB sample rate. 

 00940's design is spot on. It is exactly what I would look at doing. You could also look at the CS8421 ASRC. Building a PLL is difficult at best. 
 

When I looked at 00940's design I felt it was what I was looking at trying to do.. It basically looked like exactly what I wanted to do, except it had the ASRC chip.. but I didn't know anything about it.. know that I would want it.. so.. That's kind of good in my mind (reinforces that it's a reasonable thing to attempt doing), but also means it's sort of done.

  Quote:


 Also, I will keep emphasising, you need to make this design as a whole. Great care and attention to signal and ground return integrity throughout will be the make or break of it. Making the ASRC an option, expecially a duaghter card will wreck this. The integrity of the clock as it reaches the ASRC is exactly as critical as the clock that reaches the DAC. Any contamination on either will ruin the result. Careful layout is key. 
 

My two goals for this were to learn more about DIY electronics, and try to come up with a good robust design by getting lots of people involved in it. I'm still a bit confused about ground plane (and I know we're a ways off that yet), and how we go about having a single ground plane and not have the analogue return polute the digital ground... and yet from the Alf DAC discussion, I recall that two separate ground planes isn't really desirable.


  Quote:


  Originally Posted by *ble0t* 
_ Hrmmpff...I totally forgot about 00940's design from last spring. It seems Clutz is looking for something similar. Do we as a group want to take another stab at this? I for one am quite interested_

 

Me too. I didnm't know that 00940 already had something so far along, but I was hoping to try and get the group to take a stab at something like this. It'll give me yet another reason not to work on my PhD.

  Quote:


  Originally Posted by *guzzler* 
_Another DAC worth considering is the 1738, as used in Cyrus CD players. Fantastic sound, and fairly cheap as well (think it's ~£7)_

 

It looks pretty good too. I think then at the very least, we've agreed that differential output is the way to go?


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## ble0t

Quote:


  Originally Posted by *Clutz* 
_People will need this flexibility because people will probably want to be able to play different audio formats through their computer (i.e. other than CD Audio)?_

 

This is where the ASRC comes in...as Francis said, it should solve the issue of multiple input frequencies. Also, if I'm not mistaken, the ASRC helps to reduce jitter better than a PLL (see this thread over at diyaudio).


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## ble0t

Also, I like the idea that 00940 had in his last post in the older thread about using something like the PCM2707 for USB and then also having a CS8416 available for SPDIF/Optical/AES (using transformers where needed). Having each section (DIR/DAC/Analog) on its own board it also a great idea. How about shooting for something like this?


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## 00940

Quote:


  Originally Posted by *guzzler* 
_There is the other option of using the 179*3* which has differential voltage outputs. Aos used that in his Ally and Piccollo DACs. It's not got such a good reputation as the 1794/8, but just thought I'd mention it._

 

In the same class as the 1793, with voltage out, we could also consider the wolfson wm8740, used in the cambridge 640c, or the CS4398, used in the headroom microdac. Still, the ability to tailor your own I/V stage leads my heart towards dac such as the pcm1794/98.

 Francis : some designs using the pcm2707 and direct I2S to pcm1798have been posted earlier. http://www6.head-fi.org/forums/showthread.php?t=134125 for example. I agree that an asrc is better, but is it really a necessity ?

 A local clock can get from cheap to very expensive. Considering the budget, we cannot afford something as a tent clock. We can of course the ad1896 with only a crystal iirc.

 If we add a cs8416, transformers, and so on, costs will jump up. 

 Btw, the old schematics I posted are far from perfect. They're missing inductors on the power lines for example.


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## Clutz

Quote:


  Originally Posted by *00940* 
_In the same class as the 1793, with voltage out, we could also consider the wolfson wm8740, used in the cambridge 640c, or the CS4398, used in the headroom microdac. Still, the ability to tailor your own I/V stage leads my heart towards dac such as the pcm1794/98.

 Francis : some designs using the pcm2707 and direct I2S to pcm1798have been posted earlier. http://www6.head-fi.org/forums/showthread.php?t=134125 for example. I agree that an asrc is better, but is it really a necessity ?

 A local clock can get from cheap to very expensive. Considering the budget, we cannot afford something as a tent clock. We can of course the ad1896 with only a crystal iirc.

 If we add a cs8416, transformers, and so on, costs will jump up. 

 Btw, the old schematics I posted are far from perfect. They're missing inductors on the power lines for example._

 

My personal opinion on the matter is that if we end up trying to include SPDIF inputs, and thus need a cs8416, then we either have to up the budget substantially, or make compromises elsewhere. My goal out of this was to make a pretty good quality USB only DAC. I'm not saying I'm against it these other things, I just think we need to weigh the costs and benefits. Towards this end, my preference, I think, would be to put money towards getting a better clock, rather than multiple inputs.. but of course, I'm only one person. 
	

	
	
		
		

		
			





 Re the ASRC: Is the principle benefit that if we use it, then we are not constrained to 16bit @ 44.1kHz i2s data streams into the DAC? Because if it is, I think we have a problem further up the food chain. The PCm2707 only provides a 16bit @ 44.1khz i2s data stream (or am I wrong?)? So would we be connecting the PCM2707's SPDIF output to the ASRC?


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## ble0t

00940: I see your point. I think the idea of having seperate boards for the various aspects of the DAC would work well in this situation. I guess my thought would be to come up with a single DAC section since this should be the most standard part and then possibly differentiate the DIR section and Analog section from there. Although not completely necessary, I like the idea of having an ASRC in there if possible. Total extra cost (if using a standard crystal) would only be about $20 extra, which IMO would be worth it.


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## steinchen

I'm very interested in an DIY DAC, too. Since I already got a Piccolo DAC I'm interested in a high end DAC only.

 I advocate a modular design with input section, DAC and I/V section. The DAC section could be replaced by a board with a new DAC chip after some years. Concerning the I/V section I expect many diverging opinions and requirements (single ended vs balanced, passive vs opamp vs discrete).


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## guzzler

I would argue against a modular system. As F_V mentions too, we're dealing with fast MHz signals, and to acheive a good interfacing between the separate parts requires a good deal of thought. Also, it considerably worsens the board layout going for "flexibility" (ie, people want a board without designing it themselves). You could simply put a couple of pads on the Iout pins for people to run to their board should they so wish

 Set a definite limit on the design parameters and stick with them. There's always tradeoffs, so better to live with them than to worry about them.


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## Clutz

So then, how should we decide what the design parameters are? One important design parameter to take into consider is the budget, but that can mean many things. Is it better to go with a simpler design and build the budget assuming more expensive parts (boutique caps, fancier resistors), or assume that use good quality parts (i.e. panasonic FMs, bc components resistors, maybe vishays)?

 My design parameters would be a USB only DAC. I personally would be happy with something like the PCM2707->ASRC->PCM1794->OPA1632, or PCM2707->PCM1794->OPA1632, and possibly using some of the savings to offset a better clock? It would be mains powered- at least two seperate transformers, one with dual 5 volt secondaries to make our 5v and 3v3, and one with dual 12 volt secondaries to power the op-amps with, and possibly take the seond 12 volt tap and drop it also down to 5 if we wanted another 5 volt PSU? 

 I'd imagine we want something better than a TREAD for the PSU- maybe something like a stripped STEPS a la dsavitsk's nonos DAC's PSU?


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## ble0t

After thinking it through, it makes sense to go all one board with this as guzzler said. Modularity would be nice, but it probably makes itself impractical. I guess my vote would be for PCM2707 --> (ASRC or some type of reclock) --> PCM1798 (big cost savings over PCM1794 and still very good performance as well as diff out) --> I/V Stage. As far as the power supply is concerned, it would be possible to include it onboard if making use of chips like the reg101 and lt1764/lt1964 for the various stages.


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## dsavitsk

Quote:


  Originally Posted by *Clutz* 
_maybe something like a stripped STEPS a la dsavitsk's nonos DAC's PSU?_

 

That's the Baby Steps (TM) to you. 
	

	
	
		
		

		
		
	


	




 The problem with this design, as I have found out, is that since some of the filtering is pre transformer and some is post transformer, this requires the transformer being close to the rest of the circuit. The irony is that this adds more noise than the circuit cleans, so I think it is a net loss. This argues for making the PS on a seperate PCB or leaving out the pre transformer filters and assuming that anyone who wants them will add them seperatly.

  Quote:


  Originally Posted by *steinchen* 
_Concerning the I/V section I expect many diverging opinions and requirements (single ended vs balanced, passive vs opamp vs discrete)._

 

Don't forget hollow state options, which would need a modular board. I think this is a good place for modularity -- perhaps picking one option to put on the board, but also adding Iout pads for others.


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## 00940

-The power supply should be onboard, but not the xformers imo. CRCRC filters followed by reg101 and the like should be perfect and compact.

 - The difference in between pcm1794 and pcm1798 is mainly in the digital filter. If we arrive with a 96khz or higher signal out of an ad1896, the resampling inside the dac chip will happen further away from the signal band (see the posts in my old thread). The pcm1798 could thus be sufficient.

 - For a DAC, the use of SMD parts makes a lot of sense (for about anything but the electrolytics). It will avoid the overuse of expensive boutique stuff.


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## Clutz

Yes, the PCM1798 is quite a bit cheaper than the 94 at digikey, about $10 less. That said, I think that the PCM1794 and 1798 are pin compatible, so if someone wanted to use the 1794 instead of the 98, they would be able to do so, so we can go with either of them. But I would agree that on the budget front, we should do the costing of the project based on the 1798.


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## Clutz

Quote:


  Originally Posted by *00940* 
_-The power supply should be onboard, but not the xformers imo. CRCRC filters followed by reg101 and the like should be perfect and compact.

 - For a DAC, the use of SMD parts makes a lot of sense (for about anything but the electrolytics). It will avoid the overuse of expensive boutique stuff._

 

Makes a lot of sense to me. 

  Quote:


  Originally Posted by *dsavitsk* 
_Don't forget hollow state options, which would need a modular board. I think this is a good place for modularity -- perhaps picking one option to put on the board, but also adding Iout pads for others._

 

I really think that the Iout pads are a good idea.


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## ble0t

Sounds good to me. I would be willing to assist on the schematic/layout. What layout/PCB prog did you use for your original design 00940? Perhaps we could make that a starting point


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## 00940

like everyone, eagle free version. But I'm not sure i've much left.


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## steinchen

The free version of Eagle is limited to half eurocard size (100x80 mm = 4x3.2"), I purchased the non-profit version and am limited to eurocard size (160x100mm = 4x6.4"). The standard version has the same limits, only the professional version supports larger boards.


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## Clutz

Quote:


  Originally Posted by *steinchen* 
_The free version of Eagle is limited to half eurocard size (100x80 mm = 4x3.2"), I purchased the non-profit version and am limited to eurocard size (160x100mm = 4x6.4"). The standard version has the same limits, only the professional version supports larger boards._

 

Hrmm, well then I think we're probably going to have to get the standard version - cause I somehow doubt we can get everything on a half eurocard PCB. I'm probably willing to do it, but I've never done layout before at all before. Hell, I've never drawn a schematic in it - I've only ever drawn schematics in Adobe Illustrator.


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## steinchen

the standard version has the same limits like my non-profit version, no need to purchase such an expensive version

 nevertheless, so far the board size limit is 160x100mm, with this limit we'd have to modularize ...


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## Clutz

Quote:


  Originally Posted by *steinchen* 
_the standard version has the same limits like my non-profit version, no need to purchase such an expensive version

 nevertheless, so far the board size limit is 160x100mm, with this limit we'd have to modularize ..._

 

Well, in that case- would it make sense to move the PSU on a separate PCB? We have basically three components: power supply, dac, i/v section.. and I would think it makes the more sense to keep the DAC and i/v section close to each other, rather than the DAC and power suppy, no?

 Cheers,
 Clutz


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## 00940

In my view, we should be able to put everything quite easily on a eurocard. In my first layout attempts, I was able to fit everything but the I/V stage on half of that.


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## MegaMeee

I am also interested in a design like this... I was going to build a usb dddac (which also used the pcm2707 direct i2s), but i'm not convinced by those nonos dacs. 
 Personally the original budget $150 - $200 seems a little low to me. I think we are looking at ways to minimise cost too much considering how many people here seem to build "maxed" versions of everything.


  Quote:


  Originally Posted by *00940* 
_In my view, we should be able to put everything quite easily on a eurocard. In my first layout attempts, I was able to fit everything but the I/V stage on half of that._

 

so what did you put on your first layout attempts?

 Personally I don't care how many boards it is on (as long as it dosent lower the performance), but If everything will fit on a eurocard then why bother with more than one?

 edit: btw ignore my earlier question. This: http://focus.ti.com/lit/an/sloa099/sloa099.pdf answers it.


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## Francis_Vaughan

Chiming in again.

 Need to set a few parameters. 

 Size and configuration. I have been thinking about a DAC much like this for a while. My personal desire is a DAC that can go in the same case as my PPA. This makes for some interesting constraints. Since I built mine with the battery board there isn't a full width space anywhere. So the board would need to be long and slightly slimmer than the case to clear the power capacitors. It struck me that using the crossfeed mounting holes would be a good plan. 

 Power is an issue. Much like the 2702 DAC, a range of power feed options may be contemplated, but to use in a PPA - or similar - one needs to provide for a virtual earth - this probably means a BUF634 or similar on board. It also means possibly high regulator dissipation if used with PPA realisations with high supply voltages. It does have the nice effect that it can be used with a STEPS PS - something many people already have. 

 If laid out nicely it could use the same Molex connectors for power and audio as the PPA board, and could be configured so that it provides power loop through, enabling very neat wiring in the box. The audio could come out at the same end as the volume control in a PPA, and connect to the secondary audio input connectors right next to the volume pot.

 All this is about how to make it nice for a PPA or perhaps MMM. Clearly nothing should prevent its use in more general settings. But I bet that a huge number of PPA builders would be very interested if the pairing was thought through.

 The design by MWP points out something I had overlooked - the 2707 does provide a system clock output - although the data sheet is rather mute about its specification, it is clearly fast enough to run the 1793 OK. However the whole system is then totally dependant upon the clock integrity of the 2707 - something which is probably not really up to the task at hand. Hence the desire to add some jitter attenuation. An ASRC seems the right answer in this context. Sadly we have little information about the spectral content of the jitter from the different components, so it isn't clear which is the optimal balance. Personally I would even like to add a higher quality clock input for the 2707, on the basis that giving the ASRC as clean as possible a clock on input can only help. The data sheet for the 2707 cautions against an external clock, but only becuse the suspend function will not be able to stop the clock. If you don't intend using this, it is of no matter.

 Crystal oscillators with logic outputs and very low phase noise are available from a number of sources, often very cheaply. The Tent clock is pin compatible, so those with boutique tastes could try one to see if it is any better. Those from Valpey Fisher look very nice. 

 A layout that allows for a base I/V but also allows for that part of the board to be not populated so as to support other (off board, or daught boarded) I/V stages seems very much the right idea. Personally I am rather interested in using a coule of nice Lundhal transformers. Cost as much as the rest of the DAC however. Also discrete solutions like the Hawksford or even the Pass D1 are interesting.


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## ble0t

When you mention the clock output of the PCM2707, are you refering to the FUNC2 pin (I2S system clock output)? I would be heistant to use it as a master clock elsewhere in the system without having anymore specs on it. Perhaps someone can get in touch with TI about it?

 Also, as far as using a higher quality clock for the PCM2707 itself, I've read a couple places (and found some info here) that increasing the quality will not really do much. I would lean toward providing a very clean clock to the ASRC/DAC instead. Perhaps implementing something like this would be cheaper and work nicely for our purposes. We could possibly even look at using something like this.


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## Francis_Vaughan

Yes, I was referring to the Func2 pin. I agree - I would be very hesitant about using it. But it does explain why it is possible to interface a 2707 to a 1793 without additional componentry. That is all.

 I'm not convinced about not cleaning up the 2707 clock. There is more to the clock than the simple Spact system. I'm not convinced that the system clock does not influence the sample clock in the 2707. Improving the system clock may improve the system clcok - it may not totally cure it's ills, but it may hekp. The rationale is that the ASRC can only attenuate - not cure. Better jitter on the way in can only help - and the spectral characteristics of the jitter are not known, which makes it hard to really know.

 The quality of the system clock for both the DAC and ASRC are paramount. Jim Haggerman's clock is likely pretty good - he is a good analog designer. The Maxim clock is less so. Weasel words - "best in class" - and it quotes 21ps. We can find clocks with 3ps. Just not as flexible. But we probably don't need the flexibility.


----------



## MegaMeee

Quote:


  Originally Posted by *Francis_Vaughan* 
_Chiming in again.
 ...

 ...

 Crystal oscillators with logic outputs and very low phase noise are available from a number of sources, often very cheaply. The Tent clock is pin compatible, so those with boutique tastes could try one to see if it is any better. Those from Valpey Fisher look very nice. 

 A layout that allows for a base I/V but also allows for that part of the board to be not populated so as to support other (off board, or daught boarded) I/V stages seems very much the right idea. Personally I am rather interested in using a coule of nice Lundhal transformers. Cost as much as the rest of the DAC however. Also discrete solutions like the Hawksford or even the Pass D1 are interesting._

 

about the 2707 clock I did notice this on the dddac site: "Oh, not to forget, by comparing the Tent Clock and the normal Crystal, The Tent clock brings more defined focus..... When the gain from SPDIF to USB is 100% on some kind of imaginary scale, than this gain would be on roughly 3/4 of that scale......"
 whatever

 I agree with the idea of a base (opamp based?) i/v stage with pins or whatever so we could use our own... although I can see myself trying about 10 different things and then going back to what I started with.

 About the ppa thing would making it "ppa compatible" restrict the psu options? Oh and wouldnt it make things overly complex, because i'm assuming you would want rca outputs as well you would have a crowded back panel (not to mention a crowded inside 
	

	
	
		
		

		
			





 )


----------



## ble0t

My thought for the power supply would be to provide three options:

 1) AC powered (rectifier section, filtering, etc.)
 2) DC powered (bypasses AC rectification)
 3) Powered from external amp (use BUF634 for ground)

 This shouldn't be too difficult and/or space consuming to implement.


----------



## 00940

Bleot : it would be easy only if we use a single supply for everything and a virtual ground (to provide negative voltage for the I/V stage, you'll always need the virtual ground with a single supply).

 It makes it easier to integrate with other components, requires a smaller enclosure as stand alone (you can use a wallwart for example), and so on. However, it also makes impossible to separate digital and analog supplies and complicates the grounding scheme a lot. 

 Aiming for quality rather than ease of use, I'd say the best would be to use two boxes, eurocard sized. One for the transformers (I'd shoot for two) and basic rectification, one for the dac and the regulation.


----------



## Clutz

Quote:


  Originally Posted by *Francis_Vaughan* 
_Chiming in again._

 

I always look forward to your input. 
	

	
	
		
		

		
		
	


	





  Quote:


 Size and configuration. I have been thinking about a DAC much like this for a while. My personal desire is a DAC that can go in the same case as my PPA. This makes for some interesting constraints. Since I built mine with the battery board there isn't a full width space anywhere. So the board would need to be long and slightly slimmer than the case to clear the power capacitors. It struck me that using the crossfeed mounting holes would be a good plan. 
 

I think it's a really good idea, but it would fall on my list of "wants", not "needs", but that if it's something that can be accomodated without much sacrifice elsewhere, then it's a particularly good idea. That said, the way I will build it, will be to follow 00940's lead - one box for the DAC, one box for it's power supply. Actually, it makes me think that perhaps it'd be a good time to build a good STEPS power supply. 

  Quote:


 The data sheet for the 2707 cautions against an external clock, but only becuse the suspend function will not be able to stop the clock. If you don't intend using this, it is of no matter. 
 

Personally, I think the lack of the suspend function is of little consequence. It is something I would very happily give up for better performaince. 



> Crystal oscillators with logic outputs and very low phase noise are available from a number of sources, often very cheaply. The Tent clock is pin compatible, so those with boutique tastes could try one to see if it is any better. Those from Valpey Fisher look very nice. [/QUOTE[
> 
> This is what I would argue for, it'll allow for people to build it with a reasonable quality clock, or build it with a very high quality clock.
> 
> Clutz


----------



## ble0t

My assumption concerning the grounding scheme is one plane that is as solid as possible using ferrites on all supply lines. As long as the 'flow' of the board is done properly, this should work well. As far as the power goes, I would definitely shoot for dedicated, local regulators for all the chips. Perhaps removing the option of AC and just requiring DC would be a good compromise here. This would still allow for the use of and external PS (i.e. STEPS based, multiple TREAD based or perhaps our own design) that would be additionally regulated close to the chips.

 Also, we have agreed that having Iout pads for the I/V stage is a good idea, but are we going to have a 'standard' I/V stage on the board? If so, which one?


----------



## 00940

Quote:


  Originally Posted by *ble0t* 
_Perhaps removing the option of AC and just requiring DC would be a good compromise here._

 

Really, if we go through the troubles of working with only one single supply, providing an AC input is trivial. The big problem is to choose in between one supply line or four.


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## ble0t

Sorry...I should have made myself more clear when I was talking about DC powered. I wasn't assuming the use of a single STEPS or something of that manner. Obviously, you could use one in addition to other things, but I believe we are going to need 4 or 5 different DC supply voltages that should be further regulated onboard.

 As far as the I/V stage goes, there are a couple options I've found thus far (although they might require some modification)...

http://users.verat.net/~rogic2/1541/...screte_I-V.pdf

http://users.verat.net/~rogic2/1541/pdf/max436.pdf


----------



## 00940

Iirc, those I/V from pedja are made for the TDA1541 and won't really work with other dacs.

 I/V options I gathered :

 - opa1632
 - opa134 and the like (single, dual or even quad package)
 - Pass D1 (copyright issues, requires +/-30VDC)
 - transformers (crazy expensive)
 - 2sk389 variant of the D1 ( http://www.diyaudio.com/forums/showt...threadid=34191 )
 - balanced version of Jocko's easy I/V (http://www.diyaudio.com/forums/showt...&threadid=9910 )

 Glassman and Porksoda reworked the Hawksford I/V into something really interesting : http://www6.head-fi.org/forums/showt...=123317&page=2 Worth trying ?


 Bleot : who will have 4 different dc voltage with no common ground (to avoid a ground star where we don't want it to be) on hand ? Really, I think we have only two options in the design of this thing. Or we decide to design the PS according to our DAC, without any considerations of compatibility. Or we decide that the ability of running it from other voltage sources is important (and I can see why it could be) and then we design the DAC according to the PS. 

 The first option will get the people stuck since they will need to do it like we designed it. The second option let the people quite free to use whatever they want (let's say anything providing in between 24 and 30VDC) to power the dac.


----------



## ble0t

As usual 00940, you're probably right 
	

	
	
		
		

		
			





 EDIT: What I was thinking about was something along the lines of this where they have a single higher voltage supply for all the digital supplies (regulate down from it) and a seperate analog PS.

 Personally, I'd like to use a discrete I/V stage, so either the modified Hawksford that Glassman/Porksoda came up with or Jocko's would be nice. I guess then we could argue between feedback vs. non-feedback


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_- opa1632
 - opa134 and the like (single, dual or even quad package)_

 

I think we should ignore the opa134 in favour of the opa1632 for reasons you've previously mentioned.. 


  Quote:


 - transformers (crazy expensive) 
 

Define "crazy expensive"? 
	

	
	
		
		

		
		
	


	




 Increasing costs by 50%? 100%? 200%? 

  Quote:


 - Pass D1 (copyright issues, requires +/-30VDC)
 - 2sk389 variant of the D1 ( http://www.diyaudio.com/forums/showt...threadid=34191 )
 - balanced version of Jocko's easy I/V (http://www.diyaudio.com/forums/showt...&threadid=9910 )

 Glassman and Porksoda reworked the Hawksford I/V into something really interesting : http://www6.head-fi.org/forums/showt...=123317&page=2 Worth trying ? 
 

Do we know enough about each of the options to know the relative performance / cost of each? This is probably obvious, but I like stating the obvious, I don't think we should necessarily not consider an option just because it's expensive until we consider the relative merits of it versus a cheaper option. If it increases the project cost by 50%, but is providing that last 1 to 5%, then I think we have to ask ourselves if this is the best way to spend that money; however if one option is better than another, and it costs 10% more, and gives us 5% better performance, then I think it's worth considering. (I'm just making up numbers here.. I really have no idea what I'm talking about).


----------



## dsavitsk

Quote:


  Originally Posted by *Clutz* 
_Define "crazy expensive"? 
	

	
	
		
		

		
		
	


	




 Increasing costs by 50%? 100%? 200%?_

 

Probably $150-$200 to do it right. http://www.sowter.co.uk/acatalog/SOW...ORMERS_12.html

  Quote:


 - Pass D1 (copyright issues, requires +/-30VDC) 
 

My occasional lurks in the Pass forum suggest that if you ask really nicely and make assurances that there is no commercial intent, creating PCBs for the circuits is not uncommon. And, +/-30V is really not that onerous, so this option is probably worth pursuing.

 There is also a patent issue with AudioNote w/r/t/ the transformers, but again, a friendly request and an assurance of no commercial intent may be enough.


----------



## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_Probably $150-$200 to do it right. http://www.sowter.co.uk/acatalog/SOW...ORMERS_12.html_

 

okay, that meets my definition of crazy 
	

	
	
		
		

		
		
	


	




 but if we left output pads, then someone could still do this on their own if they wanted to, right?


  Quote:


 My occasional lurks in the Pass forum suggest that if you ask really nicely and make assurances that there is no commercial intent, creating PCBs for the circuits is not uncommon. And, +/-30V is really not that onerous, so this option is probably worth pursuing.

 There is also a patent issue with AudioNote w/r/t/ the transformers, but again, a friendly request and an assurance of no commercial intent may be enough. 
 

I don't think that +/-30V is too bad- it just means a more complicated power supply set up... But I'm still interested in a discussion of the benefits of each of these options. How much better are the better options?


----------



## dsavitsk

Quote:


  Originally Posted by *Clutz* 
_I don't think that +/-30V is too bad- it just means a more complicated power supply set up..._

 

Not really. You are still in LM317 territory here, so even a pair of treads could do this. Obviously, a higher voltage transformer (and a safety warning) would be necessary, bt otherwise not a big deal.


----------



## 00940

my grief with the +/-30v of the D1 compared to something like +/-12v for the output stage has three components :

 - we are forced to use our PS, no way to power the thing from a steps or the like. The power supply is an issue not solved yet.

 - we cannot use easily the analog supply for the analog section of the dac (needs 5V) or for an opamp for the balanced to unbalanced converter.

 - 50V caps take a lot more place than 16V caps.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_my grief with the +/-30v of the D1 compared to something like +/-12v for the output stage has three components :

 - we are forced to use our PS, no way to power the thing from a steps or the like. The power supply is an issue not solved yet.

 - we cannot use easily the analog supply for the analog section of the dac (needs 5V) or for an opamp for the balanced to unbalanced converter.

 - 50V caps take a lot more place than 16V caps._

 

That would mean we'd need to have or PSU in a seperate box, and three transformers, which would be getting rather expensive.


----------



## Francis_Vaughan

I think a step back is needed. This isn't intended to be the greatest DAC design ever. Furthmore I am generally unconvinced by the mad dash for utterly seperate power supplies for various parts. The issue is isolation, and that is addressable without going for seperate transformers.

 I would go for a single power supply. DC input, with local regulation where needed. The issue then becomes what we are prepared to provide for the I/V and output buffer. The default answer there is usually +-15v. But +-12v is easily as good. 

 So this would lead us in the direction of a design that takes either +-15 and 0v, or +30 (with the addition of an optional rail splitter) but that will be totally happy running off +-12 (or 24v with splitter). This would be a very generic spec. It should work in most applications, and has the nice advantage that it is compatible with the PPA/MMM + STEPS configuration that has proven so popular here.

 If there is demand and space, one could add additional pads and jumpers so that the insane 
	

	
	
		
		

		
		
	


	




 can break out the powerfeeds and use multiple additional supplies. Personally I very much doubt it will help.

 Using the Pass D1 I/V is always going to be a pain. Apart from the large rails it is physically quite large, and gets hot. Is is always going to be a duaghter card. Indeed there are a few designs floating about. This very much a case of someone simply needing a couple of pads in the right place on the DAC board and then being on their own. Other discrete designs are more interesting. It may be the the board can be laid out so as to make small daughter cards easy. Maybe with pin headers in carefully selected places there can be a base daughter card layout - just defines signal and power pin locations and form factor.


----------



## Clutz

Francis, I think you've made a number of good points - particularly reminding us that this isn't supposed to be the greatest DAC ever. Regarding the power supply, I think perhaps we could take note of Alf's DAC. I believe he found that they over engineered their power regulation - and that it actually measured better when one of the regulators was bypassed. 

 I think that providing pads or jumpers so that a better I/V stage can be used by those who want to should be considered a design necessity. I'm curious as to which I/V stage you would favour Francis? Without really knowing much of anything about anything, I find the OPA1632 to be quite an interesting option. it's relatively low cost and easy to implement. Are the discrete designs substantially better? My preference, in no particular order would be for the OPA1632, using Jocko Homo's balanced I/V, or Glassman & Pork Soda's discrete I/V stage.


----------



## 00940

Quote:


  Originally Posted by *Francis_Vaughan* 
_I would go for a single power supply. DC input, with local regulation where needed. The issue then becomes what we are prepared to provide for the I/V and output buffer. The default answer there is usually +-15v. But +-12v is easily as good. 

 So this would lead us in the direction of a design that takes either +-15 and 0v, or +30 (with the addition of an optional rail splitter) but that will be totally happy running off +-12 (or 24v with splitter). This would be a very generic spec. It should work in most applications, and has the nice advantage that it is compatible with the PPA/MMM + STEPS configuration that has proven so popular here._

 

We will have to run the I/V at +/-10VDC. If we allow the supply voltage to be 24V, we need these drops for final regulation and possible variations in the supply line. It's also too low for discrete designs (they require from +/-15 to +/-20V). We're pretty much stuck with opamps then. We will need a strong rail splitter : almost everything will run from the positive supply.

  Quote:


 If there is demand and space, one could add additional pads and jumpers so that the insane 
	

	
	
		
		

		
		
	


	




 can break out the powerfeeds and use multiple additional supplies. Personally I very much doubt it will help. 
 

It would be rather useless if the grounding has been optimized for a single supply.


----------



## Francis_Vaughan

I had rather deliberately ignored the additional regulator drop for the output stage. But it is an important issue. Depending upon board space there are other options - but we can at least set some parameters.

  Quote:


 It would be rather useless if the grounding has been optimized for a single supply. 
 

 Mostly true - but it doesn't seem to stop people from doing this. Worse, the designs that split the grounds up too and seem utterly unable to grasp the desperately poor ground returns created. There are designs where significant RF energy is only coupled back via the chassis. It is impossible to make any sort of quality system like that. 

 Until one is quite sure there is no RF energy in the signal one needs to maintian ground integrity - and that typically means pretty much up to the output connections - just maybe before the output buffer - but only if the _entire_ reconstruction filter is before the buffer.


----------



## DaKi][er

While you can get away with it in headphone amps, single supply virtual grounds should be avoided, they add far more problems than they fix

 Really, there is 2 ways you could design-
 Either design it to be portable, where you would want it to run off USB power or as this was set out to be, a standalone dac to run off mains with its own transformer, psu and all that with no halfway could be portable could be fixed standalone, it should be either one of the other as designing for both just makes compromises. Have pads for the AC from the transformer/s and put everything else onboard. I’m a very strong believer that the psu should be built in as part of the design and leave very little choice 

 I don’t have any personal interest in building one of these as I already did one for myself and I am not much of a fan for designing projects like these in groups as there are always far too many people wanting to put in their own input and in the end always throws it off course. I like to design by myself, for myself and everyone else is an afterthought.

 What you guy's need to do is work out exactly where you want to end up and start listing the requirements, with everyone having different ideas that will be fun


----------



## ble0t

Given everyone's responses, perhaps lets take a step (maybe a couple 
	

	
	
		
		

		
		
	


	




) back then and define what we're looking for piece by piece so we can come to an agreement. Since the PS has been the topic of most controversy, perhaps we can come to an agreement on that? I would suggest (after reading the previous posts and doing some research myself) going with a single DC supply that uses local regulators near the chips.


----------



## MegaMeee

Quote:


  Originally Posted by *DaKi][er* 
_I don’t have any personal interest in building one of these as I already did one for myself and I am not much of a fan for designing projects like these in groups as there are always far too many people wanting to put in their own input and in the end always throws it off course. I like to design by myself, for myself and everyone else is an afterthought._

 

yes but maybe with a group design we can avoid the issues you had with your amp??? personally I wouldnt know the first thing about designing a dac so I rely on threads like these.

 about the psu I think we can rule out usb powered right away can't we? does anyone here want that?

 so what does everyone think about the power issue??? need someone who knows more than me to list the options


----------



## 00940

I think, as I said earlier, two major options are in front of us.

 -1- We design the DAC as a stand alone, including its PS.

 Advantages : no need to play with a rail splitter, we can get different voltages without adding tons of regulators, we can separate the supplies, we know exactly the quality of the PS, the layout is a bit less complex.

 Problems : push the cost a bit higher, could require two enclosures or a bigger more expensive one, cannot be powered from existing power supplies, cannot be fit inside an existing amp.

 -2- We design the DAC as an add-on card, which would run from power supplies one could find in his amp. 

 Advantages : we can power the dac from existing power supplies, it does not prevent someone to use a dedicated PS, we can fit the DAC inside an existing amp, cost is a bit lower.

 Problems : makes the dac a bit more complex to design (rail splitter, more pre-regulators), restrict our I/V options.

 The real question is : is the possibility to integrate the DAC into an existing amp something important ?


----------



## ble0t

OK, that's a step even further back than I was thinking, which is good 
	

	
	
		
		

		
			





 For me, the ability to integrate it is not something terribly important, so I would vote for a PS designed to fit the DAC.


----------



## Clutz

I agree with ble0t and vote for option 1.


----------



## MegaMeee

so we are looking at a single dc compared to ac (or multiple ac in's) from 1 or more transformers?

 Personally I would prefer using transformers rather than building a steps or something to go with this...

 I guess the issue is will having a virtual ground be an issue with our uneven current draw and things??
 Also is having regululation for 5 and 3.3v silly from a 24-30v power supply?


 If we were to go with ac in from transformers would we want one transformer to provide +- 12-15? or provide other voltages as well?

 I know francis said on page 3 he thinks a single dc in would be as good... but i'm not so sure.

 I guess theres a few of us that want ac in... so if anyone wants dc in for amp compatibility I guess speak up now and we can look into that option further


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## Clutz

By self powered I would mean either DC input (either 24 or 30 volts), and then deal with converting the input voltage into the required voltages for each of the sections.


----------



## Clutz

I PM'd PorkSoda about the I/V stage that he and glassman designed, since he was planning on using it with a Monica II DAC. He said that he didn't suggest we use it, as it alone was rather large and that it would take about the same amount of time and work as a dynalo - which means if we wanted to use it as an I/V stage, we'd need to go with a two PCB solution - one for the I/V stage, and one for everything else. In my opinion, everything is pointing in the direction of op amps

 Cheers,
 Clutz


----------



## Clutz

Having given it some thought, discussed it with some friends of mine, and PorkSoda, my opinion is that we're probably best served by using the OPA1632 for our I/V stage, or going with six regular op-amps. 

 Additionally, can anyone point me to a Eagle tutorial?

 Thanks,
 Clutz


----------



## Francis_Vaughan

I would support this approach. I tend to view this DAC as the next step up for DIY DACs. Not the greatest there can be. A well understood design with a general avoidance of heroics, black magic, and wildly opinionated design ideas should be the goal. Something solid.

 There are issues with any op-amp based I/V, but with care they can be ameliorted to a reasonable amount. There will always be the school that dislikes op-amps in this role, and that is fine. But these issues are best served by the step beyond. Which isn't this design. (I'm building up a HagDAC - PCM 1704 based - and am sooo tempted to try a transformer output a bit later. But that is comeasurate with the price and goals of Jim's DAC. It would not be for this one.)

 Some care is needed. The nature of the I/V stage means that SMD is going to be pretty much crucial. The idea of socketing op-amps and going for op-amp rolling in an I/V stage is simply not on. Success will come from very careful layout and control of the ground and power. Isolation of the stages will be key. Similarly the reconstruction filter will need careful thought and layout. There is much to be said for passive designs here.


----------



## eVITAERC

Regarding the OPAMP I/V, have you guys looked at what Peufeu implemented in his TDA1545 based DAC? His approach used a simple resistor tor I/V, but with a bunch of stuff tacked on to make sure that the resistor did not change the voltage that the DAC is seeing at its output.

 Link: http://peufeu.free.fr/audio/extremis...plement_4.html


----------



## 00940

I've been rethinking this thing a bit.

 - An interesting I/V could be opa1632 followed by INA134. We could get a path without capacitors, the INA134 as a nulling offset pin. It's exactly what Bel Canto used (they have a servo on the offset pin though).

 - I'm reconsidering the dedicated PS. It would require everyone to play with high voltage AC to hook up the transformers. Not sure it's such a good idea.

 - I've actually been playing a bit in Eagle. I remade a little library with interesting parts, such as PCM1794, pcm2707, ad1896, reg101, tps79333, opa1632. I'll upload it tomorrow. First layout tests show it should be quite easy to fit everything on an eurocard.


----------



## MegaMeee

Dedicated psu or dc in I don't really care... I don't see the difference with playing with ac when building a steps or something and playing with ac in the dac.

 I've just got an irrational dislike of virtual grounds for some reason


----------



## Clutz

Quote:


  Originally Posted by *Francis_Vaughan* 
_There are issues with any op-amp based I/V, but with care they can be ameliorted to a reasonable amount._

 

I'm curious, what are the typical issues of an op-amp based I/V? Is it that we're passing an analog signal through more components than we need to (e.g. make a design only as complicated as it needs to be), and thus increase the amount of noise accumulated?

  Quote:


 There will always be the school that dislikes op-amps in this role, and that is fine. But these issues are best served by the step beyond. Which isn't this design. (I'm building up a HagDAC - PCM 1704 based - and am sooo tempted to try a transformer output a bit later. But that is comeasurate with the price and goals of Jim's DAC. It would not be for this one.) 
 

And we can leave Iout pads from the DAC, so that people could wire a transformer in if they were so inclined.. 

  Quote:


 Some care is needed. The nature of the I/V stage means that SMD is going to be pretty much crucial. The idea of socketing op-amps and going for op-amp rolling in an I/V stage is simply not on. Success will come from very careful layout and control of the ground and power. Isolation of the stages will be key. Similarly the reconstruction filter will need careful thought and layout. There is much to be said for passive designs here. 
 

I understand (or more correctly, I'm aware of) the problems associated with socketting DAC chips, but what are the problems with socketting the op/amps in the I/V? Minimizing unwanted resistance? The OPA1632 comes in an SOIC-8 package, so that isn't a problem there.


----------



## 00940

the problem with socketed opamps is not resistance but inductance.

 the problems with opamps as I/V converters are documented in those papers from Barrie Gilbert :

 Opamp myths : http://archive.chipcenter.com/analog/c007.htm
 Spicing up the opamps : http://archive.chipcenter.com/analog/c011.htm
 Are opamps really linear ? : http://archive.chipcenter.com/analog/c014.htm

 From this thread, http://www.diyaudio.com/forums/showt...threadid=55422 , it was made clear that one of the most important parameters of an opamp performing I/V would be gain bandwith.

 The interesting point then is that the OPA1632 looks also more interesting than the opa627 on that parameter. For small signals, the opa1632 achieves 180MHz of bandwith against 16MHz for the OPA627. The OPA1632 only achieves such performances however with very small signals (one tenth of the volume needed for full line out).


----------



## Clutz

Thanks for the links, I'll give them a read. I'm kind of curious of trying a resistor based I/V, but if we include an Iout pad too, we should be able to try it. 
	

	
	
		
		

		
		
	


	




 Cheers,
 Clutz


----------



## ble0t

Perhaps passive I/V would be the way to go then if op-amps seem inferior...


----------



## eVITAERC

I agree with the passive I/V route, there really is nothing wrong except for the fact that the DAC will see a varying voltage on its Iout. But that can be remedied with Jfets.


----------



## Clutz

00940 provided some links about the cons of using an op-amp based I/V, but what about the cons of going passive? I would prefer to go with an op-amp based I/V, and provide Iout pads from which someone could wire in a passive I/V section if they wanted to go that route, or would that again lead to problems with inductance? 

 I'm going to try and do some reading about passive vs op-amp based I/V today.


----------



## Clutz

After doing (briefly) some reading on the internet about the benefits / drawbacks of an opamp versus a passive i/v stage on a DAC, a few things occurred to me.

 First, we must remember that this is probably going to be plugged into a headphone amplifier, and then through headphones into our heads. If one of the main advantages of an op-amp based I/V section is that we get greater voltage out (2Vrms), and thus when listening we don't have to turn the volume up as high, then I'm not sure this is a benefit worth while trading off for better sound quality (as has been argued) - I can always just increase the gain on my amp. A passive I/V would also be cheaper (???), which would mean we could possibly re-route any budget that would'v ebeen spent there elsewhere (???).

 But on the other hand, I'm not sure I buy the argument that the passive necessarily sounds better. Is it because in practice, a good passive I/V is much more cost effective to implement than an opamp I/V? If the signal is ultimately going to be passed through an amplifier again anyway (headphone amp, ore preamp-> poweramp), does it really matter if we have an opamp gain stage in this design? 

 One potential major benefit of going with passive, even if we didn't add anything else to the design anywhere else - is that it might take up a lot less PCB space- and we could get it on a half eurocard - which might be a sufficient benefit itself. 

 Let me be clear, I'm not against a passive I/V, but at this point I think I'd prefer to build an opamp based I/V, esp. if a passive I/V could be implemented offboard by connecting up to a set of Iout pads. I'd like to be able to hear the difference myself, and I think if we can design a PCB using an opamp I/V, and allow Iout pads so individuals can implement their own passive (or other) Iout, than this is a better solution, because if we go with only a passive I/V on the PCB, then an opamp I/V is pretty much out of the question for anyone who is interested, right?


----------



## 00940

A passive I/V is not a lot cheaper. Its driving abilities are way too low. You need to follow the resistors with an active section anyway. 

 Furthermore, designers tend to output 2V or higher with opamps because it's standard but also because it's reducing the noise/signal ratio.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_A passive I/V is not a lot cheaper. Its driving abilities are way too low. You need to follow the resistors with an active section anyway. 

 Furthermore, designers tend to output 2V or higher with opamps because it's standard but also because it's reducing the noise/signal ratio._

 

So then your vote is for an opamp i/v section as well?


----------



## 00940

Quote:


  Originally Posted by *Clutz* 
_So then your vote is for an opamp i/v section as well?_

 

Since I've been reading this : http://www.diyhifi.org/forums/viewto...er=asc&start=0 , yes.

 I found it making an unrelated research on google. Andy_c brought a convincing explanation on why all the articles I brought here earlier are perhaps correct but not applicable to our I/V. Well worth reading.

 So I'm putting water on the fire I lighted. After all, the opamps could be perfectly ok as I/V.


----------



## ble0t

Very informative thread....I knew there was a reason I liked that forum 
	

	
	
		
		

		
			





 It seems to me that it's really a matter of preference. A passive stage would be cheaper though if we're looking to keep costs down.


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## Clutz

As i count it, those in favour of active:


 Francis_Vaughn;
 00940(???);

 those who seem to be in favour of a passive:
 dsavitsk;
 eVITAERC;

 I'm not entirely sure were ble0t, MegaMee, or guzzler stand on the issue. It's been a while since we've heard from steinchen. 

 Depending on how the layout works, would it be possible to implement both some form of active I/V, and a simple passive I/V? Or are we entering the territory of too many trade offs here? I digress..

 As far as an active I/V is concerned, from what I've read it seems that some people think a discrete solution is better than an op-amp solution. I haven't seen many people who've argued that an op-amp i/v is better than an active discrete one. That said, with limited space, are we going to be able to achieve a discrete I/V that will outpace an opamp? If so, will it out perform it sufficiently to warrant the extra complications? 

 I think there are good reasons to go with an opamp based I/V; at least there are reasons I can somewhat follow. Anyway, i'm going to leave it at that. I think we need to come to a decision about this, or should we start to work on the schematics for the other sections first, and then revisit it after we've all had some more timeto think?


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## Francis_Vaughan

I'm happy with op-amp mostly as it should provide a solid, well understood design. Something I think this DAC should strive for. Passive here means a single resistor - and that is a bit more contentious. There may be much scope for experimentation, and a revised design. Part of the issue with the passive designs is that they seem hard to get right without tweaking. A couple of transformers is about $160 - so we can ignore that option.

 Something the Barrie Gilbert articles also outline is how crucial the power supply is here. Get this wrong and the I/V will not be good. As usual for audio the issue is not the ultimate in accurate voltage, but in getting a stable, well decoupled, noise free PS. One that is happy across a wide bandwidth. This is something best addressed with very careful layout and decoupling - and is not well served by the multiple transformer style. It is importnat to note that noise imposed onto the I/V supply by the I/V's operation must be kept away from the earlier circuits. There has often been an attitude that the digital circuits don't matter, and that all care must be lavished upon the "analog" stages. The reality is that they are all equally crucial.

 There is much work that can be done without getting too worried about the final I/V realisation. The most important part is the clock. The precise layout is very important, as are the power supplies. The I/V stages will be easy in comparison.


----------



## dsavitsk

Quote:


  Originally Posted by *Clutz* 
_those who seem to be in favour of a passive:
 dsavitsk;_

 

But I wouldn't take my vote too seriously. I am working on a more sophisticated nos dac than my last one right now, so I likely won't build this one. So, for the people that like the passive I/V, I think providing an Iout pad is sufficient.


----------



## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_But I wouldn't take my vote too seriously. I am working on a more sophisticated nos dac than my last one right now, so I likely won't build this one. So, for the people that like the passive I/V, I think providing an Iout pad is sufficient._

 

You may not build this one, but I value your input. 

  Quote:


  Originally Posted by *Francis_Vaughn* 
_Something the Barrie Gilbert articles also outline is how crucial the power supply is here. Get this wrong and the I/V will not be good. As usual for audio the issue is not the ultimate in accurate voltage, but in getting a stable, well decoupled, noise free PS. One that is happy across a wide bandwidth. This is something best addressed with very careful layout and decoupling - and is not well served by the multiple transformer style. It is importnat to note that noise imposed onto the I/V supply by the I/V's operation must be kept away from the earlier circuits. There has often been an attitude that the digital circuits don't matter, and that all care must be lavished upon the "analog" stages. The reality is that they are all equally crucial._

 

So at the very least, this would involve careful placement of decoupling capacitors on the power and ground pins of the opamps used in the I/V section then, correct? Perhaps a pair of decoupling caps in parallel, something like 0.1 and 0.01, and then in series with that a ferrite bead? (+ it would have it's own reg102/105/whatever)?

  Quote:


  Originally Posted by *Francis_Vaughn* 
_There is much work that can be done without getting too worried about the final I/V realisation. The most important part is the clock. The precise layout is very important, as are the power supplies. The I/V stages will be easy in comparison._

 

If the most important part is the clock, then is it truly overkill to go with an X0 clock?


----------



## guzzler

For this, I'd be happy with an opamp design. 

 F_V, is it possible to use a single resistor with a differential output?

 As for clocking, I would think that putting in space for an 8dil oscillator with a good regulated supply devoted solely to it would be the best plan without going overboard. The extra pads that puts on also would allow something like a Tent Clock XO2 to be put on, if a oscillator didn't cut it for you. The 8dil is the most common variety, and you can get lots of good x-tals (including tent xo) in that package


----------



## MegaMeee

opamps for me 
	

	
	
		
		

		
		
	


	




 the question is which ones


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## 00940

Guzzler : of what clock/oscillator are you speaking ? The one feeding the PLL in the PCM2707 or the clock feeding directly the AD1896 and the DAC ? I would put an oscillator for the pcm2707 and a canned clock for the second.

 Clutz : yes, I'm in opamp camp. We should perhaps consider the THS4631, but I'm wondering if we can get that beast under control. The datasheet of that opamp interestingly goes on at length about I/V functions.

 I wanted to get a grasp of the size of this thing. Here attached is an eurocard sized pcb (100X160mm). You can see routed the whole section PCM2707+AD1896, with its local regulation. To give you an idea, most other dac, opamps, regulators are also on board, and the big caps on top are 2200uf/25V. Looks like we can fit a lot on such a board.


----------



## DaKi][er

Quote:


  Originally Posted by *00940* 
_I wanted to get a grasp of the size of this thing. Here attached is an eurocard sized pcb (100X160mm). You can see routed the whole section PCM2707+AD1896, with its local regulation. To give you an idea, most other dac, opamps, regulators are also on board, and the big caps on top are 2200uf/25V. Looks like we can fit a lot on such a board._

 

For a bit more perspective, this is just a tad over 1/2 a eurocard in size, 100mm wide and about 90mm long from memory
http://www.mypage.tsn.cc/dakiller/dac1/Images/Board.png
http://www.mypage.tsn.cc/dakiller/da...Assembled2.JPG

 Fit my whole dac on there, and I think I got a lot more on mine than you plan to have


----------



## Francis_Vaughan

Quote:


 s it possible to use a single resistor with a differential output? 
 

I can't see why not. The design after still remains a little more complex than a single sided I/V but there may be much to be said for this. Just hang a resistor across the outputs, then build a passive reconstruction filter - still differential - then to a proper balanced line receiver stage. Could work very well. 

 The issue with this is that we strike out into some level of the unknown. This means multiple rounds of protoypes.


----------



## guzzler

Quote:


  Originally Posted by *00940* 
_Guzzler : of what clock/oscillator are you speaking ? The one feeding the PLL in the PCM2707 or the clock feeding directly the AD1896 and the DAC ? I would put an oscillator for the pcm2707 and a canned clock for the second._

 

Same

 F_V, I'm still curious about using resistors for I/V. Iout +/- each with a resistor to ground, so your output is balanced voltage then to a summing opamp? Feasible, or completely wrong?


----------



## Clutz

With the apparent gobs of PCB space we're going to have, I think we have a question to ask: Is it possible to implement both a passive and an opamp based I/V on one PCB, and then just populate the I/V section the builder wants to? 

 Even if we can, do we want to - or would it be better to save on the PCB costs and go with a half eurocard?


----------



## guzzler

Quote:


  Originally Posted by *Clutz* 
_With the apparent gobs of PCB space we're going to have, I think we have a question to ask: Is it possible to implement both a passive and an opamp based I/V on one PCB, and then just populate the I/V section the builder wants to? 

 Even if we can, do we want to - or would it be better to save on the PCB costs and go with a half eurocard?_

 

The general impression was not for flexibility, just outputs direct from Iout for people that want to experiment. Putting two seperate things on the board will only complicate things, and make the layout harder and less coherent


----------



## Clutz

Quote:


  Originally Posted by *guzzler* 
_The general impression was not for flexibility, just outputs direct from Iout for people that want to experiment. Putting two seperate things on the board will only complicate things, and make the layout harder and less coherent_

 

Alright, forget that idea then.


----------



## ble0t

So are we agreed on the flow of the DAC (PCM2707 --> AD1896 --> PCM179*) at least?


----------



## steinchen

is the spdif-input definately axed ? just for saving 7 bucks ? 

 I don't want to start the whole discussion all over, but why didn't you consider ad1955 or pcm1704 for the DAC ? Since you are going to use lt1764/lt1964 (which I regard as the best 3 terminal regulators atm) money can't be such a big concern.


----------



## ble0t

Well, the purpose (as we've come to define it) was to design a USB based DAC. SPDIF was discussed, but it wasn't the price difference persay, but the implementation and board size that was the main deciding factor.

 Also, the difference between other voltage regs and the lt1763/lt1964 is something like $2. The difference between a PCM1798 and a PCM1704 is upwards of $10 and that's just for the U version, not the J or K which could be as much as $18 more. The AD1955 looks to be more around the price range that I think we are looking at, although I haven't seen many reviews of it around.


----------



## Clutz

Quote:


  Originally Posted by *steinchen* 
_is the spdif-input definately axed ? just for saving 7 bucks ? 

 I don't want to start the whole discussion all over, but why didn't you consider ad1955 or pcm1704 for the DAC ? Since you are going to use lt1764/lt1964 (which I regard as the best 3 terminal regulators atm) money can't be such a big concern._

 

I'm probably going to over answer this, but whatever.

 I think we axed the SPDIF input because we had to decide what our design focus was, and it was to make a USB DAC that was better tha 2702/4 designs, but not trying to go for the absolute best USB DAC ever. As far as I'm concerned, nothing is really set in stone until we send whatever PCB files off to the PCB manufacturer. If we can easily include an SPDIF, then I'd be agreeable to it- but the thing. As you point out the CS8614 is pretty inexpensive, $6.18 or so from digikey when buying in single quantities. I don't think the major cost here is buying the chip, but will including it result in us having to have a lesser PCB layout to accomodate it. IMHO, that's the cost I care more about. 

 Regarding using the AD1955, I don't see any reason why we couldn't use it - it's a stereo chip in the same price range ($12); but why do you prefer it to the 1798 - on the specs, it doesn't seem to be that much better. Yes, it can do SACD decoding and the PCM1798 can't, but that only makes a difference if we have external SPDIF input, because the PCM2707 can only output 16bit@44.1khz, so we couldn't take any benefit. So I'm a bit lost here.

 But the PCM1704 is a single channel chip that costs quite a bit more. If we used them, instead of the cost of the DAC(s) chip being between $10 and $20, it would be between $40 and $80, and add another $20 for the digital filter and we're at $100. At that point we're basically re-inventing DaKiller's wheel (which looks like a pretty good wheel mind you). Instead of going with the PCM1704s, I'd rather include a really good clock, or use the money for another project. More than double of the cost of using the PCM1794 DAC chips. So, from my perspective, the PCm1704s are sort of a non-starter.

 I'll just do a quick running total of the chip parts, from Digikey

 PCM2707 $6.80
 AD1896 $18.75 each
 PCM1798 $10.85 each

 So that's $36.40 / DAC under the current 'design philosophy'. Add $10 or so if you're planning on using the PCM1794 (which is pin compatible, just slightly better specs). Or another $2 if we used the AD1955 instead. Add another $6 for the CS8614 and we're at $42. We'd also have to include one or two more voltage regulators - so add another $2 to $4 again. 
 SPDIF receiver 

 CS8614 - $6.18



 If we did include SPDIF input, then how would the set up work?
 (PCM2707 or CS8614)->AD1896->PCM1798

 or would it be

 PCM2707 (SPDIF out) -> CS8614 -> AD1896 -> PCM1798 or AD1955?
 external SPDIF input -^

 The way DaKiller has done it, requires some control circuitry to choose between USB and SPDIF input, which would also have extra costs to implement it. An

 Prices of the linear regulators

 LT1964ES5-5-ND -5 volts 220mA $2.70 each
 LT1964ES5-BYP Adjustable 220mA $2.70 each (out of stock)

 And various other DACs mentioned above. 

 AD1855 $12.50 each
 PCM1704U $20 each (we'd need two)
 PCM1704U-J $24 each
 PCM1704U-K $36 each
 DF1704 - $16
 DF1706 - $20


----------



## Clutz

Here is an opinion about the 1704 versus 1794. They don't seem to believe that the 1704 is necessarily better than the 1794, in what sounds like a similar design.

http://www.audioasylum.com/scripts/t.pl?f=KandK&m=2101


----------



## dsavitsk

I think that using a chip that is at its best with a microprocessor, and then not using a microprocessor, is probably a little silly. I believe that the 1704 is in this class. Done well, the 1704 is really good sounding, but my guess is that without external control it is not worth the money. It is also worth pointing out that TI won't send you samples of this chip anymore 
	

	
	
		
		

		
		
	


	




 so I'd look toward the cheaper ones.


----------



## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_I think that using a chip that is at its best with a microprocessor, and then not using a microprocessor, is probably a little silly. I believe that the 1704 is in this class. Done well, the 1704 is really good sounding, but my guess is that without external control it is not worth the money. It is also worth pointing out that TI won't send you samples of this chip anymore 
	

	
	
		
		

		
		
	


	




 so I'd look toward the cheaper ones._

 

Agreed, but I didn't think the 1704 needed a microcontroller. I've read the datasheet for the 1704 and I don't see anything about a microcontroller - did you mean the 1792?


----------



## dsavitsk

I think you're right, my mistake. I was thinking about the processing done in my dac (the one I own, not theone I designed) that is done before the 1704.


----------



## Clutz

No more thoughts on the matter? Any other responses to steinchen's idea of including an spdif in? Which opamp solution to go with for i/v? 
	

	
	
		
		

		
		
	


	




 I'm still trying to get some reading done at the same time that I'm trying to write part of my PhD thesis.


----------



## DaKi][er

Quote:


  Originally Posted by *Clutz* 
_No more thoughts on the matter? Any other responses to steinchen's idea of including an spdif in? Which opamp solution to go with for i/v?_

 

spdif requires a receiver chip to decode it, then you would need a mux to switch between the receiver I2S output and the usb chip output, quite easy to do but it adds more parts and complications there

 opa627 is probably one of the more popular chips for i/v conversion for high end gear, others you could look at are the ths4601 and 4631 which are faster and also cheaper to buy


----------



## 00940

- I'm a bit reluctant towards the OPA627. It is very expensive for this design (we need at least 6 !). And I don't think it's worth all this money. It's indeed included in many high end dac or cdp, but mostly in what I'd call the "lazy" high end, constructors who just pick the more expensive parts and put them together. It's good, I like it, but not enough to drop 90$ on it (1/2 of the budget planned)

 - 2 PCM1704 + 1 DF1704 would make the price skyrocket. And we have to scrap the balanced option (thinking of it, should we consider balanced output as a necessary option ?).

 - SPDIF input. I "hate" the CS8416, it's a real nightmare to layout and it needs billions of pull-down/up, taking up a lot of board space. The TI DIR1703 is still available and is easier to work with, but has problems switching frequencies on the flight (reason it's not advertised anymore by TI). If we include a hardware reset for when it doesn't lock properly, it shouldn't be a problem for a diy dac. Its specs and price are better than the CS84** serie too.

 - The LT regulators are best for higher voltage. For 5V and below, the specs of the TI parts (reg101, tps793**) are better iirc (to check).

 Just my 2c.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_- I'm a bit reluctant towards the OPA627. It is very expensive for this design (we need at least 6 !). And I don't think it's worth all this money. It's indeed included in many high end dac or cdp, but mostly in what I'd call the "lazy" high end, constructors who just pick the more expensive parts and put them together. It's good, I like it, but not enough to drop 90$ on it (1/2 of the budget planned)_

 

I'm reluctant to use the OPA627's here too,but the THS4631 would work, and are quite a bit cheaper. What about the OPA1632's that you had suggested earlier? Another approach that we could go with, another would be to do something like the AD844 following Pedja's design with a TDA1541, available here http://users.verat.net/~pedjarogic/a...a1541a_dac.htm, as has been suggested on DIYHifi. 

  Quote:


 - 2 PCM1704 + 1 DF1704 would make the price skyrocket. And we have to scrap the balanced option (thinking of it, should we consider balanced output as a necessary option ?). 
 

I don't think we should use the PCM1704s either. I asked for some input about it on DIYHifi (PCM1704 vs PCM1794/8 with a good clock), and Jocko Homo said that he felt that the PCM1794 was better than the 1704 anyway. Jim Haggerman said that he felt that the PCM1794 was a good chip- that he just prefered the 1704 cause it gave him more control over some of the details. So I think that this issue is settled. PCM1798 (/4)

 [quopte]- SPDIF input. I "hate" the CS8416, it's a real nightmare to layout and it needs billions of pull-down/up, taking up a lot of board space. The TI DIR1703 is still available and is easier to work with, but has problems switching frequencies on the flight (reason it's not advertised anymore by TI). If we include a hardware reset for when it doesn't lock properly, it shouldn't be a problem for a diy dac. Its specs and price are better than the CS84** serie too.[/quote]

 Then no CS8416, easy. I can't find the product sheets for the DIR1703. I'm kind of iffy on SPDIF in if for only one reason, we're trying to go for a USB DAC, and how much is sacrified in terms of layout by trying to include a SPDIF in? 

 As far as I'm concerned, the following has been agreed to

 PCM2707 -> ASRC (AD1896) -> PCM1798 (/4)

 What remains to be determined is, will we also have an SPDIF in, and the I/V. SPDIF in is really a yes no question. If we say yes, then is it a TOSLink or Coax, or both? If it was included, then I'd probably use it as a DAC for my Onkyo CDP, but I would plan on using it primarily as a USB DAC. 

 Regarding I/V, we seem to have stopped considering the OPA1632- any particular reason for this? It seemed like a pretty interesting option to me. The THS4631 is still a pretty expensive chip at $10 a pop.


----------



## steinchen

Quote:


  Originally Posted by *Clutz* 
_Another approach that we could go with, another would be to do something like the AD844 following Pedja's design with a TDA1541, available here http://users.verat.net/~pedjarogic/a...a1541a_dac.htm, as has been suggested on DIYHifi._

 

I'd try to avoid caps in the signal path as far as possible and particulary electrolytics at all cost.


----------



## 00940

Regarding the output capacitor. We have a balanced outputs DAC. When we summ the two outputs to go single ended, most of the offset should go away naturally. Bel Canto obviously uses a servo on the INA134 doing the summing. Maybe could we do something similar ?

 No option is out for me yet, including the opa1632 
	

	
	
		
		

		
			





 The THS4631 worries me a bit since it has been reported as quite unstable.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_Regarding the output capacitor. We have a balanced outputs DAC. When we summ the two outputs to go single ended, most of the offset should go away naturally. Bel Canto obviously uses a servo on the INA134 doing the summing. Maybe could we do something similar ?

 No option is out for me yet, including the opa1632 
	

	
	
		
		

		
		
	


	




 The THS4631 worries me a bit since it has been reported as quite unstable._

 

The INA134 seems pretty interesting. Figure 2 on page 8 of the product information sheets shows how the chip can be connected up to adjust offset, which would be a way to minimize DC offset without any capacitors in the signal path, no?


----------



## 00940

http://www.diyhifi.org/forums/viewto...4a5b743a9adea8

 We'd better follow this closely.


----------



## ble0t

Agreed....just saw that thread myself and it's nice to have someone who can take some good measurements. Hopefully, the PCM2* series does live up to its billing


----------



## SnoopyRocks

I've been following this thread for a while and it seems promising. If there will be spidf input, it would be something I'd like to participate in. Otherwise, I'll pass.

 There is nothing special about how the INA134 sets the output common mode voltage. It's nothing more than a nice opamp with well matched resistors in the same package. With a noninverting opamp, the output common mode can be set arbitrarily with the voltage at the positive input.

 I would not read too much into the measurements in the linked thread from diyhifi. All that they do is confirm that a low performance part lives down to expectations. Performance is not a priority for an audio dac with a THD+N of 74.3 dB (~12.5 bits). Why would the clock be that much better than the dac in the CM108? If it was a Burr Brown, ADI, etc. part, then it might mean something.


----------



## 00940

Snoopyrocks : in the thread on diyhifi, there are graphs about the TI TUSB3200 and they're not good at all. The TUSB3200 is used for example in the USB add-on card of the Apogee mini-dac or inside the m-audio sonica.


----------



## Francis_Vaughan

The TUSB3200 is also used in the M-Audio Audiophile USB, and probably a great deal more of their products. I must say that I am reasonaly impressed with the sound of mine - which actually surprises me a bit - despite the name it isn't exactly high-end in terms of componentry.

 The interesting this about this device is that it uses a software controlled PLL that vary the divider rates of various frequency syntesisers. The precise parameters of the PLL control possibly vary from implementation to implemetation. The very wide smooth spectrum of jitter may be a result of the very fine grained frequency changes. The unfortunate spread may be the result of hunting for the USB sample rate - which is partly a function of the PC and OS doing the sending.

 Measuments of a SPACT based unit will be very interesting. In principle it should be much more resistant to the wide spread. But on the whole you can see why I am so enthusiastic about the use of an ASRC to help control jitter. What will really matter will be the spectrum of the jitter, and the jitter attenuation spectrum of the ASRC. We may be lucky and get a good match, or there may be residual components that are very difficult to control.


----------



## Clutz

I think the last page of this thread is also going to be interesting. http://www.diyhifi.org/forums/viewto...t=288&start=75
 Cheers,

 Clutz


----------



## Clutz

I've been giving this some more thought over the past few days. We're aiming for a DAC that is going to be a substantial improvement over a simple PCM2702/4 implementation. At the same time, if we want to incorporate an SPDIF input, then I think we're entering the realm where we want to design a serious DAC and give extra consideration to the I/V section? Snoopy, what is your take on opamps for I/V? I am really curious about the jitter performance of the SpAct system - hopefully we'll have some results at diyhiif soon that we can examine. 

 One of the big disadvantages of using SPDIF in, isn't including the extra $6 SPDIF input reciever, it's the extra circuitry we'll need to control which input we're using and we'll have to feed the ASRC using SPDIF instead of I2S. I2S seems to have less jitter problems than SPDIF- at least if I'm understanding things from diyhifi properly.. but maybe I'm not. I'm not sure if it matters at that point, cause we're reclocking it and then sending the signal out from the ASRC via I2S.. but, still..just thinking (mental vomiting?) outloud. 


 Francis, at diyhifi, there was something mentioned about how an ASRC ends up turning jitter into some other sort of audio error - they questioned the benefit of using an ASRC. I didn't understand any of it, can you explain it? I'm trying to find a link for it - but coming up blank. 

 I'm still interested in this, and in way over my head. If nothing else, I'll probably try to put together something based on the examples in the datasheets and then lay it out on a PCB board. One reason that that SPDIF in is somewhat appealing, is it would possibly make my Onkyo CDP sounda lot better.


----------



## SnoopyRocks

Yes, this would be a serious DAC. That is the point after all. To get performance head and shoulders above a PCM2702, significant effort is required. However, the choices of an ASRC, OpAmp I/V, and using the digital filters built-in to the DAC greatly simplify things to a point where it is manageable. The hard work (chip design) has been done for us, all we have to do is put the pieces together.

 OpAmps are the cheap and easy way to accomplish the I/V. For a first prototype, it is a proven solution that is trivial to implement and difficult to botch. Since the DAC has current output, it is perfectly fine to run this off the board to another without meaningful signal loss to try more elaborate solutions. My opinion is that OpAmp I/V is one less thing to worry about. There are lots of cute other ways to make the transimpedance stage...worry about this later. There is no reason to get too greedy...yet 
	

	
	
		
		

		
		
	


	




 Selecting inputs is simple to implement with logic. Don't worry about it now. The hard part is getting motivated to actually do the work (at least for me). Right now that means deciding on the basic components and the power source.

 Some comments on jitter/reclocking since Clutz brings it up:

 There are two ways to traditionally handle the jitter problem: reclocking with a good PLL or with an ASRC. Of the two, an ASRC is _much_ easier to implement because it basically only needs to be fed a high quality clock to work well, something that can be done without too much difficulty and would be necessary regardless. Reclocking with a PLL is more effort because the loop filter must be carefully designed to balance the tradeoff between minimizing the jitter from its own VCO and the jitter in the incoming clock. What I have seen of this (PLL that is) in the DIY community is basically trial and error, with a lot of tuning (you can read between the lines here).

 Yes, ASRCs do filter jitter on the incoming clock. Basically, ASRCs reconstruct (interpolate) the signal to great precision (think analog) in the digital domain. From the reconstructed input, the output is taken at the appropriate time, chosen by the high quality clock. Timing errors (jitter) are accumulated and filtered. Instead of showing up as errors in time, the jitter results in amplitude (digital word) errors at the output, only at a much lower level than they came in at. What is important to note is that an instaneous error in the timing of one clock edge is spread over many samples instead of just the one. Naturaly, jitter on the clean clock, if any, remains unaltered though.


----------



## 00940

- The design of a secondary slow PLL is way to complicated for us. We have to stick to ASRC or forget any jitter reduction.

 - The AD1896 requires to be fed with I2S. If we have to add SPDIF in, we will either have to switch in between two I2S lines (one coming from the spdif receiver, one coming from the usb receiver) or to get spdif out from the usb receiver and switching spdif lines at the input of the spdif receiver. The last option create more jitter according to the post of John Swenson. Does anyone know how to properly switch I2S lines ?


----------



## ble0t

Wouldn't a multiplexer work? It would require some additional logic, but it should be up to the task.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_- The AD1896 requires to be fed with I2S. If we have to add SPDIF in, we will either have to switch in between two I2S lines (one coming from the spdif receiver, one coming from the usb receiver) or to get spdif out from the usb receiver and switching spdif lines at the input of the spdif receiver. The last option create more jitter according to the post of John Swenson. Does anyone know how to properly switch I2S lines ?_

 

This is what I was worrying about. This is where the real cost of implementing an SPDIF in comes from, imho.


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_Wouldn't a multiplexer work? It would require some additional logic, but it should be up to the task._

 


 Sorry ble0t, didn't see this post before I had responded. A multiplexer would seem to work, if we can find an appropriate part.


----------



## 00940

The 74HC257 was mentionned on diyaudio for similar applications.


----------



## ble0t

So, the question then is how to get the multiplexer to decide which signal is present. We could come up with some sort of digital logic (not using a uC) to automatically switch it or use some analog circuitry based on a toggle switch.


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_So, the question then is how to get the multiplexer to decide which signal is present. We could come up with some sort of digital logic (not using a uC) to automatically switch it or use some analog circuitry based on a toggle switch._

 

My vote would be for a toggle switch. DaKiller had some simple circuit that he used so that he could use a simple push button to switch between sources. I think some sort of automatic detection switch is overly complicated. I'll be building two anyway- once for my computer, one for my CDP. Once they're set up, I probably won't be changing either of them very often.


----------



## 00940

what would be great is to find a way to let the default position to the I2S lines coming from the SPDIF receiver and switch automatically when an USB connection is established. Cannot we use the sspnd flag of the pcm2707 for that ?


----------



## ble0t

Quote:


  Originally Posted by *00940* 
_what would be great is to find a way to let the default position to the I2S lines coming from the SPDIF receiver and switch automatically when an USB connection is established. Cannot we use the sspnd flag of the pcm2707 for that ?_

 

Having a default is a must in this case because it would be impossible to pick one over the other without a uC if both were 'on'. In any case, the one thing to note about the SSPND on the PCM270* series is the fact that if it doesn't receive any data for 5ms sec, it automatically suspends. This could cause it to go back and forth between the two if for instance playback is stopped/started and might not be the best option.

 Since this was primarily intended to be a USB DAC, perhaps we can go the other way and use something like the AUDIO or NV/RERR pins on the CS8416 instead since those are more consistent outputs.


----------



## Clutz

An unrelated question / issue; it says in the datasheet for the 8416 that the reset pin has to be held low while the device is being powered on until the power supply and inputs are stable. How do we do that? Just a simple timer circuit? Or do we just not worry about it?

 And to use the AUDIO or NV/RERR outputs, we'd need to use the QFN package, right?


----------



## ble0t

I wouldn't be terribly concerned about that, although a quick check of DaKiller's design might be of interest. As for the package, we don't have to use the QFN...you just have to check further down in the datasheet. It has a 'Hardware Mode' section for the SOP version.


----------



## DaKi][er

Quote:


  Originally Posted by *Clutz* 
_An unrelated question / issue; it says in the datasheet for the 8416 that the reset pin has to be held low while the device is being powered on until the power supply and inputs are stable. How do we do that? Just a simple timer circuit? Or do we just not worry about it?

 And to use the AUDIO or NV/RERR outputs, we'd need to use the QFN package, right?_

 

Just an RC on the reset pin, I used the same one to reset the flipflops that was used on the input selection as well

 Also a note, if you are going to be using a pushbutton selection similar to mine, add some proper clock debouce (either a RC feeding a schmitt triggered input of a logic gate or max6816 which I ended up modding into mine)

 If you need any help trying to understand the CS8416, just ask, I spent hours going over that datasheet

 Jus one last thing, when picking parts make sure you can actually obtain them, the CS8416 is not an easy part to get hold of depending on where you live (like Australia 
	

	
	
		
		

		
		
	


	




)


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## SnoopyRocks

TI's SRC4392 and SRC4382 look like they would do everything. Too bad they are only preview now - not production yet. This means there will be absolutely no whining about an unobtainable TQFP package. 
	

	
	
		
		

		
		
	


	




SRC4392 page
SRC4382 page


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## 00940

I'm not sure those are such a good thing for us. TI's SRC4392 and SRC4382 require control through ICs and are not as flexible as the AD1896 when it comes to the ASRC and I2S lines.


----------



## ble0t

Great info in this thread and this post especially concerning the PCM270*

http://www.diyhifi.org/forums/viewtopic.php?p=7777#7777

 Seems like we don't even need to worry about powering the headphone section of the DAC (saves some headache for sure)


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_Having a default is a must in this case because it would be impossible to pick one over the other without a uC if both were 'on'. In any case, the one thing to note about the SSPND on the PCM270* series is the fact that if it doesn't receive any data for 5ms sec, it automatically suspends. This could cause it to go back and forth between the two if for instance playback is stopped/started and might not be the best option.

 Since this was primarily intended to be a USB DAC, perhaps we can go the other way and use something like the AUDIO or NV/RERR pins on the CS8416 instead since those are more consistent outputs._

 

I looked over the data sheet for the CS8416 again, and I saw the hardware mode- but I don't how were we can get a signal from NVERR or AUDIO? I'm sure I've missed something, but can you point it out?


----------



## DaKi][er

How would you have this hooked up? The pcm270* and cs8416 both outputting i2s into a mux and you want a way to switch between them? What about utilizing the 4 inputs on the cs8416 as well?

 You can't really decide on the input selection till you make up your mind on the rest of the circuitry first

 What comes to mind about selecting the USB as default is to use the V_bus 5v input off the usb plug itself to know if the usb it plugged in or not if there is no other way to get reliable status out of the pcm270* itself


----------



## 00940

Quote:


  Originally Posted by *DaKi][er* 
_How would you have this hooked up? The pcm270* and cs8416 both outputting i2s into a mux and you want a way to switch between them? What about utilizing the 4 inputs on the cs8416 as well?_

 

Exactly like that. According to the measurements of John Swenson, getting spdif out of the pcm270* then putting it into a spdif receiver increases jitter. Regarding the 4 spdif inputs, is it really worth the extra work ? Beside testing transports, does it have a real use ?


----------



## ble0t

I would tend to agree with 00940...I think a single SPDIF connection is all that is needed. As for switching the I2S signal, I don't think there's any reason this isn't feasible given the right design.


----------



## Clutz

There may be an argument to be made for using the multiple SPDIF input of the CS8416 to control the source. For one thing, it simplifies the circuit a bit, cause we wouldn't need another chip in the signal path. It's another chip that we have to worry about it having a good power supply for, being able to sink it's own currents effectively. e.g. Won't using an external mux possibly result in longer data traces to the chip, and thus more noise in the signal? If this isn't a significant concern, then I think going with the mux is definitely the way to do it.


 That said, I think the VBus idea is a really good one. Thanks DaKiller!


----------



## SnoopyRocks

I'm with Clutz on using the 8416 to select the input. We could then have a choice of USB (spdif output from the pcm2704), optical, coax (rca), or coax (bnc). It is then up to the builder's discretion to install the desired connectivity. Isn't maximum input flexibility a good thing? Besides, the jitter will be there regardless and, for the scope of this project, we need to trust that the ASRC will take care of.

 What's the problem with a manual input select switch? Why the unnecessary complication? 
	

	
	
		
		

		
		
	


	




 If the user isn't bright enough to select the correct input, he/she isn't worthy of listing to such a fine DAC.


----------



## DaKi][er

Quote:


  Originally Posted by *SnoopyRocks* 
_I'm with Clutz on using the 8416 to select the input. We could then have a choice of USB (spdif output from the pcm2704), optical, coax (rca), or coax (bnc). It is then up to the builder's discretion to install the desired connectivity. Isn't maximum input flexibility a good thing? Besides, the jitter will be there regardless and, for the scope of this project, we need to trust that the ASRC will take care of.

 What's the problem with a manual input select switch? Why the unnecessary complication? 
	

	
	
		
		

		
		
	


	




 If the user isn't bright enough to select the correct input, he/she isn't worthy of listing to such a fine DAC. 
	

	
	
		
		

		
		
	


	


_

 

That just about describes the input of my dac to a TEE, the usb, optical, coax rca and coax bnc all on the cs8416
 I see there are really only 2 choices of input selection of something like that, either a push button to cycle through the 4 or a rotary switch
 Just the push button setting is volatile, so you loose the position on power down, one day I’ll get a super capacitor to run the flip-flop IC to remember the position setting on mine, was calculating a few days out of a 5V 1F cap


----------



## 00940

I'm a bit reluctant to trust completly the ASRC. My point of view is the lower the jitter to begin with, the better. John Swenson's measurements show only 75ps of jitter for I2S direct out of 2707 compared to more than 200ps for the spdif out. Which is line with the measurements of the Grace's engineers.

 Furthermore, if we add a spdif in, let's do it correctly. Which means we have to buffer the inputs of the CS8416. Doing it for one input is ok; doing it for four is getting a bit bothersome. See : http://www.diyhifi.org/forums/viewto...r=asc&start=15 , bottom.


----------



## 00940

ok, here are the two possible conceptual drawings :


----------



## MegaMeee

Was it only me who thought that half the point of this dac (or half the point of using usb in the first place) was to go straight from usb to i2s and avoid spdif???
 personally I would prefer the top option because I wouldn't stuff the spdif part of the board anyway.

 edit: can we use te same setup as the dddac mk2???


----------



## ble0t

Quote:


  Originally Posted by *MegaMeee* 
_Was it only me who thought that half the point of this dac (or half the point of using usb in the first place) was to go straight from usb to i2s and avoid spdif???_

 

Exactly...I think switching the I2S is a much better choice.

 EDIT: I did a quick look over at TI and I think something like this would work...


----------



## steinchen

I vote for the I2S-switch version, too. If you don't want or need the spdif-in just omit the parts. I'm only in if the DAC has an spdif-in.


----------



## 00940

http://focus.ti.com/lit/an/szza030/szza030.pdf TI note on selecting switches.


----------



## Clutz

So that seems to be decided then, the first conceptual model and going with an I2S switch.


----------



## ble0t

I was looking at perhaps using a different SPDIF receiver since the CS8416 has 8 inputs (somewhat excessive for our application) and seems to be a bit of a pain to implement (all the external resistors). I came across the AK4117. It is only a two input receiver and would probably work just as well and it's implementation seems much easier as well.

 Just a thought...

 EDIT: Upon further review, it appears that this would need a uC, so I guess it's not really an option...back to looking...


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_I was looking at perhaps using a different SPDIF receiver since the CS8416 has 8 inputs (somewhat excessive for our application) and seems to be a bit of a pain to implement (all the external resistors). I came across the AK4117. It is only a two input receiver and would probably work just as well and it's implementation seems much easier as well.

 Just a thought..._

 

Hmm - just glancing over the datasheet after the gym. Haven't read it closely yet, but it seems like a good idea to me. Good find ble0t.


----------



## 00940

The AK4117 was used in AOS' DACs. It seems he didn't use a micro controller with them, perhaps do they run ok without one ? It could be good to send him an email about it.

 See for the exemple the ally dac board schematic : http://www.aoselectronics.com/ally.html

 Edit what worries me a bit is how to get the AK parts. They're not in digikey, mouser, farnell, allied electronics, farnell's stocks.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_The AK4117 was used in AOS' DACs. It seems he didn't use a micro controller with them, perhaps do they run ok without one ? It could be good to send him an email about it.

 See for the exemple the ally dac board schematic : http://www.aoselectronics.com/ally.html

 Edit what worries me a bit is how to get the AK parts. They're not in digikey, mouser, farnell, allied electronics, farnell's stocks._

 

I just contacted AOS by email. He lives in Vancouver, and it's still early here so perhaps we'll hear back from him soon. I'll post his comments when I get them. 

 If it's hard to get the Ak parts, then maybe we shoudln't use it. Maybe we just need to look for another SPDIF receiver?


----------



## Clutz

From aos:
  Quote:


 ...I can tell you that AK4117 works well in hardware mode, without a
 microcontroller because I used it in both PPA and Piccolo standalone. You might run into some problems as it seems to behave
 sometimes in the ways not in the datasheets, but you shouldn't have trouble to get it working with at least 44/48 kHz but since you
 likely want it for USB and possibly CD use, that is all you care anyway. 
 

But since it's difficult to find this part, maybe we should consider DIR1703E? They have them on hand at digikey.. The only difficulty is finding a datasheet for it...

 Edit: No, it's quite easy to find the data sheet for the DIR1703E. I'm an idiot. Anyway, I asked aos about acquiring the AK4117s. He said go to the AKM website. He thought it might be Allied Electronics or All Electronics though.

 All American Electronics seems to carry it- but their website says that it is currently down for maintenance, sooo...? No - it's back up. So that's a potential source for the AK4117, which could be nice. 

 The DIR1703E from TI is a lower jitter part, 75ps versus 200 for the AK4117; so the DIR1703 is a better part, and it is still available from Digikey- though they only have 613 in stock, and it doesn't look like TI is producing them anymore, so... 

 Another possibility is that AK makes a part that is both a DIR and an ASRC is the ak4122. I haven't finished downloading the datasheet for it yet- but it might work then to replace both the CS8416, a mux, and the ASRC. But I',m talking out of my ass, so.. wait and lets look at the data sheet.


----------



## 00940

the dir1703 should be tested for real life performances in the coming weeks : http://www.diyhifi.org/forums/viewto...8ed5a68aacc6db


----------



## ble0t

Looking a bit more closely at the AK4117 datasheet, it isn't technically necessary to digitally control it at all. However, that leaves you with using all default settings which are as follows:

 -Low Power mode
 -MCKO and SDTO source are recovered by PPL from RX input
 -MCKO is 256fs where fs is 44.1kHz
 -Clock/crystal used must be 11.2896MHz
 -Input is RX0
 -Output is 24bit, Left Justified

 I'm assuming the SPDIF is only intended to be used for CD playback, so having fs at 44.1kHz is fine. The only thing that differs is the output mode. On the PCM1798, all that requires changing in order to have it accept left justified (as opposed to the I2S from the PCM2707) is to put FMT0 high. Since we are already planning on implementing some logic to cause the mux to switch, it would simply be an extension of this.


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_Looking a bit more closely at the AK4117 datasheet, it isn't technically necessary to digitally control it at all. However, that leaves you with using all default settings which are as follows:

 -Low Power mode
 -MCKO and SDTO source are recovered by PPL from RX input
 -MCKO is 256fs where fs is 44.1kHz
 -Clock/crystal used must be 11.2896MHz
 -Input is RX0
 -Output is 24bit, Left Justified

 I'm assuming the SPDIF is only intended to be used for CD playback, so having fs at 44.1kHz is fine. The only thing that differs is the output mode. On the PCM1798, all that requires changing in order to have it accept left justified (as opposed to the I2S from the PCM2707) is to put FMT0 high. Since we are already planning on implementing some logic to cause the mux to switch, it would simply be an extension of this._

 

There is a good chance I'm just not getting something, but, doesn't that make this a less than ideal part to use then? Cause we wanted to use I2S to feed the DAC because it has less jitter, and the CS8416 can output I2S? Otherwise we have a PCM2707 feeding I2S into the mux, the mux feeding the PCM1798, and the AK4117 feeding left justified (left justified what?) into the mux to the PCM1798 and switch both the PCM1798 to accept a left justified input while switching the inputs on the mux? 

 What about the DIR1703? I can send the datasheet to everyone...?


----------



## ble0t

24 bit, Left Justified is just another form of transmitting serial data. It has a different format than I2S, but it isn't necessarily worse. Since we're using a mux, which is in essence a kind of switch, it doesn't really matter what we're sending through it whether it be I2S or something else. We would just need to make sure that the DAC chip is switched to accept either data format.

 It's just a thought if we're looking at using the AK4117....


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_24 bit, Left Justified is just another form of transmitting serial data. It has a different format than I2S, but it isn't necessarily worse. Since we're using a mux, which is in essence a kind of switch, it doesn't really matter what we're sending through it whether it be I2S or something else. We would just need to make sure that the DAC chip is switched to accept either data format.

 It's just a thought if we're looking at using the AK4117...._

 

Ahh. 
	

	
	
		
		

		
		
	


	




 Okay. 
	

	
	
		
		

		
		
	


	




 I thought that it was something like SPDIF, but then I figured it wasn't..

 I was just confused.. Thanks for clearing itup.


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_24 bit, Left Justified is just another form of transmitting serial data. It has a different format than I2S, but it isn't necessarily worse. Since we're using a mux, which is in essence a kind of switch, it doesn't really matter what we're sending through it whether it be I2S or something else. We would just need to make sure that the DAC chip is switched to accept either data format.

 It's just a thought if we're looking at using the AK4117...._

 

Ahh. 
	

	
	
		
		

		
		
	


	




 Okay. 
	

	
	
		
		

		
		
	


	




 I thought that it was something like SPDIF, but then I figured it wasn't..

 I was just confused.. Thanks for clearing itup.


----------



## 00940

I suggest we wait for John Westlake measurements on the DIR1703. It still is my favorite option for now, unless the measurements are very bad.

 And don't forget we're not feeding the pcm1798 directly, there's the AD1896 in between the mux and the dac.

 Edit : I suggest we start slowly to draw schematics to fill in the operating blocks. And there is the question of the power supply to consider then.
 Edit2 : I'll upload a library with some useful parts later on today. It can be improved I guess and must be checked.


----------



## webzeb

Quote:


  Originally Posted by *00940* 
_I suggest we wait for John Westlake measurements on the DIR1703. It still is my favorite option for now, unless the measurements are very bad.

 And don't forget we're not feeding the pcm1798 directly, there's the AD1896 in between the mux and the dac.

 Edit : I suggest we start slowly to draw schematics to fill in the operating blocks. *And there is the question of the power supply to consider then.*
 Edit2 : I'll upload a library with some useful parts later on today. It can be improved I guess and must be checked._

 

I think that it would be relevant to focus on the power supply and the output stage (obviously it implies the components of the DAC to be selected
	

	
	
		
		

		
		
	


	




 ).

 From my point of view that's crucials part of the DAC,and they might be more limatating than the converter itself.

 Fred


----------



## ble0t

Arg...I completely forgot about the little ASRC guy sitting in the middle there. Chalk that one up to a busy day at work...my mind must have been drifting 
	

	
	
		
		

		
		
	


	




 I am also eager to see the results of the DIR1703 measurements. I was surprised when I finally cracked open the datasheet and saw that is uses a SPACT system as well


----------



## 00940

here is the library; replace .txt by .lbr


----------



## ble0t

BTW, 00940, nice find on that jitter article over at diyhifi. I guess I would definitely lean toward using the DIR1703 at this point, although I'm anxious to see what JohnW comes up with.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_here is the library; replace .txt by .lbr_

 

Found a very small error in it -

 In the library, the we have a device named TPS7933, but a symbol for TPS79333. 

 How did you add these parts to the library?

 Cheers,
 Clutz


----------



## Clutz

Haggerman suggested that instead of going with multiple reg101's, we use one reg101 for a 3.3 volt psu, one reg101 for a 5 volt psu and do a good job of local bypassing at all of the power supply connections. 

 The only real downside I can see of this is it may make the routing of voltage traces more difficult. I'm going to try and start laying out the schematic.


----------



## ble0t

I already have a schematic almost fully done as well as a start on a layout if you'd like me to post it. I've been designing it to fit in a Hammond 1455J1201BK enclosure (approx 4" x 3" x 1").


----------



## 00940

Bleot : please post the schematic (in .sch and .jpg), so we can start discussing it. 

 Power : I prefer to have different regulators for all chips. It makes it easier to keep the sections independent and separate. Btw, what are we using for supply ? One single supply and buffered ground ? Or dual supply ? AC or DC ?

 Layout : how small do we go for the smd parts ? 1206, 805 ?


----------



## SnoopyRocks

Things seem have progressed a bit since I last checked in.

 I take it no one else uses Pads2005. What PCB software is being used?

 I prefer 0805 (or even smaller) caps for chip supply decoupling. Otherwise 1206 is fine. Is the plan to make this entirely SMT?

 For the power supplies, I'm in favor of requiring a separate dual DC supply (e.g. +-12V) from which all other voltages can be derived on board. The alternative would be to have a single DC supply to generate the other voltages, but would require AC output coupling and low voltages opamps.


----------



## Clutz

I'm with Snoopy on this. 0805 for decoupling caps. 1206 is just too large when you've got to have a bunch of them in close proxmity to each other - just too hard to work with. I also agree with requiring a +/- 12 volt DC input PSU required.


----------



## MegaMeee

sounds good to me.


----------



## ble0t

OK...just let me clean them up a bit 
	

	
	
		
		

		
			





 EDIT: OK...I have a .pdf made and I have the .sch (I use EAGLE) zipped up, but both are too large to post on the forum 
	

	
	
		
		

		
		
	


	




 Anyone related to the project have some web space to post on?


----------



## Clutz

I have crappy webspace I can put it on, on my own server - yes. It's nothing fancy, but it'll work.

 I'll PM you ble0t.


----------



## ble0t

YGM


----------



## Clutz

Come and get it.

http://twtch.zoology.ubc.ca/


----------



## 00940

Ok, got the schematics.

 - good idea for the usb esd protector. 

 - it'd be best to have resistors on the I2S lines between each chips

 - I wonder if it is good to have the ferrite before the electrolytic caps. Considering the electrolytics will be further away from the chips, we allow high frequencies to wander a bit. Wouldn't it be better to have the ferrite in between the electrolytic and the ceramic ?

 - On the pcm1798, I would connect vcomr and vcoml to +5V and not to VCC1, through its own ferrite.

 - We need an hardware reset of the DIR1703.

 - you cannot use the REG101 straight from the supply. Its max input is 10V. We need preregulation. I would use one LM317 at 7V for all digital chips and a pair of LM317/337 for +/-9V for the I/V and the analog supply of the PCM1798.

 Looks fine otherwise.


----------



## ble0t

Quote:


 - good idea for the usb esd protector. 
 

Thanks, it works nicely on my PCM2704 DAC I just got finished.

  Quote:


 - it'd be best to have resistors on the I2S lines between each chips 
 

Yep...I've been working on getting the PCM2707 and DIR1703 (now that we've decided on it) sections up and going before anything else.

  Quote:


 - I wonder if it is good to have the ferrite before the electrolytic caps. Considering the electrolytics will be further away from the chips, we allow high frequencies to wander a bit. Wouldn't it be better to have the ferrite in between the electrolytic and the ceramic ? 
 

It's not a big deal either way. I'll move them inside as per your suggestion.

  Quote:


 - On the pcm1798, I would connect vcomr and vcoml to +5V and not to VCC1, through its own ferrite. 
 

Isn't that what I did? 
	

	
	
		
		

		
		
	


	




  Quote:


 - We need an hardware reset of the DIR1703. 
 

No problem...I'll add an RC section to that.

  Quote:


 - you cannot use the REG101 straight from the supply. Its max input is 10V. We need preregulation. I would use one LM317 at 7V for all digital chips and a pair of LM317/337 for +/-9V for the I/V and the analog supply of the PCM1798. 
 

Ahh...thanks for noting that. I'll put an LM317 section to drop it down to 7V before feeding it to the REG101's.

 Work has been a bit hectic lately, but hopefully I'll get some time to improve upon it over the next couple days. Thanks for the suggestions!


----------



## 00940

Bad news gentlemen : http://www.diyhifi.org/forums/viewto...=288&start=120


----------



## ble0t

Yep...saw that this morning as well. 

 But, even given that information, what choice do we have if we would like to use USB as an option? We're including SPDIF and implementing an ASRC (running from a different master clock) to help the situation as much as we can.


----------



## Clutz

This is definitely disappointing news. Does this leave us with any other options? Are there better USB to SPDIF chips available? I can't imagine another TI chip performing substantially better if they're all using SpACt. Is it time to go and search the other semiconductor companys for a different USB receiver?

 I suppose alternately we could then think about it as an SPDIF input DAC that also has the benefit of having a USB receiver with it?


----------



## ble0t

Quote:


  Originally Posted by *Clutz* 
_Are there better USB to SPDIF chips available?_

 

It's actually USB --> I2S...and I'm not sure who else makes them. Perhaps AKM?


----------



## SnoopyRocks

Lets not get carried away by the measurements on the other site. There's not much more that can be done about them (if they are in fact represenative) given the scope here anyway. The basic system here is fundamentally sound and reasonably addresses the problem. If done well, it should sound great. Forget about the measurements and get back on task. If the clock out of the ASRC looked like that, then we'd have problems.


----------



## dsavitsk

Quote:


  Originally Posted by *SnoopyRocks* 
_Lets not get carried away by the measurements on the other site._

 

This is right. 

 This test was bus powered with no noise filtering. How do we know this is representative of anything. Maybe it is, but one measurement of a questionable setup does not a result make (aren't some of you guys scientists?) Further, if you are all going to get caught up on overthinking every aspect of this project, it is never going to get done. I have used a pcm2707 into a tda1543, and it sounds good. Could it sound better? Sure, but the point of this project, as I understand it anyway, was never to build a perfect DAC with 0 jitter. It was to build a reasonably priced DAC that sounds reasonably good. The PCM2707 will do that, and it will likely do it better than any other way you plan to get digital audio out of your computer (except maybe via squeezebox, but it is already over the proposed budget.) I think perhaps the test shows that Wavelength Audio is full of it, but we knew that anyway, so it seems to not be such a startling result. Oh, and even though mine is not bus powered and filters every aspect of the usb signal, using a usb cable with a ferrite on it makes a difference as does the computer used, so a well controlled experiment is in order if you want results that matter.

 Now, get back to work.

 -d


----------



## ble0t

Quote:


  Originally Posted by *dsavitsk* 
_Now, get back to work._

 

You slave driver you


----------



## MegaMeee

Quote:


  Originally Posted by *dsavitsk* 
_Now, get back to work.
 -d_

 

I agree 
	

	
	
		
		

		
		
	


	




 I pretty much expected results like these... And I pretty much don't care. As said if worst comes to worst we still end up with a good value spdif dac that you can also plug into any pc.


----------



## pipp

I just finished a usb dac. Based on pcm2706 i2s and stacked TDA1543's. Maxed out with Black Gates and tantalum resistors. Power is 2 x Regulated AC -> 9v and 2 x separate 9v to 5v supplys. It plays great !


----------



## Clutz

New PCM2706 data over at diyhifi..

http://www.diyhifi.org/forums/viewtopic.php?p=8251#8251

 Much, much better looking now.


----------



## Francis_Vaughan

Pretty aggressive layout in terms of bypassing - but a bit of a "I keep telling everyone this" for me too 
	

	
	
		
		

		
		
	


	




 Got to really push the power supply de-coupling as hard as you can.


----------



## 00940

Would this be sufficient ? PCM2707 receiver section, based on Bleot's schematic.


----------



## SnoopyRocks

I've decided to do parallel developement with PADS2005 instead of Eagle since learing a new software suite is not that appealing to me right now among other reasons. The schematic with the symbols I've created is attached.

*number of pcb layers* 
 IMO, this should be a 4 layer board. What does everyone else think?

*board size*
 Do we have a target for the board dimensions? Is there a particular case in mind?

*power section*
 The voltage regulator section is out of hand. Separate supplies (regulators) for analog and digital is reasonable. A regulator per chip is rediculous. This application is not that sensitive and any performance benefit would be mininal. Regulators don't isolate well at the the digital switching frequencies anyway. Be reasonable.

*physical dimensions of the usb jack*
 Can someone please direct me to a mechanical drawing of the usb jack so I can make a symbol with the right footprint?

 I have more comments and questions, but this is enough for now.


----------



## 00940

Quote:


  Originally Posted by *SnoopyRocks* 
_*number of pcb layers* 
 IMO, this should be a 4 layer board. What does everyone else think?_

 

Way too expensive for small run production, especially since the price must stay around 200$.

  Quote:


 *board size*
 Do we have a target for the board dimensions? Is there a particular case in mind? 
 

Eurocard sized I'd say ?

  Quote:


 *power section*
 The voltage regulator section is out of hand. Separate supplies (regulators) for analog and digital is reasonable. A regulator per chip is rediculous. This application is not that sensitive and any performance benefit would be mininal. Regulators don't isolate well at the the digital switching frequencies anyway. Be reasonable. 
 

Well, the PLL sections of both the spdif and usb receiver /are/ very sensitive. And the evaluation board of the AD1896 uses two seperate regulators for the digital and analog supplies of the AD1896 (both 5V). The amount of regulators is thus not crazy in my view, we could do crazier. 
	

	
	
		
		

		
		
	


	




 But the scheme probably needs to be modified. A practical problem in the layout is that we have 3.3v and 5v digital lines. The dir1703 and pcm2707 need 3.3v, so does the pcm1794. In the middle we have the multiplexer and the ad1896 and their 5v. A big 7v line and local regulators make it easier to layout at first sight. A compromise would be to use one 3.3v regulator for both pcm2707 and dir1703, a 5v regulator for both the multiplexer and the ad1896 and one 3.3v regulator for the dac digital section.

 edit : another advantage of multiple regulators is to use the reg101 which can output at most 100ma, and preferably with not too much current, to stay in its most linear operating range. If we use the same regulator for more chips, we probably have to load it more or pass to a reg102, with lesser specs.


----------



## Clutz

Quote:


  Originally Posted by *SnoopyRocks* 
_*number of pcb layers* 
 IMO, this should be a 4 layer board. What does everyone else think?_

 

This would be a lot more expensive, and I think we need to have a target budget for this project or else it will be very easy to go overboard. What are the benefits of going with a 4 layer board? How much more expensive is a four layerboard than a two layer board for a small run? 

 Maybe we should start to add up the parts costs at this point, so that we know how much of the budget we've spent so far?

  Quote:


 *power section*
 The voltage regulator section is out of hand. Separate supplies (regulators) for analog and digital is reasonable. A regulator per chip is rediculous. This application is not that sensitive and any performance benefit would be mininal. Regulators don't isolate well at the the digital switching frequencies anyway. Be reasonable. 
 

I agree with 00940 here about the reg101 versus reg102, but I think that 00940's compromise seems like a good one.


----------



## SnoopyRocks

Is the goal here a $5 pcb? The pcb is one of the most important components of the system. Moreover, since the sentiment is towards abundant power bypassing, lets do it right with propper power planes. The cost is not exporbitant and IMO money is better spent here than on boutique opamps.

 For a frame of reference, my last board to test an IC I designed was 3.9 x 6.9 in. (slightly larger than eurocard) with considerably more holes because of generous stiching. This was a 6 layer board. The cost was a little over $900 for 10 boards. 4 layers would have been about $550. I don't think that ~$50 for the pcb is unreasonable. With more boards, the price per boards goes down quite a bit. Is $35 for a 4 layer board unreasonable?


----------



## dsavitsk

I think there is a disconnect here. The only way you are going to get people on this or any other forum to put down that kind of money for a DAC, it had better be prototyped and it had better work way better than the other projects out there that can be built for about $100. This means that you are going to have to sell a ton of boards, you are going to have to eat the cost of prototyping, you are going to have to get it pretty much right on the first try to avoid several rounds of prototyping, and you are going to have to beat Yeo's board. Any one of these seems difficult, but doing all is probably an unreasonable thing to ask. Go with a 2 layer board, send it to batch PCB to make a handful, and be done with it. This is a fun learning project, not a best DAC ever made project. I think (and correct me if I am wrong) when Clutz says $150-200 he means that's what he wants to spend, not that he wants to put down $1000 to develop a DAC whose intellectual property rights and sunk costs he is going to give away so he can sell $35 to $55 DAC boards.


----------



## mikechai

Quote:


  Originally Posted by *pipp* 
_I just finished a usb dac. Based on pcm2706 i2s and stacked TDA1543's. Maxed out with Black Gates and tantalum resistors. Power is 2 x Regulated AC -> 9v and 2 x separate 9v to 5v supplys. It plays great !_

 

What is the total cost to build? Any pictures?
 Any comparison with commercial DACs or sound cards?


----------



## Francis_Vaughan

Sadly the reality of designing and prototyping the board is pretty much as has been outlined. The cost of entry into the four layer board arena is enough to exclude efforts like this. As far as I can see group buys are still prohibited on this forum (despite guarantees that a clarification would be forthcoming some months ago.) Even then, it is quite a few people taking a pretty deep breath to stump up the cash to run the prototyping. With enough enthusiasm and will it could be done. But I think the whole design forum needs to prove itself a little more. Eventually I might hope we will see such a thing.

 Be careful about the justification for four layers. One thing we don't need is a power plane. This isn't a high speed digital design. It has a curious mixture of needs, and it has much more the needs of an RF design than a digital one. We don't need to supply lots of digital switching, rather we need to very carefully control the switching energy that is present, which leads us to lots of bypassing, and lots of low pass filtering on the power. Sure, four layers would make life easier, but providing a power plane would not be one of the uses of a layer.


----------



## Clutz

FYI: A running total of the putative parts costs so far (note, these parts were priced from Digikey.ca, not digikey.com, so the prices are in Canadian Dollars, and have been converted back to US assuming an exchange rate of $0.85. Parts costs using the PCM1794 DAC, excluding all the other important bits like ferrites, resistors, capacitors, diodes, in $US (which was where my $150 to $200 came from) $78, using the PCM1798 were is $68. A good clock like the Tent XO is about $30 (something I will probably spend money on, but woudln't count this into the budget)- I don't know how much an Elso Kwak clock would cost though. 



 DIR1703$5.31 15.31
 PCM2707$8.44 18.44
 OPA1632$4.89 14.89
 REG101$2.95 38.85
 AD1896$23.27123.27
 PCM1794$25.42 125.42
 PCM1798$13.44 00
 CS8416$7.97 17.97
 LM317$2.00 24
 LM337$2.00 24
 Multiplexer ??? ??? ???



 CDN Sub$78.40 
 US Sub$66.64 


 The biggest concern I have with a four layer board is 'debugging' it. If we have ten people who were willing to participate in the PCB (just assuming for the moment), then I don't think that even $50 is necessarily outrageous for the PCB. My guess is that even a two layer PCB is going to cost about $20 to $30 for the number of boards we're going to be making, the extra $30 itself doesn't make me shrink. If it's a four layer PCB and we have a problem with the board,our abilities to debug and/or fix it are substantially reduced relative to the case of a two layer PCB, right? Anyway, I have no goals to turn this into a group buy. Even if I wanted to do that, I just don't have the money up front to buy a whole bunch of boards to sell to others later on.

 I don't really understand the ins and outs of the importance of a ground plane- that's really something that is lost on me from reading the various threads on DACs.

 Anyway, that's just my thoughts on the matter.


----------



## 00940

Quote:


  Originally Posted by *dsavitsk* 
_I think there is a disconnect here. The only way you are going to get people on this or any other forum to put down that kind of money for a DAC, it had better be prototyped and it had better work way better than the other projects out there that can be built for about $100. This means that you are going to have to sell a ton of boards, you are going to have to eat the cost of prototyping, you are going to have to get it pretty much right on the first try to avoid several rounds of prototyping, and you are going to have to beat Yeo's board. Any one of these seems difficult, but doing all is probably an unreasonable thing to ask. Go with a 2 layer board, send it to batch PCB to make a handful, and be done with it. This is a fun learning project, not a best DAC ever made project. I think (and correct me if I am wrong) when Clutz says $150-200 he means that's what he wants to spend, not that he wants to put down $1000 to develop a DAC whose intellectual property rights and sunk costs he is going to give away so he can sell $35 to $55 DAC boards._

 

I broadly agree. While the final cost of a 4 layers board is reasonable (at eurocircuit, you can get eurocard 4 layers for around 25€ by 20), it's the prototyping which gets expensive fast.

 The goal is to make the most of the budget. This being said, I think the project will appeal to different diyers than Yeo's boards. Aggressive oversampling/upsampling vs nos, last generation dac vs old philips TDA154*, ASRC vs reclocking, passive I/V vs opamps. That's like the opposite philosophy.


----------



## Francis_Vaughan

OK, I've being doing a bit of reading. The TUSB3200 supports Isochronous transfers in Asynchronous mode. This means it is possible to slave the PC data source to the DAC clock. So, after all the worry, it may be that this is actually the best answer.

 Not home free. The TUSB3200 is a much more complex device than a PCM2704. The internal microcontroller must be programmed. But there is an SDK. (Available for free on the TI website.) I have not had a chance to look at it yet - but one hopes there is most of the code needed ready to roll - including asynchronous mode. 

 Also - one needs to know if the USB sound driver on the PC end will support asynchronous mode. 

 I have also been looking at the possibility of opto-isolation of the USB. It is a pain, but maybe achievable within the constraints of this project.


----------



## 00940

Francis : drivers side should be fine under windows. Read this reply by Thomaspf : http://www6.head-fi.org/forums/showp...46&postcount=3

 I know John Swenson on audioasylum is currently working on a usb asynchronous interface based on the tas1020b


----------



## pipp

Quote:


  Originally Posted by *mikechai* 
_What is the total cost to build? Any pictures?
 Any comparison with commercial DACs or sound cards?_

 

As for price it all depends of parts chosen, as in all DIY projects. Im sure it can be buildt with fairly good components for say 50-100USD using the usb bus for power. However I would not recommend that. When trying that here there where some bakground noize that came of the usb bus power.

 Im werry carefull when it comes to rating stuff I have buildt, because when you build something you're not always impartial. With that said, I have sold my ECD1 
	

	
	
		
		

		
			





 Im not going to say that the DAC is bether then the ECD1 thats for others to decide for themself. However I prefere the USB dac over the ECD1.

 edit: Oh and pictures, I will post some when I have it all cased up and it looks all neat and nice


----------



## ble0t

Here's my interpretation of the schematic thus far...

 I added a Neutrik XLR connector for the voltage rails and an LM317/LM337 section. In light of the recent posts concering PCM2706, I'll probably move the bypass caps in a bit closer as well. You can see the DIR1703 and the multiplexer on the lower right.


----------



## 00940

Who here has a commercial version of Eagle ? We will need it for the final board. I suggest the best way to proceed is to have a few different versions of each operating blocks, fusion those version (which could be an interesting lesson in pcb layout) and then send the final version to someone with a commercial version of eagle to put all the blocks on the final pcb. We need some method, otherwise we are going to go crazy fast


----------



## Clutz

I thought it was steinchen?


----------



## SnoopyRocks

I've been outvoted so I'll drop the 4 layer request and spare everyone my responses. Back to work. 
	

	
	
		
		

		
		
	


	




 The schematic has grown quite and keeping track of the different versions is not as easy as it needs to be. It would be nice to have a regularly updated "official" version so that we're all on the same page. I think this is pretty important if different layout blocks done individually are going to match up and be merged later as 00940 suggests. Opinions?


----------



## 00940

Should we set up a wiki ?


----------



## dsavitsk

Quote:


  Originally Posted by *00940* 
_Should we set up a wiki ?_

 

How about a CVS system?


----------



## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_How about a CVS system?_

 

I don't mind hosting the files on twtch. I've never set up wiki or cvs, but I could create an account for someone. It's not the fastest machine in the world, an old OS X 10.3.9 (which can be upgraded to 10.4, if need be) G4 733 w/ 768MB RAM. I occasionally use it to run Mathematica, Matlab, R or C simulations on- but not that often.


----------



## ble0t

I have the Standard edition of EAGLE, so I can do a Eurocard size layout since I think that is what we'll need.


----------



## 00940

allright then. Is there anyone actually willing to layout, beside Bleot and I ? If not, we can work together and then submit a common design to criticism here.


----------



## ble0t

That works for me if everyone else is OK with it.


----------



## steinchen

Quote:


  Originally Posted by *Clutz* 
_I thought it was steinchen?_

 

I purchased the non-profit version of Eagle (limited to Eurocard-size, 2 layers and, of course, non-profit use)

 Since ble0t and dsavitsk are far more involved into this project than I am the project is far better served in their hands. If there is something I can do to support you I'll gladly do, just let me know.


----------



## MegaMeee

If you 2 guys are willing then great... For a while I was wondering if there was anyone who knew how


----------



## Clutz

Quote:


  Originally Posted by *ble0t* 
_That works for me if everyone else is OK with it._

 


 Two thumbs up from me.


----------



## dsavitsk

Quote:


  Originally Posted by *steinchen* 
_Since ble0t and dsavitsk are far more involved into this project than I am the project is far better served in their hands._

 

?

 I think you must have me confused with someone else. I am, at best, offering comments from the sidelines, and at worst just a pain. That leaves ble0t and 00942, though, I think maybe Clutz ought to do it.


----------



## SnoopyRocks

I can do layout but there are some caveats. First, like 00940, I only have the free version of Eagle. Moreover, I'm not familiar with Eagle and I presume that it is just as difficult to pick up as other PCB suites. I'm willing to give it a shot though. As a side note, I do have a full version of Mentor Pads2005, which I do know how to use, that has no restrictions on board size or layers, not that it matters in this case. 

 Right now we need a floorplan for the layout. Things to consider are general signal routing and power bus placement for starters. 

 For the synchronization issue, my proposition is to send changes through ble0t (full version of Eagle), who can then forward updates to Clutz for web hosting. Are weekly (biweekly) updates reasonable?


----------



## ble0t

Interesting link just got posted over at diya...

http://www.cypress.com/portal/server...n_device_9.pdf


----------



## steinchen

Quote:


  Originally Posted by *dsavitsk* 
_?

 I think you must have me confused with someone else. I am, at best, offering comments from the sidelines, and at worst just a pain. That leaves ble0t and 00942, though, I think maybe Clutz ought to do it._

 

sorry, you're right, I meant 00942


----------



## Clutz

Hello all

 I was under the weather the past few weeks so I hadn't been doing much Head-Fi'ng (other than waiting for my K701s). I've lost track of where we were at. Is anyone else still around and kicking? 

 Cheers,
 Clutz


----------



## 00940

Bleot and I have been working quite a lot (but Bleot was gone one week). The layout of the analog section is about done, the digital section well under way, we still have to work on the supply. I've to work on the values of the output filter. We have to fix ourselves on a new spdif receiver however, since the DIR1703 is not living up to the expectations.


----------



## MegaMeee

So whats wrong with the dir1703?


----------



## 00940

For the DIR1703 : 
http://www.diyhifi.org/forums/viewto...=asc&start=105


----------



## Clutz

Well, then that would seem to bring us back into CS8412 territory, wouldn't it?


----------



## 00940

The CS8412 is discontinued since way too long, hard to get 
	

	
	
		
		

		
		
	


	




 Even the CS8414 is getting difficult to get from common reselller. We're working with the CS8416 currently.


----------



## MegaMeee

That sounds good to me.


----------



## ppl

Quote:


  Originally Posted by *DaKi][er* 
_While you can get away with it in headphone amps, single supply virtual grounds should be avoided, they add far more problems than they fix

 Really, there is 2 ways you could design-
 Either design it to be portable, where you would want it to run off USB power or as this was set out to be, a standalone dac to run off mains with its own transformer, psu and all that with no halfway could be portable could be fixed standalone, it should be either one of the other as designing for both just makes compromises. Have pads for the AC from the transformer/s and put everything else onboard. I’m a very strong believer that the psu should be built in as part of the design and leave very little choice 

 I don’t have any personal interest in building one of these as I already did one for myself and I am not much of a fan for designing projects like these in groups as there are always far too many people wanting to put in their own input and in the end always throws it off course. I like to design by myself, for myself and everyone else is an afterthought.

 What you guy's need to do is work out exactly where you want to end up and start listing the requirements, with everyone having different ideas that will be fun 
	

	
	
		
		

		
			



_

 

I disagree that a quality DAC can not be made using a virtual ground providing the Virtual ground is done correctly and using a fair amount of output current. I like the LT-1210 set up as a low bandwidth unity gain VF for a virtual ground. Using this configuration I measured the noise floor between the supply rail and the virtual ground and obtained -135 Db. Using this DAC chip http://www.cirrus.com/en/pubs/whiteP...s43122wp-1.pdf
 And the DIR-1703 you’re using, so I would not abruptly discount the virtual ground system and portable use. 

 I have just found this thread and find this quite interesting. Regarding a DAC for the PPA AOS once made one http://www.aoselectronics.com/pace.html so you might give it a look over, this DAC is no longer available however.

 Someone posted about using a pre regulator ahead of the TPS-79101 regs that are only good to 6 volts max even for the adjust type have this been investigated more


----------



## 00940

The power supply section is not completly fixed yet. Only the final regulation is onboard (using reg101 for 5v supplies and tps79333 for 3.3v supplies). Since we are using only one supply for digital/analog, I must say a virtual ground makes some sense. 

 ppl : there will be a serious imbalance in between the positive and negative supply. Would you advice to put a load on the negative side to balance the draws ?


----------



## ppl

with the LT1210 no with the BUF-634 stacked in a pair of two I get balanced supply with ref o the virtual ground of no more than 300 mV and this is quite ok for a Split rail supply regardless of how it is derived. With the LT-1210 the balance is within 20 Mv. Both with the Digital supply pulling 92 am from the pos rail to the Virtual ground. Also I like the over sampling idea I also am working on that however a different chip that allows bypassing so as to conserve battery life when over sampling is not required. It appears to my measurements that beyond ICQ conditions that the current drain of the digital circuits are in direct proportion to the data rate. http://www.cirrus.com/en/pubs/proBulletin/CS8421_PB.pdf
http://www.cirrus.com/en/pubs/rdData...RD5381_RD1.pdf
 BTW I am still waiting to hear how long you could run that TPS regulator on 9 volts with out falure.
 .


----------



## 00940

We will have preregulation at 5.5V for the digital section, with a simple lm317. 
 The dac will be wall powered, inefficiency is no big deal.


----------



## ppl

Is this Dac to be euro-card size? If so I assume an external AC supply is the thinking? If this is true I would give serious consideration to some isolation of the analog stage or OP-AMP’s used on the DAC’s output. If an I/V type is used then power supply integrity is of more importance than it is with a voltage output DAC. At the Minimum an R/C network should be inserted in the supply rails powering the op amps. A simple 10 ohms series resistor with a 0.1uF cap is the absolute min however a good 10uF of so of quality Electrolytic is technically more suitable hear as a large value bypassed with a low value film cap. Is a better solution. A PPA style JFET TLE and caps are what I would do if using a virtuial ground supply however if AC only operation is considered then my favor is twards a normal split rail supply a Jung type should be great hear, as the Virtual ground is great in a portable unit if a true split rail supply is available then it should be used, remember it only is (1) more wire and the cost of the power supply parts go way down as the LT-1210 is not a low cost part.


----------



## 00940

The DAC will fit on an eurocard (it could actually be a bit smaller, we'll see). The aim was to provide for regulation onboard. So only DC would have to be provided. The interest of the rail splitter would be to allow for a wider range of PS to be used.

 Yes, we use a dedicated I/V (OPA1632), since the chosen DAC is the PCM1798. However, even a voltage out DAC needs a good, clean analog supply to feed the opamps inside the DAC (or its analog circuitry).

 The LT1210 is expensive but wouldn't the LT1206 be sufficient ? It's already 4$ cheaper, for 250ma (minimum) output current. And do we actually need so much current ? If we put a constant negative load on the splitter, of roughly the value of the idling digital section, we probably need way less, more like 50ma, to balance the splitter under all conditions. A pair of paralled njm4556 would give 4X70ma peak, for less than 1$.

 We will certainly use RC networks. And all power pins on board are bypassed by a small ceramic cap (0805), another bigger ceramic or tantalum cap (1206) for the digital chips and a small electrolytic (around 47uf). And each power pin in the digital section is fed through a ferrite.

 In the end, the ideal solution would be to use three supplies. A digital (+) and two analog (+/-). Since this solution involved at least three transformers (or the expensive three outputs elpac WM113), it was ruled out for cost considerations (we're aiming for 200$ without case). Thus the interaction in between the analog and digital supply will happen no matter what, rail splitter or not.


----------



## ble0t

Well, it's been a while, but 00940 and myself have been hard at work and we've finally come up with a preliminary design. First, here are the links to the schematic as well as shots of the board layout:

 WARNING: The file sizes aren't that big (.gif format), but I kept the resolution very high to keep the finer details easy to see.

Schematic

Top Layer

Bottom Layer

Both Layers w/Top Silk

 We have attempted to design this to be as flexible as possible for anyone who wants to build it. 

 There is the option of either USB (PCM2707) or SPDIF (CS8416) input which can either be selected via a soft-button switch (i.e. momentary switch instead of a toggle) or set specificaly via a jumper. There is even the option to input I2S directly and use the other input as an additional digital in (i.e. use the board inside of a CD player).

 The intended signal path is the same as what had been discused earlier:
 PCM2707 / CS8416 --> AD1896 --> PCM179* --> OPA1632/OPA134 based I/V stage

 The AD1896 and PCM179* will be clocked via a low jitter Crystek oscillator.

 The I/V stage contains pads in various places to allow for the use of whatever I/V and/or filter stage is desired.

 The power supply was designed for a 15V DC input (most likely from a wallwart) that is then regulated to 11V using an LM317 and then split to +/-5.5V using an LT1206 for virtual ground. Each digital chip has its own regulator(s) that are fed from the +5.5V supply.

 Finally, we do have some space in the lower right portion of the board and we would like input as to what we should use the space for. Our thoughts at this point in time are either a balanced output section or perhaps a small preamp/line driving section. Any and all input/suggestions/comments are welcome.

 EDIT: I almost forgot to mention the PIC12F509...I am currently working with someone on getting code for this. I have used a DIP version of the PIC to make things much easier as far as programming is concerned. My thought at this point in time is that I could offer to program the PIC for anyone who would like to build this DAC (as long as the number of people was reasonable).


----------



## SnoopyRocks

It's nice to know that progress is being made. 
	

	
	
		
		

		
		
	


	




 At a cursory glance, it looks ok.

 I am not a fan of the power supply arrangement and the virtual ground in particular. It just seems like more trouble than it's worth for a project like this. This linear regulated +12@0.3A,-12@0.3A,+5@0.86A DC supply uses a 5 pin DIN connector and is only $9.69 at Jameco. Yes, less than $10. It seems ideal for this application and would decrease the board size, parts count and cost, while making it easier to build and troubleshoot.


----------



## MegaMeee

Quote:


  Originally Posted by *SnoopyRocks* 
_It's nice to know that progress is being made. 
	

	
	
		
		

		
		
	


	




 At a cursory glance, it looks ok.

 I am not a fan of the power supply arrangement and the virtual ground in particular. It just seems like more trouble than it's worth for a project like this. This linear regulated +12@0.3A,-12@0.3A,+5@0.86A DC supply uses a 5 pin DIN connector and is only $9.69 at Jameco. Yes, less than $10. It seems ideal for this application and would decrease the board size, parts count and cost, while making it easier to build and troubleshoot._

 

kind of a problem for us folks in 240v countries 
	

	
	
		
		

		
		
	


	




 I kinda like the current psu arrangement. The Vin is probably just a tiny bit too high for 12v sla batteries though 
	

	
	
		
		

		
		
	


	




 . Do you guys have any recommendations for a psu.. What sort of current rating do you think is necessary?

 edit: forgot to mention how much I appreciate the effort you guys are putting in


----------



## ppl

Quote:


  Originally Posted by *SnoopyRocks* 
_It's nice to know that progress is being made. 
	

	
	
		
		

		
		
	


	




 At a cursory glance, it looks ok.

 I am not a fan of the power supply arrangement and the virtual ground in particular. It just seems like more trouble than it's worth for a project like this. This linear regulated +12@0.3A,-12@0.3A,+5@0.86A DC supply uses a 5 pin DIN connector and is only $9.69 at Jameco. Yes, less than $10. It seems ideal for this application and would decrease the board size, parts count and cost, while making it easier to build and troubleshoot._

 

 I tend to agree that if tis DAC is to be AC powered then a true split supply is in order however if battery operation is at all desired then the LT1206 is a good solution however i would put equial capacitence from both pos and Neg rails because as it is will all the capacitence on the Pos rail then instability in the LT1206 may result. have you figured out what operating current your going to Bias the LT1206 at? also why not use the higher current LT1210?


----------



## 00940

Ok, here's the reasoning behind the rail splitter.

 - there are very few dual (or even triple) outputs wallwarts. And for 220-230VAC inputs, it's even worse. If we design the dac around one of those wallwarts and it is discontinued, it will force people to build a complete PS for it.

 - On the other hand, a 15VDC wallwart is easy to find for about anyone.

 - Noone forces you to actually use the rail splitter. If you want to connect a multiple taps PS, feel free to do so. We will change the layout to put jumpers on, to allow such modularity. 

 - I don't know well batteries. The problems is that we have a very small window of operation. The input of the railsplitter must be constantly kept in between 12VDC and 10.4VDC. More than 12VDC and we exceed the rating of the TPS79333 (max 6V input), less than 10.4VDC and the REG101-5 for the analog section of the PCM1798 cannot regulate it anymore.

 - The LT1206 is rated at "at least" 250ma, I'm not sure it is really needed to go further with the LT1210. I've to check if they're pin compatible though.

 - I will calculate the total current draw of that thing this afternoon.

 Update : estimated current draw : around 130mA on the positive line, around 20mA on the negative line. Details : 

 CS8416 : 5.7 + 5.9 + 2.8 : 14.4ma @3.3V
 PCM2707 : 23ma @3.3V
 AD1896 : 30ma @3.3V
 PCM1798 : 13ma @3.3V, 19ma@5V
 OPA1632 : 2X 15ma @11V
 OPA134 : 2X 5ma @11V

 on the -5.5V : 20ma

 on the 5.5V : 14.4+23+30+13+19+15+5 : 119.4ma


----------



## ppl

I would use these regulators from Analog devices thay are good with inputs twice that of the TI part 12 volts vs 6 volts for the TI part. this allows greater power supply voltage flexibility, http://www.analog.com/en/prod/0%2C28...3301%2C00.html


----------



## 00940

they are awfully similar to the REG101, aren't they ? Same cost too, compared to the cheaper TPS79333.


----------



## 00940

Another option would be to use one more beefy REG102 for the whole digital section, except the "analog" section of the CS8416 which would have its own REG101 regulator to safeguard PLL performances. Then, people using batteries could be a bit more free. The rail splitter should see in between 10.4VDC (a minimum) and 18VDC.

 I guess that would also please those who think that there are too many regs onboard


----------



## ble0t

I'll update the PS section to be more modular and thus accomodate anyone who wants to feed the +/- DC voltages in directly.

 As for the capacitance existing on the positive rail, I will add some to the negative rail to balance it out a bit. Good catch on that one 
	

	
	
		
		

		
		
	


	




 As for the other voltage regs, I agree with 00940...the Analog part looks like a competing part to the REG101...and it definitely is more expensive.


----------



## Clutz

What about the trace lengths between the ASRC and the PCM1798? Can we shift the ASRC to the right to reduce the length of those traces, or is that a non-issue?

 Re: Power Supply - I like the 15VDC input setup. I think, like 00940 said, it provides an ease of use and it is capable of being flexible enough to use multiple supplies. I'll probably start out using a 15VDC (a tread, maybe a STEPS?) and then possibly later rig up three psu with three independent psus or something.


----------



## 00940

The length of those tracks is a non-issue for me. Actually, having the AD1896 and its noisy decoupling a bit apart of the DAC seems a good thing.

 I was however wondering if the I/V doesn't risk offset at the output. Should we add a trimmer to take care of it ? And if yes, how to do it best ? I was thinking of having a buffered voltage divider feeding one of the inputs of the output opamps. It will add like 5$ but it's still cheaper than expensive output caps


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_The length of those tracks is a non-issue for me. Actually, having the AD1896 and its noisy decoupling a bit apart of the DAC seems a good thing._

 

Alright 
	

	
	
		
		

		
		
	


	




  Quote:


 I was however wondering if the I/V doesn't risk offset at the output. Should we add a trimmer to take care of it ? And if yes, how to do it best ? I was thinking of having a buffered voltage divider feeding one of the inputs of the output opamps. It will add like 5$ but it's still cheaper than expensive output caps 
	

	
	
		
		

		
		
	


	



 

I think it seems like a good idea. Good quality output caps are going to cost more than $5 - and keeping caps out of the audio path seems like a good idea too.


----------



## ppl

Quote:


  Originally Posted by *00940* 
_The length of those tracks is a non-issue for me. Actually, having the AD1896 and its noisy decoupling a bit apart of the DAC seems a good thing.

 I was however wondering if the I/V doesn't risk offset at the output. Should we add a trimmer to take care of it ? And if yes, how to do it best ? I was thinking of having a buffered voltage divider feeding one of the inputs of the output opamps. It will add like 5$ but it's still cheaper than expensive output caps 
	

	
	
		
		

		
		
	


	


_

 

You probaly need to adjust the DC offset however once you find the right resistence a pot is no longer required and all remaining boards can use this predetermond resistor value as thay will all be close to one another. this makes the cost and complexity do way down


----------



## 00940

In "theory" there shouldn't be significant offset I think. The differential outputs of the DAC are at 2.5V if my memory's good and this common mode voltage should disappear with the balanced to unbalanced converter. If offset there is, it will be mostly coming from mismatch in I/V resistors and the opamp themselves (an opa134 is specified for 4mV offset iirc). So the offset should be (very?) low but variable for any DAC build. The Belcanto DAC2 (similar topology but with the PCM1738) has no output caps but apparently uses an offset servo with the INA134. 

 Btw, can someone check my maths ? With a 44.1KHz sampling frequency, with a signal band of 20KHz, with "upsampling" (the AD1896) to 96KHz and 8X oversampling (the pcm1798's digital filter), the first alias should be at 8 x 96 - 20 = 748KHz ?


----------



## Clutz

00940 said:
			
		

> In "theory" there shouldn't be significant offset I think. The differential outputs of the DAC are at 2.5V if my memory's good and this common mode voltage should disappear with the balanced to unbalanced converter. If offset there is, it will be mostly coming from mismatch in I/V resistors and the opamp themselves (an opa134 is specified for 4mV offset iirc). So the offset should be (very?) low but variable for any DAC build. The Belcanto DAC2 (similar topology but with the PCM1738) has no output caps but apparently uses an offset servo with the INA134.
> /QUOTE]
> 
> Why not add the DC Servo circuit to the design - but before implementing it, try running the DAC without installing any of the servo's parts and measure the DC offset (i.e. after the opamp I/V, but before the servo). If it's too high, then we can put the required parts in, and if it's not needed then the that circuit just isn't completed- no harm, no foul, right?


----------



## ble0t

After getting some feedback from some of you about the possible desire to use this with batteries or other power sources, we've redesigned the power supply to be a little less picky and also cut down on the component count. Bascially, instead of using individual regulators for each digital chip, we've gone to using a single LT1763 with the already existing passive decoupling. The only thing to maintain its own regulation is the analog supply of the CS8416. Also, I've added some pads to the supply lines for anyone who wants to supply their own +/- voltages. Here are the updated files:

Schematic

Top Layer

Bottom Layer

Both Layers w/Silk

 00940 is also working on implementing the servo to minimize offset. If any additional space is left, we will most likely implement an optional balanced output on the board as well. The next update will most likely contain these two additions.


----------



## dsavitsk

Page 16 of the PCM2707 datasheet says this "To avoid back voltage in self-powered operation, the device must not provide power to the pullup resistor on D+ while VBUS of the USB port is inactive."

 Are you doing anything about this, or just sugesting that people not power the DAC with the USB cable disconnected, or nothing at all? The 2707 on one of my DACs burned up, and I am not sure of the cause, but I am wondering if this might be it?

 -d


----------



## Clutz

When the device is connected to a USB bus, then VBUS is at +5V. Couldn't we use a VBUS to close a relay and power on the rest of the DAC, only when connected to a USB BUS? That way if the DAC was turned on while not plugged into a USB bus, the DAC itself would never be powered?


----------



## dsavitsk

That seems reasonable to me, although you might also just connect the pullup resistor to the usb's power which seems easier. As I say, I am not sure what caused one of mine to die, and while this seems like a possibility, I've not heard of anyone else with this as a problem, and there are some other things I could have done.


----------



## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_That seems reasonable to me, although you might also just connect the pullup resistor to the usb's power which seems easier. As I say, I am not sure what caused one of mine to die, and while this seems like a possibility, I've not hears of anyone else with this as a problem, and there are sme other things I could have done._

 

It does seem easier to connect VBUS to the pullup. The relay idea seemed like a better idea to me at the time, but now I have no idea why that is, except that I think I'm the type of person who would over engineer things. 

 Anyhoo.


----------



## 00940

The DDDAC for example uses indeed a small system to disconnect the pull-up. I overlooked the issue since the other chips of TI don't need it. We're gonna have a look on that.


----------



## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_That seems reasonable to me, although you might also just connect the pullup resistor to the usb's power which seems easier. As I say, I am not sure what caused one of mine to die, and while this seems like a possibility, I've not heard of anyone else with this as a problem, and there are some other things I could have done._

 

Page 29, "D+ must not activate (HIGH: 3.3V) while the device is detached from USB".

 One problem with this is that D+ is supposed to be pulled up to 3.3V and VBuS is 5.0V, so we'd have to make use of a voltage divider or something. Not that it's a terribly difficult thing to do, I'm just pointing it out for the sake of the design.

 So this makes me wonder - what happens if the device is in self-powered mode, we connect D+ through VBUS using a voltage divider, and the usb cord gets yanked? What are the consequences of D+ being pulled high? If this causes damage to the chip, what how long can it safely tolerate this? I don't know how to get the answers to these questions, but it sort of makes me think that perhaps tying VBUS to D+ via a voltage divider and using a relay to turn the whole thing off might not be a bad idea. On the other hand, could quickly turning it off with the relay be damaging?

 Edit: I was just looking at the schematic and I noticed that the D+ isn't pulled up at all, or is it me?


----------



## 00940

The pullup is included in the emi filter iirc.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_The pullup is included in the emi filter iirc._

 

I wondered if that was the case. I coudln't find any datasheet for that part. 
	

	
	
		
		

		
		
	


	




 Coo coo I say.

 Cheers,
 Brad


----------



## ble0t

I've added something nearly identical to what the DDDAC uses (i.e. an AND gate) to account for this...thanks for catching that dsavitsk


----------



## dsavitsk

Which part did you use? There is a single AND gate at Digikey made by toshiba, but they only have 80 in stock and it is being discontinued. Mouser has a similar one from Fairchild, but the sot-23 size is out until september and the only one they have is a so-70 size (I am unable to find anyting in Eagle with that footprint.) Did you go with the full size 4 gate version?


----------



## ble0t

TI to the rescue again...$0.39 part at Digikey and they have thousands in stock...

http://focus.ti.com/docs/prod/folder...74ahc1g08.html


----------



## Clutz

Any updates?


----------



## Clutz

How are we going to go about programming this thing.. It is a PIC isn't it? 

 Other than that, excluding the costs of various resistors, capacitors, a DC power source, the crystal and the crystek oscillator, it looks like the cost for the parts is going to be just under $100 ($110CDN). It seems to me that it's pretty much on track for cost. 
	

	
	
		
		

		
		
	


	




 Re: IC7 and IC12 - do you think the THS4631 would work well here, or would it have a problem with oscillations? I've got a bunch of these, thinking I was going to try them out in my PPA (but I haven't yet.. I don't really feel like buggering around with it, swapping resistors, etc), so I figured that this might be a good use for this - or is there going to be any benefit from using an opamp that's better than the 134? (e.g. 8610, which I also have two of, or 2227/2228)? My concern about anything other than the 132/134 is that they might oscillate, and since they're SOIC, it'd be a real PITA to swap them off if they did do that. 

 Sorry ,I'm just getting really excited about this. I have parts coming in for a TDA1541 DAC, and the dynalo.


----------



## ppl

Quote:


  Originally Posted by *Clutz* 
_How are we going to go about programming this thing.. It is a PIC isn't it? 

 Other than that, excluding the costs of various resistors, capacitors, a DC power source, the crystal and the crystek oscillator, it looks like the cost for the parts is going to be just under $100 ($110CDN). It seems to me that it's pretty much on track for cost. 
	

	
	
		
		

		
		
	


	




 Re: IC7 and IC12 - do you think the THS4631 would work well here, or would it have a problem with oscillations? I've got a bunch of these, thinking I was going to try them out in my PPA (but I haven't yet.. I don't really feel like buggering around with it, swapping resistors, etc), so I figured that this might be a good use for this - or is there going to be any benefit from using an opamp that's better than the 134? (e.g. 8610, which I also have two of, or 2227/2228)? My concern about anything other than the 132/134 is that they might oscillate, and since they're SOIC, it'd be a real PITA to swap them off if they did do that. 

 Sorry ,I'm just getting really excited about this. I have parts coming in for a TDA1541 DAC, and the dynalo._

 

that THS4631 is made for this application with the exception of having a high input impedance as oposed to the low impedance that DA chip likes otherwise speed and open loop gain are all good as is the voltage noise whitch is similer to the AD-8610 however the input bias current of the Ti part is about an order of magnatude greater than the AD8610 however since you are operating at low impedances voltage noise is of more importence. its a trade off because you need high open loop gain so that eliminates most high speed opamps with the exception of this TI chip.

 However i think the ideal Op amp for an I/V convertor stage is somthing like this http://focus.ti.com/lit/ds/symlink/opa860.pdf


----------



## Clutz

Quote:


  Originally Posted by *ppl* 
_that THS4631 is made for this application with the exception of having a high input impedance as oposed to the low impedance that DA chip likes otherwise speed and open loop gain are all good as is the voltage noise whitch is similer to the AD-8610 however the input bias current of the Ti part is about an order of magnatude greater than the AD8610 however since you are operating at low impedances voltage noise is of more importence. its a trade off because you need high open loop gain so that eliminates most high speed opamps with the exception of this TI chip.

 However i think the ideal Op amp for an I/V convertor stage is somthing like this http://focus.ti.com/lit/ds/symlink/opa860.pdf_

 

Hi ppl,

 Thanks for the feedback. I think I'm going to go ahead and use this THS4631 chip then. I'm not worried about the THS4631 having a high input impedence, because the DA chip is running into the OPA1632, so the OPA1632 will see high input impedence of the THS4631, not the DA chip itself.. Or do I have that wrong?

 Cheers!
 Clutz


----------



## ble0t

We're very close to presenting the 'final' version of board.

 Yes, that is a PIC to control the CS8416...I have a friend who can easily take care of the code and burn a limited number of chips for our use. Even if the board is finalized by the end of the weekend, it's going to take a bit to get a proto or two done and fully tested, especially with the PIC involved. But, things are finally starting to move along.

 I'll keep you all posted


----------



## Clutz

Great news ble0t! I'm very excited to see the revisions. Let me know how much it'll cost you to prototype the boards and I'll send some money your way to help offset the costs. I can either send it to you paypal, or if you prefer- via a cheque. 

 Looking forward to it!

 Cheers,
 Brad


----------



## MegaMeee

Quote:


  Originally Posted by *Clutz* 
_Great news ble0t! I'm very excited to see the revisions. Let me know how much it'll cost you to prototype the boards and I'll send some money your way to help offset the costs. I can either send it to you paypal, or if you prefer- via a cheque. 

 Looking forward to it!

 Cheers,
 Brad_

 

Yeah I will as well... Although only if you can it via paypal.


----------



## 00940

Clutz, PPL : the I/V function is performed by the OPA1632. Its specs are not that bad compared to the ths4631 : voltage noise at 1.3nV compared to 7nV for the ths4631, open loop gain is at 78db to 80 for the THS4631, gain bandwith is 180mhz compare to 210 for the THS4631.

 The opa134 afterwards is only a differential receiver to turn the signal to unbalanced. We used it rather than a drv134 to allow people to configure it as a second filter if they wish so and to change the output opamps to tailor the sound of the dac to their liking. If cost is no object you can use there any opamp which is comfortable with +/- 6VDC rails. Bypassing is quite good, so stability shouldn't be too much of a problem.


----------



## Clutz

Hi 00940,

 Thanks for clearing that up. I knew that the OPA1632 was doing the I/V, but I had forgotten about converting balanced to unbalanced. I'm only going to use the THS4631 because I've got a bunch of them lying around (but I also have a bunch of 227s) in the SOIC format, and I don't have any 134s - so might as well! It'd still leave me with three - plenty to get some browndogs and try sticking them in my PPA at another time. 

 Re: schematic and layout.


----------



## SnoopyRocks

The input impedance is being misinterpretted here. With closed loop operation, you cannot simply take the input impedance. In fact, the impedance seen looking into the opamp small, roughly (Z_f+Z_o)/A_o, where Z_f and Z_o are the feedback and OpAmp output impedances. A_o is the loop gain. The impedance seen by the DAC would be the parallel combination of the actual input impedance and this effective impedance, resulting in a small overall impedance.


----------



## Clutz

Thanks Snoopy! 


 On an unrelated note, has anyone heard anything from Francis Vaughn lately?


----------



## MegaMeee

I'm sure he's busy doing at uni doing his job... I know i'm busy doing the assignments he has written


----------



## ppl

Quote:


  Originally Posted by *SnoopyRocks* 
_The input impedance is being misinterpretted here. With closed loop operation, you cannot simply take the input impedance. In fact, the impedance seen looking into the opamp small, roughly (Z_f+Z_o)/A_o, where Z_f and Z_o are the feedback and OpAmp output impedances. A_o is the loop gain. The impedance seen by the DAC would be the parallel combination of the actual input impedance and this effective impedance, resulting in a small overall impedance._

 

this is true that your resultent impedence is what ever you design it to be however it is my understanding that a current output dac likes to see a real low AC impedance


----------



## Clutz

Quote:


  Originally Posted by *MegaMeee* 
_I'm sure he's busy doing at uni doing his job... I know i'm busy doing the assignments he has written 
	

	
	
		
		

		
			



_

 

Off topic: Interesting. What course is are you taking? Is he a professor?

 Cheers,
 Clutz


----------



## ble0t

Ok, here's the 'final' version of the sch/brd. Obviously, minor fixes and tweaks are still an option, but any new additions/major changes are pretty much done. We've added an optional balanced output section that can be jumpered to from the pads following the I/V stage. For your viewing pleasure...

Schematic

Top Layer

Bottom Layer

Both Layers w/Silk


----------



## 98venom

Is there a proposed group buy on this? I would seriously be interested in getting a board or two for a future project.......


----------



## Clutz

Deleted.


----------



## MegaMeee

Quote:


  Originally Posted by *Clutz* 
_Off topic: Interesting. What course is are you taking? Is he a professor?

 Cheers,
 Clutz_

 

Im doing comp science and mechatronic engineering double.

 As for Francis I don't have him for any lectures. The assignment I did that he wrote was a computer architecture assignment. 
 He's listed on the Uni of Adelaide website as an adjunct senior lecturer. From the Comp Science staff page we only have three "professors", those people being the heads and deputy heads of school. Most people on the academic side are listed as either senior lecturers or lecturers.

 I'm guessing that he basically is the computer architecture guy at the Adelaide Uni, if that makes sense.


----------



## Clutz

Hi ble0t,

 It looks good, very good, to me. Why are there through-hole resistors between the PCM2707 and the reclocker, and the CS8416 and the reclocker instead of SMD? Just to keep SMD count down?

 How important do you expect the power supply to be? e.g. Will a tread be sufficient - or is something approaching a steps going to provide substantial improvements? 


 I'm really excited 
	

	
	
		
		

		
		
	


	




 Cheers,

 Clutz

  Quote:


  Originally Posted by *ble0t* 
_Ok, here's the 'final' version of the sch/brd. Obviously, minor fixes and tweaks are still an option, but any new additions/major changes are pretty much done. We've added an optional balanced output section that can be jumpered to from the pads following the I/V stage. For your viewing pleasure...

Schematic

Top Layer

Bottom Layer

Both Layers w/Silk_


----------



## 00940

The through hole resistors are actually there to allow you to easily connect alternative I2S inputs.

 There is the equivalent of a tread onboard, so power supply shouldn't be too critical.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_The through hole resistors are actually there to allow you to easily connect alternative I2S inputs._

 

Ahh, very clever. 
	

	
	
		
		

		
		
	


	




  Quote:


 There is the equivalent of a tread onboard, so power supply shouldn't be too critical. 
 

Alright. 
	

	
	
		
		

		
		
	


	




 Anxiously awaiting when the boards are ready to go.


----------



## SnoopyRocks

Nice work. 
	

	
	
		
		

		
		
	


	




 Some comments:

*spdif receiver*
 Is it too late to reconsider the SPDIF receiver chip choice? A PIC controller seems like an unnecessary complication. Hardware programming is preferable. Right? 
 (I must have been asleep when the change was made. 
	

	
	
		
		

		
		
	


	




 )

*Differential outputs*
 It's nice to see this option added. 
	

	
	
		
		

		
		
	


	




 However, some more thought needs to go into the differential filtering though. The fundamental issue is that the filtering is not symmetric. To understand why, look at the location of Pads 3,4,9 & 10 (i.e. the location that the differential outputs are forwarded from). The positive input gets a first order LPF. While the negative input does get filtered also, _it reduced by the loop gain_. A solution would be to move Pads 3,4,9, & 10 to the other side of R10/R13 & R29/R31. This would leave no image reject filtering on the differential outputs. I think that, at the very least, capacitors should be added in parallel with R47, R48, R49, & R51, giving a first order LPF on the differential outputs. Let's think this through more carefully.


----------



## 00940

Snoopy : don't we have a lpf anyway with C37, C40, C51, C53 ? Considering how high the first alias will appear, isn't sufficient actually ?

 I agree it would be best to move the pads, to avoid assymetric interaction with the output lpf.


----------



## ble0t

I'll see what I can do about moving those pads...

 As for the PIC, I personally think it makes the design easier and more flexible. There are many more options that can be explored and the actual board layout is simplified/improved greatly by using it. Don't be frightened by the programmable logic


----------



## 00940

If the PIC's code is made available, anyone can easily program one using this kind of diy programmer : http://www.janson-soft.de/pic/pic.htm . This one can be greatly simplified for our use for example.

 I did something similar for the at90s1200 but working on the parallel port. As long as you don't have to write the code, it's very easy.


----------



## ble0t

That is also why I went with a DIP package for the PIC...it makes it easy to program using a variety of cheap programmers out there. I do plan on releasing the code once it gets to a stable, working point.


----------



## ble0t

OK...the pads have been moved as per SnoopyRocks' suggestion. I tend to agree with 00940 on the LPF question...I believe we already have that covered as far as I can tell.

 Are there any other design related issues to get into? Speak now or forever hold your peace


----------



## Clutz

Just to say that I think the differential outputs are great. It may mean that my next amp will be a dynamid.. and then converting my K701s and HD580s to balanced.


----------



## MASantos

This is probably a dumb question, but what is the purspose of the PIC?

 Also, is there a ETA on the boards?


----------



## 00940

The pic is a function selector for the cs8416. It avoids using a forest of jumpers.

 There is no ETA on the boards. It must be first prototyped and troubleshoot and its sound evaluated before any further development is considered.


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_There is no ETA on the boards. It must be first prototyped and troubleshoot and its sound evaluated before any further development is considered._

 

Might I suggest posting the schematic and layout at diyhifi? They seem like they'd be good for getting some more feedback on layout / design. The more eyes that look at it, the more likely that any errors that may exist will be caught. How much do you figure it's going to cost you to get the prototype boards made? 

 Cheers,
 Clutz


----------



## SnoopyRocks

Quote:


  Originally Posted by *00940* 
_Snoopy : don't we have a lpf anyway with C37, C40, C51, C53 ? Considering how high the first alias will appear, isn't sufficient actually ?

 I agree it would be best to move the pads, to avoid assymetric interaction with the output lpf._

 

Yes, that network gives us a single pole. A single pole will only give 20 dB per decade and consequently very little attenuation in the second Nyquist band since this is less than a decade after the filter corner frequency. Why not give the differential output drivers the same filter as the I/V? It's just 4 more caps and there is plenty of board space for them. 

 Does 1st order filtering seem right for the differential output when the single-ended output has 3rd order filtering?
  Quote:


  Originally Posted by *ble0t* 
_As for the PIC, I personally think it makes the design easier and more flexible. There are many more options that can be explored and the actual board layout is simplified/improved greatly by using it. Don't be frightened by the programmable logic 
	

	
	
		
		

		
		
	


	


_

 

Ok. I just wanted to know the thinking behind the choice.


----------



## ble0t

As 00940 has said, there really is no ETA on the boards. For that reason, I can't even begin to guess what the cost per board would be. Although the main design is complete, the tweaks are not. Please be patient


----------



## Clutz

I just noticed that there is no reverse voltage protection. If you miswire your PSU and connect the power up incorrectly it could make for a rather expensive oops. How about dropping a single diode in there - a 1N4004? Even if it isn't likely to happen (which with me, it is) it's cheap enough to implement - and if people don't want to they can jumper it.


----------



## Clutz

Is it me, or did you guys read Guido Tents' article on decoupling ICs?


----------



## 00940

Good point on the reverse protection.

 I've read the paper of Guido, it's somewhere on my harddrive, why ?


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_Good point on the reverse protection.

 I've read the paper of Guido, it's somewhere on my harddrive, why ?_

 

The way you've connected the power and ground pins up on the ICs seems reminiscent to me of what he recommends in that article. I was happy to see it, that's all.


----------



## ble0t

Quote:


 The way you've connected the power and ground pins up on the ICs seems reminiscent to me of what he recommends in that article. 
 

That was the goal 
	

	
	
		
		

		
		
	


	




 I'll put a diode in for the section ahead of the LM317....putting them anywhere else would affect the voltage on the rails.


----------



## Clutz

Well,fwiw, the more I look at it, the more impressed I am with it.


----------



## ble0t

Just to keep everyone updated, 00940 and Snoopy are working on some filter related stuff (as per the discussion previously in the thread). As soon as they square that away, I'll have what will hopefully be the final version of the board.


----------



## Clutz

Thanks ble0t,

 Does anyone have any suggestion as to where I can go to read about filters?

 Thanks,
 Clutz


----------



## 00940

there are two chapters on that in the book on opamps applications published by Analog Design under the direction of Walt Jung. I posted the link somewhere on this board to download it.


----------



## SnoopyRocks

http://www.national.com/an/AN/AN-779.pdf
http://www.circuitsage.com/filter.html

 These should get you started.


----------



## Clutz

I just thought I'd take a stroll through the thread and say "Hey, what's up fella's?"

 So, hey- what's up fellas? Feeling good? How's life? Any progress to report?


----------



## ble0t

I've got the filter changes (balanced output section) as well as a couple of other corrections made. I've also been doing some cleanup here and there. 00940 is out of town until next week and I'm waiting to run the updates by him as well as some others to see what they think before I post another update.

 Don't worry...I want to see a proto board as bad as you do, but we also want to make sure the design is as optimized as possible 
	

	
	
		
		

		
		
	


	




 PS - Thanks to Snoopy for the input on the filters


----------



## Clutz

I fully agree - optimize the design as much as possible before going to proto. I was just curious as to how things were going! 

 Cheers,
 Clutz


----------



## MegaMeee

I too thought this thread was in serious need of a bumping...

 BTW life's great, Uni sucks and I'm just packing up to go on on hols with the family for Easter.


----------



## ezkcdude

Can AD1794 be used (instead of 98) without any modifications to the schematic?


----------



## ble0t

Quote:


  Originally Posted by *ezkcdude* 
_Can AD1794 be used (instead of 98) without any modifications to the schematic?_

 

I'm going to assume you mean the _PCM_1794, and the answer is yes, it is pin compatible and can be used in place of the PCM1798.


----------



## Clutz

Quote:


  Originally Posted by *ezkcdude* 
_Can AD1794 be used (instead of 98) without any modifications to the schematic?_

 

It's a PCM1794, it is pin compatible just like ble0t said. In fact it is exactly what I plan on using. I think it's false economy to use the PCM1798 over the 1794 to save $8, when that's a very small fraction of the total cost of putting this DAC together in the end and it's specs are significantly better. That said, there are some very high end CDPs that are using the PCM1732 which isn't as good as the 1798 on paper.


----------



## ezkcdude

Thanks, for correcting me (I must have been drunk when I wrote that). I'm interested because I would like to design a similar DAC, except it would be for a SqueezeBox, so I would replace the USB receiver with a CS8416 to receive SPDIF. The SqueezeBox outputs 16/44.1(48) for FLAC/WAV files at 100% volume, and 24/44.1(48) for all other formats (e.g. mp3) and when attenuating with the digital volume control. I've asked this over at diyaudio but never got a satisfying answer. According to the 1794 spec sheet, you can use an external digital filter, therefore, bypassing the internal one. I think it is best to avoid any additional digital filtering altogether (after the ASRC), and only use analog I/V filtering. If I do HAVE to use the 1974 filter, I would use the slow rolloff mode. Any thought/opinions about this are welcomed.


 Edit: Oops! Man, this is a long thread. I just read back starting on page 12, and realized you do plan on using CS8416. Great! I will stay tuned.


----------



## 00940

I'm back 
	

	
	
		
		

		
		
	


	




 ezkcdude : You can only bypass the internal digital filter if you use the pcm1794 in mono mode (p.17 of the datasheet). Using two pcm1794-8 and the following I/V would drive the cost up significantly and it's not considered in the frame of this project.

 re: the pcm1794 instead of the pcm1798. The output current of the pcm1794 is twice the output of the pcm1798. You will have to adapt the I/V resistors and the paralleled filter's caps.


----------



## ezkcdude

Quote:


  Originally Posted by *00940* 
_ezkcdude : You can only bypass the internal digital filter if you use the pcm1794 in mono mode (p.17 of the datasheet). Using two pcm1794-8 and the following I/V would drive the cost up significantly and it's not considered in the frame of this project._

 

Thanks! I guess slow rolloff mode will have to do. Is that what you guys are planning?


----------



## 00940

for now, it's set in sharp roll off. Reason is simply ease of layout, the CHSL pin is very isolated...


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_I'm back 
	

	
	
		
		

		
		
	


	




 ezkcdude : You can only bypass the internal digital filter if you use the pcm1794 in mono mode (p.17 of the datasheet). Using two pcm1794-8 and the following I/V would drive the cost up significantly and it's not considered in the frame of this project.

 re: the pcm1794 instead of the pcm1798. The output current of the pcm1794 is twice the output of the pcm1798. You will have to adapt the I/V resistors and the paralleled filter's caps._

 

Nice to see you back 00940! 
	

	
	
		
		

		
		
	


	




 I'm planning on using the PCM1794 instead of the 1798, so I guess I'll have to change the I/V resistors. Paralleling the filter's caps isn't too big a deal.


----------



## MegaMeee

yep i'll be using the 1794 as well I think...


----------



## steinchen

same here


----------



## ezkcdude

Hey, all. Hope your design is progressing. I am posting a schematic for using CS8416 in hardware mode. I am trying to design a DAC similar to the one 00940 and the others are working on, except it will only have an SPDIF input (from my SqueezeBox). Apparently, there's not too many designs out there using CS8416, and all the ones I've seen use it in software mode. Anyway, if any of you have time to take a look at it, I'd appreciate any advice/help/comments. I'm not sure about many of the resistor values (R), or if it's even correct to have those (mainly for the output clocks). Also, I'm not sure about the reset pin (RST). Right now I just have it tied to AGND. I have set up the function pins so that I2S is output (hopefully). Once I get this part squared away, I think the setup for AD1896 and PCM1794 will follow pretty close to the design you guys are working on. Thanks, in advance!


----------



## DaKi][er

Quote:


  Originally Posted by *ezkcdude* 
_Hey, all. Hope your design is progressing. I am posting a schematic for using CS8416 in hardware mode. I am trying to design a DAC similar to the one 00940 and the others are working on, except it will only have an SPDIF input (from my SqueezeBox). Apparently, there's not too many designs out there using CS8416, and all the ones I've seen use it in software mode. Anyway, if any of you have time to take a look at it, I'd appreciate any advice/help/comments. I'm not sure about many of the resistor values (R), or if it's even correct to have those (mainly for the output clocks). Also, I'm not sure about the reset pin (RST). Right now I just have it tied to AGND. I have set up the function pins so that I2S is output (hopefully). Once I get this part squared away, I think the setup for AD1896 and PCM1794 will follow pretty close to the design you guys are working on. Thanks, in advance!_

 

My dac uses the cs8416 in hardware mode, the datasheets certainly dont make it easy to follow on how to set it up, I think I read it about 10X over and over untill it started to make sence
http://www.mypage.tsn.cc/dakiller/dac1/


----------



## ezkcdude

Quote:


  Originally Posted by *DaKi][er* 
_My dac uses the cs8416 in hardware mode, the datasheets certainly dont make it easy to follow on how to set it up, I think I read it about 10X over and over untill it started to make sence
http://www.mypage.tsn.cc/dakiller/dac1/_

 

Thanks, for responding. I had actually seen your schematic before, and was thinking about sending you a PM. So, your schematic answers a few questions for me. It looks like I don't need the resistors on OLRCK or OSCLK. Other than that, it looks like I had it mostly correct, I think. I'm still a little confused about the RST pin. In my schematic, I have it pulled low. Will that work or do I need to do something else?


----------



## DaKi][er

Quote:


  Originally Posted by *ezkcdude* 
_Thanks, for responding. I had actually seen your schematic before, and was thinking about sending you a PM. So, your schematic answers a few questions for me. It looks like I don't need the resistors on OLRCK or OSCLK. Other than that, it looks like I had it mostly correct, I think. I'm still a little confused about the RST pin. In my schematic, I have it pulled low. Will that work or do I need to do something else?_

 

You don’t need resistors on ORLCK and OSCLK but I’m sure it would be beneficial to have about 20-30 ohms there 
 Don’t take all of what I’ve done as the 'way it should be' as I’ve learnt quite a lot more since designing that
 For the RST pin, you want an RC on it that will on startup slowly bring the pin to ground so that it holds it in reset to let the power supplies stabilize and for the setup resistors to be read properly. I didn’t draw it too well on my schematics, but the reset pin is connected to the same reset circuitry that is on my pushbutton selection circuitry, there is a label 'reset' that points out where it connects. A value that is R*C = about 100mS would be plenty here


----------



## ezkcdude

I'm still confused about the RST pin. From the data sheet:

  Quote:


 When RST is low, the CS8416 enters a low power mode and all internal states are reset, including the control port and registers, and the outputs are muted. In Software Mode, when RST is high, the control port becomes operational and the desired settings should be loaded into the control registers. 
 

To me, it seems like RST should only be set high in software mode, and since I'm using HW mode, it shouldn't matter whether RST goes to ground immediately or has a short delay. I guess it can't hurt to add an RC, but I wish the data sheet was more helpful. As for the resistors for the clock outputs, they should be set to 100 Ohms, according to the CS8416 Eval Board data sheet.


----------



## dviswa

Hi Dakiller,

 That is a very interesting DAC design. According to your sig, you have not completed it yet. Is that still true or your sig's too old? If so how does it perform, specially compared to Alien DAC.

 Sorry guys, I am not trying a thread jack, but the idea of having 24 bit DAC, on board isolation is just too tantalizing 
	

	
	
		
		

		
			





 Don't you all want to know?


----------



## MegaMeee

edit: damn new page


----------



## dviswa

Megamee,

 Sorry, for not being clear. I was referring to 
  Quote:


  Originally Posted by *'DaKi* 
_http://www.mypage.tsn.cc/dakiller/dac1/_

 

I thought this is a different DAC from the one being designed/discussed here in this thread.


----------



## Clutz

Quote:


  Originally Posted by *dviswa* 
_Megamee,

 Sorry, for not being clear. I was referring to 

 I thought this is a different DAC from the one being designed/discussed here in this thread._

 

It is quite different; it's based on the PCM1704 and the one discussed here is based on the PCM1794.


----------



## DaKi][er

Quote:


  Originally Posted by *dviswa* 
_Hi Dakiller,

 That is a very interesting DAC design. According to your sig, you have not completed it yet. Is that still true or your sig's too old? If so how does it perform, specially compared to Alien DAC.

 Sorry guys, I am not trying a thread jack, but the idea of having 24 bit DAC, on board isolation is just too tantalizing 
	

	
	
		
		

		
		
	


	




 Don't you all want to know?_

 

Well it is completed as much as that I’ve been using it as my primary source for the past few months and not completed as far as there are quite a few things I’ve learnt along the way that could be done better in it just that I haven’t gotten around to implementing in this

 I can’t compare it directly with the alien dac, but I do have one of the original guzzler usb dac's. I don’t have the greatest ears to compare and I try and keep out of the subjectiveness off it all especially when it comes to DIY but I will say that it is better than the guzzler dac and for 8X the cost it ought to be
 But then I’m not fully in it just to get something that sounds better, its the educational journey of designing it as well that I enjoy as well as after getting some experience in all this diy stuff, I have my own views on how to go about designing/building these things (which I have already voiced early on in this thread) that led me to go my own way as such (I like to do things my way)

 Any further progress on my dac is pretty much stopped for now, I’ve got other things that take up enough of my time/interest that I don’t really want to do anything more on it. But feel free to PM me if you are interested in taking something like this up yourself as I’d be glad to help you with it


----------



## dviswa

DaKiller,

 Thanks for the update.

 Regards,
 Dinesh


----------



## Clutz

Are you guys here? 
	

	
	
		
		

		
		
	


	




 Any news?


----------



## 00940

Yes, and we're both sleeping. Who are you, mortal, to dare waking us up ?

 Ok, let's be serious. 
	

	
	
		
		

		
		
	


	




 We're waiting for comments by some people who accepted to comment on the design. They seem very busy sadly and we're waiting for their comments to order a prototype's pcb.


----------



## ezkcdude

Quick question. Where will you get OPA1632 from, and will it be the SOIC (1632D) or MSOP (1632DGN) version? I see that Mouser only has DGN in stock, and Digikey doesn't have either. The DGN package has that PowerPad feature which looks kind of difficult to work with. I assume you would try to go for the other one, but it looks like it may be hard to get.

 Edit: Just to update my own post. I found the SOIC version at Newark InOne. They have 112 left. Might want to stock up on those.


----------



## Clutz

Or order samples from Texas Instruments directly.


----------



## ble0t

*yawn*

 I was sleeping so nicely 
	

	
	
		
		

		
			





 I'll send a friendly nudge to our reviewers and see how they are coming along. I didn't mean for it to take this long. Hopefully we can get some protos ordered soon


----------



## Clutz

Getting some reviews done is an excellent idea, especially with a fairly complicated layout and schematic like this. I'm still pretty excited about it though.


----------



## dviswa

Quote:


  Originally Posted by *Clutz* 
_Getting some reviews done is an excellent idea, especially with a fairly complicated layout and schematic like this. I'm still pretty excited about it though. 
	

	
	
		
		

		
		
	


	


_

 

Second that


----------



## MegaMeee

thirded
	

	
	
		
		

		
		
	


	




 although I don't think the schematic is that complicated... It could be much worse...

 Just had a thought...

 This thing is going to be tough to solder though don't you think.. I think i need to invest it some proper parts.. a magnifying glass for one 
	

	
	
		
		

		
		
	


	




 I bought a couple of 41hz.com amp 3's to try some surface mount soldering but only 1 of the two works and I have no idea why (not that I really care, I also bought a charlize which sounds a bit better IMO 
	

	
	
		
		

		
		
	


	




 )
 The discrete components were fine to solder after i watched tangents tutorial but the tripath 2021 chip was pretty tough, so I think I will need to get some more appropriate tools for soldering IC's in the future..


----------



## dviswa

Quote:


  Originally Posted by *MegaMeee* 
_I think I will need to get some more appropriate tools for soldering IC's in the future.._

 

Fine Desoldering braid's your friend. Speaking for myself, it is impossible not to put some solder bridges on those smd ics. There might be some with super duper dexterity and talent here, but I am not one of them. Taking care of inevitable bridges is the key.


----------



## DaKi][er

Flux paste is even better, you get it so the chip will only draw as much solder as it needs off the iron, and if anything bridges you can literally wipe them off with the iron, just keep the flux up to it


----------



## dviswa

Hi Guys,

 Have'nt we waited enough for that review. 

 +1 for a prototype run 
	

	
	
		
		

		
		
	


	




 Regards,
 Dinesh


----------



## ezkcdude

Well, since someone resurrected this thread yet again, I have a lingering I/V question. What are the advantages, in terms of audio, in using OPA1632 over individual op amps? Clearly, it is a simpler solution because fewer components are needed. But there are other op amps with better specs (e.g. lower Ib, Vo, etc). I suppose a disadvantage to using separate op amps for balanced output is that they would have to matched carefully. Was this a real concern? I guess I'm just curious as to the rationale that was used for making the design decision to use OPA1632. It's not that I'm trying to be critical, more like devil's advocate here. To me, the I/V issue is by far the toughest thing to settle on, so I'm hoping to gain some enlightement from more experienced designers/DIYers than myself. O.k., you can go back to sleep now


----------



## 00940

Quote:


  Originally Posted by *ezkcdude* 
_But there are other op amps with better specs (e.g. lower Ib, Vo, etc). I suppose a disadvantage to using separate op amps for balanced output is that they would have to matched carefully. Was this a real concern? I guess I'm just curious as to the rationale that was used for making the design decision to use OPA1632._

 

I know very, very few opamps with better specs for our application. Look for settling time, slewrate, noise and gain bandwith. The only opamp we identified as a good competitor was the ths4631. And the difference is not big.

 Add consideration about layout and availability and you have our reasons.


----------



## Radfahrer1

This project looks really interesting. I have been looking for DIY USB DAC in this price range for quite a while. So if its not too hard to build and there will be a pcb board group buy count me in.
 I am sorry my knowledge in eletronics in not enough to contribe to the development


----------



## Radfahrer1

What's the status on this project? You guys haven't posted in a long while. I hope it is still alive


----------



## ble0t

My apologies...'real life' has been making an intrusion into my DIY time 
	

	
	
		
		

		
		
	


	




 Honestly, I hope to get a proto ordered ASAP. 000940 and myself are ironing out the very last little tweaks and I hope to post the latest version of the project soon.


----------



## ble0t

OK...here is the 'final' version of the board. I'm looking to order a couple protos sometime early next week, so if anyone sees anything, let me know. Enjoy 
	

	
	
		
		

		
		
	


	




Schematic

Top Layout

Bottom Layout

Top + Bottom Layout

Top + Bottom + Top Silk Layout


----------



## steinchen

great! I got nothing to say but thanks for all your hard work, I'm really looking forward to build this project.


----------



## ble0t

OK...I'm going to need to order at least five proto boards in order to make this cost effective i.e. not ridiculously expensive. Four of the boards are already taken (00940, myself, my friend who is doing the PIC code, and dviswa), but I am willing to do an order of up to 8 boards. For this reason, anyone who wishes to give this a shot can PM me. 

*Disclaimer:* This is a prototype design that has NOT been tested. Although we hope we did a good job designing this board, we cannot guarantee that you won't be left with more than just a large coaster. Also, these boards (due to their size) will cost at least $35 a piece, so keep this in mind!

 EDIT: I totally goofed on the price...it's fixed now. Also, as far as the board house is concerned, we have very tight requirements and a majority of cheaper board houses would not be able to accomodate us in this regard, which is why it might be a tad more expensive.


----------



## Clutz

Who are you ordering the PCB from? dsavitsk and I ordered a bunch of PCBs from somewhere and it was pretty inexpensive. Do you want me to see if I can find out where it was and forward you the info?


----------



## steinchen

you always get what you pay for, I'm fine spending a couple of dollars more and be sure that I get decent quality, particulary with so thin traces.


----------



## 00940

Bleot : have a look here : http://www.thepcbshop.com/ 

 That's what Guzzler used for the original USB DAC and the house is reliable afaik. The requirements are ok (they're working in 200micron, a bit less than 8mil).

 Please anyone interested, have a look at the schematics, especially the settings of pins such as reset or mute, all the maintenance stuff that can quickly go wrong


----------



## Clutz

Quote:


  Originally Posted by *steinchen* 
_you always get what you pay for, I'm fine spending a couple of dollars more and be sure that I get decent quality, particulary with so thin traces._

 

Well, not always. I'd rather have a Toyota Camry than a Range Rover - and a Range Rover is a lot more expensive. But your point is well made. 

 I'm really torn about this. I would really like to order one of these boards, but I owe a crapload in taxes.

 Did you ever get your friend to comment on the design 00940/ble0t?

 Cheers,
 Clutz


----------



## Clutz

Quote:


  Originally Posted by *00940* 
_Bleot : have a look here : http://www.thepcbshop.com/ 

 That's what Guzzler used for the original USB DAC and the house is reliable afaik. The requirements are ok (they're working in 200micron, a bit less than 8mil).

 Please anyone interested, have a look at the schematics, especially the settings of pins such as reset or mute, all the maintenance stuff that can quickly go wrong 
	

	
	
		
		

		
		
	


	


_

 

I'll try and check out the schematics later today, if I get bored at work. 
	

	
	
		
		

		
		
	


	




 Cheers,
 Clutz


----------



## ble0t

Quote:


  Originally Posted by *00940* 
_Bleot : have a look here : http://www.thepcbshop.com/_

 

The site has a bunch of errors in both FireFox and IE and I can't get any prices or submit anything (seems to be some .NET errors) 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *00940* 
_Please anyone interested, have a look at the schematics, especially the settings of pins such as reset or mute, all the maintenance stuff that can quickly go wrong 
	

	
	
		
		

		
		
	


	


_

 

Yes...I would again encourage anyone to take a look at the design and see if they can catch something we've looked over...


----------



## ezkcdude

So, you guys are going straight from schematic to PCB? That takes some balls. Good luck, guys!


----------



## 00940

Bleot : yes, it is messed up here too for now. It was working earlier though. Try tomorrow maybe ?

 ezkcdude : well, it's only a small serie of prototypes. And with a design of this complexity, it's difficult to do otherwise, unless you're using a bunch of adapters for the smd ICs, which ends up costing quite a lot abd doesn't give you a full idea of the potential of the design. Ordering more than one prototype is also needed, so that we don't throw the design for an error due to one builder in particular. "Large(r) scale" PCB orders (especially at the destination of less experimented builders) won't happen until the design is proven to work and sound good, until someone is willing to do it (I'm not really interested in managing that), until the details are worked out with the head-fi.org management, until real demand is there, until .... (fill out yourself 
	

	
	
		
		

		
		
	


	




 )


----------



## steinchen

just a minor wish: I'd like to try several I/V-converters with the DAC, but once you populated the opa1632 you can't disconnect them, the resistors (R7, R8, R57, R58, ...) are smd and thus cannot be socketed. Two DAC-I-out pads in line of each channel would be great, so one could easily jumper/break the trace to the I/V-stage.


----------



## 00940

Quote:


  Originally Posted by *steinchen* 
_just a minor wish: I'd like to try several I/V-converters with the DAC, but once you populated the opa1632 you can't disconnect them, the resistors (R7, R8, R57, R58, ...) are smd and thus cannot be socketed. Two DAC-I-out pads in line of each channel would be great, so one could easily jumper/break the trace to the I/V-stage._

 

Actually, R7-8, R57-58 are resistors leading to the balanced output. There are no resistors in between the opa1632 if the I/V (IC11 and 16) and the DAC outputs. However, there are pads in line with the outputs. But indeed, you will have to cut tracks if you wish to use another I/V after soldering the opa1632.


----------



## threepointone

kinda new to this thread, and I can't figure out much about the DAC after skimming through the first few pages--am I correct that this was the oversampling 24bit/192khz DAC that was being designed a long time ago? It's a USB DAC with line level outputs and all, right?

 is it _really_ $35/board? Or is it less in bulk when there's a group buy or something?


----------



## Clutz

Quote:


  Originally Posted by *threepointone* 
_kinda new to this thread, and I can't figure out much about the DAC after skimming through the first few pages--am I correct that this was the oversampling 24bit/192khz DAC that was being designed a long time ago? It's a USB DAC with line level outputs and all, right?

 is it really $35/board? Or is it less in bulk when there's a group buy or something?_

 

Well, lets put it into perspective.

 A PCM1794 costs $25. The ASRC chip, the AD1896 costs about the same. The PCM2707 costs $8. The CS8416 costs- probably around $20? I don't klnow though. The OPA1632s, which you need 4 of - cost $4.75 each. Then there is the PIC, which I have no idea how it will cost. There are the various voltage regulators, Then you have the various little bits, like the voltage regulators, capacitors, resistors, and various logic ICs.. There is probably around $150 in parts. So, if $60 for a well designed, well implemented board is making you cringe, then this probably isn't a good project for you.

 I'm thinking about implementing my own DAC, and I expect the parts to cost $140 in addition to what I've already spent (PCm2707, pcm1794, ad1896, opa1632)...


----------



## MegaMeee

good to see this project looks to be getting under way... I'd join in with the proto's but I am flat out for the next 3 months doing both work experience and studying at the same time... 
	

	
	
		
		

		
		
	


	



 Still good luck with the proto's.. Hopefully if the proto's work (From what I can see they should, but you never know) there will be some new interest in this project and a second batch of boards will follow.


----------



## ezkcdude

Quote:


  Originally Posted by *Clutz* 
_Well, lets put it into perspective.

 A PCM1794 costs $25. The ASRC chip, the AD1896 costs about the same. The PCM2707 costs $8. The CS8416 costs- probably around $20? I don't klnow though. The OPA1632s, which you need 4 of - cost $4.75 each. Then there is the PIC, which I have no idea how it will cost. There are the various voltage regulators, Then you have the various little bits, like the voltage regulators, capacitors, resistors, and various logic ICs.. There is probably around $150 in parts. So, if $60 for a well designed, well implemented board is making you cringe, then this probably isn't a good project for you.

 I'm thinking about implementing my own DAC, and I expect the parts to cost $140 in addition to what I've already spent (PCm2707, pcm1794, ad1896, opa1632)..._

 

SRC4192 can be substituted for AD1896, and it will save you about $5.00 at digikey. It doesn't have an onboard clock, but that doesn't matter in this case. It's not much of a saving, but these things add up, especially if you fry a few. By the way CS8416 costs $6.42 at digikey (just ordered a few myself over the weekend).


----------



## 00940

Quote:


  Originally Posted by *ezkcdude* 
_SRC4192 can be substituted for AD1896, and it will save you about $5.00 at digikey. It doesn't have an onboard clock, but that doesn't matter in this case. It's not much of a saving, but these things add up, especially if you fry a few. By the way CS8416 costs $6.42 at digikey (just ordered a few myself over the weekend)._

 

The SRC4192 CANNOT be subsituted to the AD1896 without a big hit in jitter rejection. It has been shown its filter value is set too high.


----------



## ezkcdude

Quote:


  Originally Posted by *00940* 
_The SRC4192 CANNOT be subsituted to the AD1896 without a big hit in jitter rejection. It has been shown its filter value is set too high._

 

Hmmm...Do you have a link for that?


----------



## 00940

http://www6.head-fi.org/forums/showt...hlight=src4192

 among others... make a search on diyaudio too.


----------



## threepointone

Quote:


  Originally Posted by *Clutz* 
_Well, lets put it into perspective.

 A PCM1794 costs $25. The ASRC chip, the AD1896 costs about the same. The PCM2707 costs $8. The CS8416 costs- probably around $20? I don't klnow though. The OPA1632s, which you need 4 of - cost $4.75 each. Then there is the PIC, which I have no idea how it will cost. There are the various voltage regulators, Then you have the various little bits, like the voltage regulators, capacitors, resistors, and various logic ICs.. There is probably around $150 in parts. So, if $60 for a well designed, well implemented board is making you cringe, then this probably isn't a good project for you.

 I'm thinking about implementing my own DAC, and I expect the parts to cost $140 in addition to what I've already spent (PCm2707, pcm1794, ad1896, opa1632)..._

 

nah, $150 seems fine--I guess I've been getting used to looking at all the new ~$5 tiny SMD boards

 and so it seems like this is just a really good dac with low jitter, not one of the new designs that can oversample?


----------



## ezkcdude

Quote:


  Originally Posted by *00940* 
_http://www6.head-fi.org/forums/showt...hlight=src4192

 among others... make a search on diyaudio too._

 

00940, I'm not trying to give you a hard time or anything, but all I saw was a lot of speculation. Somewhere in there gmarsh had a comment about the cutoff frequency, but I couldn't find anything in the data sheet about that. It would be nice to see real jitter tests comparing these two chips. Anyway, no big whoop. I was just proposing a way for people to save a few bucks, especially during prototyping. I'm definitely not trying to convince anyone that SRC4192 is better than or even equivalent to AD1896, just that they are pin-compatible. Of course, one could just as easily try both.


----------



## ble0t

OK...it seems that using The PCB Shop will be cheaper (and easier for that matter). Currently, I'm looking at ordering 5 boards which would put the cost of each board at $35 a piece. This does include the price of getting it shipped to me and the price of sending it out as well. I'll be placing the order tomorrow, so if anyone wants to get on board, let me know asap.


----------



## 00940

ezkcdude : The frequency cut off of the filter of the src4192 is indeed not in the datasheet. However, from the people who asked TI directly (John Siau from Benchmark, Gmarsh, among others), we know that it is higher than for the AD1896. The quality of a digital PLL is directly linked to how low the cut off frequency is.

 threepointone : this DAC oversamples quite a lot to be honnest. It first uses the AD1896 asynchronous reclocker to reduce the jitter making it to the DAC. It is under this respect similar to the Benchmark DAC1. At this stage, it "upsamples" the incoming flow to 96KHz/24bits and feeds it to the digital filter of the pcm1798 (or pcm1794 for higher quality). This filter then oversamples it eight times. Both chips are fed by a low jitter crystek clock, working at 24.576MHz (96KHz*256fs). So a 44.1KHz incoming signal will end up at 768KHz, oversampled by a factor of almost 18. It allows us a relaxed analog filter. It is good to note that the DAC will only accepts inputs up to 24bits/96KHz with a 24.576MHz clock. You would need a higher clock to get 192KHz inputs.


----------



## ble0t

Quote:


  Originally Posted by *00940* 
_It is good to note that the DAC will only accepts inputs up to 24bits/96KHz with a 24.576MHz clock. You would need a higher clock to get 192KHz inputs._

 

Just to expand a bit further here, the title of the thread is in fact 'USB DAC Design questions', so the aim was to make a very good quality USB DAC. The addition of the SPDIF section was included to add more flexibility to the design. Given that the current generation of USB --> I2S ICs only output at a max of 16bits/48kHz, the design should work nicely for the intended purpose.


----------



## threepointone

Quote:


  Originally Posted by *00940* 
_ezkcdude : The frequency cut off of the filter of the src4192 is indeed not in the datasheet. However, from the people who asked TI directly (John Siau from Benchmark, Gmarsh, among others), we know that it is higher than for the AD1896. The quality of a digital PLL is directly linked to how low the cut off frequency is.

 threepointone : this DAC oversamples quite a lot to be honnest. It first uses the AD1896 asynchronous reclocker to reduce the jitter making it to the DAC. It is under this respect similar to the Benchmark DAC1. At this stage, it "upsamples" the incoming flow to 96KHz/24bits and feeds it to the digital filter of the pcm1798 (or pcm1794 for higher quality). This filter then oversamples it eight times. Both chips are fed by a low jitter crystek clock, working at 24.576MHz (96KHz*256fs). So a 44.1KHz incoming signal will end up at 768KHz, oversampled by a factor of almost 18. It allows us a relaxed analog filter. It is good to note that the DAC will only accepts inputs up to 24bits/96KHz with a 24.576MHz clock. You would need a higher clock to get 192KHz inputs._

 

that's good--ignore the 192khz nonsense, I got confused with the numbers, 96khz is what i meant. It's just that I recently did a few blind tests with foobar on my sound card and I could tell a definite difference between 24/96 and 16/44.1


----------



## steinchen

I need AD1896 + CS8416, Farnell Germany doesn't sell to private persons (business clients only) and digikey charges a fortune for shipping. I'd very appreciate if someone can hook me up on his order.


----------



## ble0t

I placed the order with The PCB Shop yesterday, but I am still waiting for verification/billing information from them (they have a screwy system over there 
	

	
	
		
		

		
			





 ). I'll keep everyone updated when I hear more...


----------



## dviswa

Quote:


  Originally Posted by *ble0t* 
_I placed the order with The PCB Shop yesterday_

 

Wow, that is great news 
	

	
	
		
		

		
		
	


	




 . Time I rolled up my sleeve and got going. For this round can we get those PIC programmed by your friend Ble0t? If so any idea what that might cost?

 I don't remember anybody doing a BOM. Please let me know if one exists or else I will start one.

 Regards,
 Dinesh


----------



## dviswa

BTW, guys, don't we need a name? USA DAC v1.0 hardly does it any justice.

 Any proposals for a catchy name?


----------



## 00940

The truly magnificent Bleot and 00940's DAC ? 
	

	
	
		
		

		
		
	


	




 The NNS-DAC (no non-sense DAC) has my vote.
 I imagine we could also use the HHTS-DAC (horribly hard to solder DAC)


----------



## dviswa

Quote:


  Originally Posted by *00940* 
_The truly magnificent Bleot and 00940's DAC ?_

 

You mean TMBO DAC?
 How about TomBoy DAC


----------



## ezkcdude

Well, that's better than TomKat! 
	

	
	
		
		

		
		
	


	




 Here's a name I have "reserved": PteraDAC...

 just cuz I likes my dinosaurs.


----------



## dviswa

Quote:


  Originally Posted by *ezkcdude* 
_Here's a name I have "reserved": PteraDAC..._

 

What?!!!? The DAC already extinct... Even before it was made??? 

 Just kidding


----------



## MegaMeee

horribly hard to solder DAC gets my vote... I'm getting nightmares already


----------



## ezkcdude

Quote:


  Originally Posted by *dviswa* 
_What?!!!? The DAC already extinct... Even before it was made??? 

 Just kidding
	

	
	
		
		

		
		
	


	


_

 

Here's one that will really blow your mind:

 "BAD DAC" stands for...drum roll please...

 Bad Acronym DAC


----------



## ble0t

Quote:


  Originally Posted by *dviswa* 
_For this round can we get those PIC programmed by your friend Ble0t? If so any idea what that might cost?_

 

The PIC itself costs around $1 from Digikey...I'm working with my friend to get the code done, although making sure it's working will take a near complete board, so be patient on the SPDIF section


----------



## ble0t

Doh! I just got the design back with errors...it appears that the vias need to have a greate diameter-to-pad ratio. Who knew? 
	

	
	
		
		

		
		
	


	




 I'm updating the design and will resubmit it later today...


----------



## Clutz

I'm curious about the use of the LT1793 instead of a REG101.33?


----------



## 00940

The LT1763 is feeding a lot of chip, the 100ma of a RG101 were a bit too limited. And IIRC, the LT1763 was more interesting than the REG102 or 103


----------



## Clutz

Interesting: Why did you think that the LT1763 was better than the REG102? It does seem to have substantially lower ripple than the REG101 or REG102, so why not use it in place of the REG101 too? 

 I'm thinking about using a REG102-3.3 and REG102-5, but I'm now wondering if the LT1763 would be better.


----------



## Clutz

I'm curious about the use of the LT1793 instead of a REG101.33?


----------



## ble0t

The LT1763 can supply 500mA of current as 00940 alluded to above. The use of REG101 in the other two positions was because we didn't need that amount of current and thus its cheaper I believe.

 On a side note, I wouldn't get all that hung up over the minute details of the regulators. Yes, it is important to have good filtering and low noise, but in the end I think the layout and other component choices make a larger difference.


----------



## ble0t

Sorry for the lack of updates here...

 Basically, I've been going back and forth with the board house on a couple small issues. I'm hoping to get everything resolved by early next week so we can get this show on the road


----------



## steinchen

May you post a schematic that contains the resistor and capacitor values please? Alternatively, is there a preliminary BOM already available ?


----------



## ble0t

As soon as I get the board situation squared away, I'll take a look at the BOM and get that posted. Sorry for the delays...


----------



## ble0t

Woohoo! The design has (finally) been accepted. We should hopefully have some protos in a couple weeks 
	

	
	
		
		

		
		
	


	




 I'll keep everyone updated and post some pics as soon as they arrive!


----------



## MegaMeee

lol I bet that that board ordering didn't turn out to be as simple as you expected...
 Still very much looking forward to your thoughts when you build the proto...


----------



## ble0t

The boards came in today! My camera is currently in another state (the girlfriend is borrowing it for a trip), so I won't be able to get any pics done until the middle of next week unless I attempt to use the camera phone. I can tell you that they look great and I'm excited to build one!

 I've been trying to get a partial BOM done, but I've been quite busy with "real life" again, so it's not as far along as I'd like. I'll try to get that done by the end of next week.

 Finally, I had only ordered 4 boards but they sent 5 of them, so I have an additional board for anyone who wants it. If someone claims it, it will lower the per board cost for everyone that is prototyping since I can spread out the savings 
	

	
	
		
		

		
		
	


	




 I will try to get the boards in the mail as soon as I can. If you haven't sent me your addresses (both mailing and Paypal), drop me a PM or e-mail.

 EDIT: OK, I pulled out the phone camera and took the best shots I could...here ya go!

PCB Top

PCB Bottom

 2nd EDIT: All boards are spoken for! Thanks everyone!


----------



## ble0t

OK...here is the BOM thus far for Digikey. I'm working on the one for Mouser and should hopefully get that posted either tonight or tomorrow.

 EDIT: The Mouser BOM has now also been added. Please note that certain parts are missing in relation to the I/V stage and balanced stage. I am working on getting those values and should be able to post updated versions soon. Thanks!


----------



## ble0t

Just an update for you guys...

 I've been really, really busy lately (best man in a wedding this past weekend and out of town training most of this week), so I will try and get the boards out as soon as I possibly can.


----------



## ble0t

OK...sorry for the delay on everything as of late. I finally got the boards mailed out as of Thursday this past week. I just bought a house (my first one...yay) about two weeks ago, so things have been hectic and money is going to be a bit tighter, but I hope to start building one of these after I've moved in (which is at the end of the month). Hopefully a couple of the other prototypers will have something before then. Also, please let me know if anything is missing from the BOM aside from the I/V and balanced stuff.


----------



## steinchen

How is the progress with programming the PIC chips ? Without them the DAC won't sound too good I figure ;P


----------



## ble0t

The code there is progressing a bit slower than I would like. I would point out though that this only affects the SPDIF portion of the DAC...the USB portion will work just fine (you can use a jumper to hard set it to only use the USB signal). Let me know when you guys get your boards


----------



## steinchen

received the board safe and sound today, looks very good 
	

	
	
		
		

		
		
	


	




 the legend print of the board contains part outlines but no part numbers or ids, please post an eagle layout with tNames and bNames enabled


----------



## ble0t

Sorry about that...the labels got so tight in the design, it was going to be impossible to have them on the board and have them be legible. I'm working on putting the board labels into both BOM's and also working on getting the I/V and balanced parts in there as well. I'll post everything as soon as I can.


----------



## ezkcdude

Quote:


  Originally Posted by *steinchen* 
_received the board safe and sound today, looks very good 
	

	
	
		
		

		
		
	


	




 the legend print of the board contains part outlines but no part numbers or ids, please post an eagle layout with tNames and bNames enabled_

 

Are pics ensuing?


----------



## steinchen

Quote:


  Originally Posted by *ezkcdude* 
_Are pics ensuing?_

 

when populated, yes. a bare pcb without part labeling on the silkscreen isn't really interesting


----------



## ezkcdude

Quote:


  Originally Posted by *steinchen* 
_when populated, yes. a bare pcb without part labeling on the silkscreen isn't really interesting_

 

Oh. Well, I guess I wasn't interested then


----------



## dsavitsk

Quote:


  Originally Posted by *ezkcdude* 
_Oh. Well, I guess I wasn't interested then 
	

	
	
		
		

		
		
	


	


_

 

Look at post 391


----------



## ezkcdude

What transformer (part number) is being used for the SPDIF input?


----------



## ble0t

Sorry for the delay, but things have been busy for me as of late. Here are shots of the top and bottom with names and a new BOM that has all the values associated with the names. The BOM spreadsheets for the specific distributors do need to be updated to reflect some changes and also to reflect the addition of the I/V section and the balanced section. I'll try and get those done next.

Top

Bottom

*Also, there is one minor error in the silkscreen on the board, but it is very important: C67 and C71 (caps connected to VCOM on the DAC) should be reversed i.e. the polarity is wrong on the silkscreen.*

 EDIT: Oops...thanks for catching that link problem steinchen...it's been fixed now


----------



## steinchen

both links point to silk_top, this is the link for silk_bottom: http://img.photobucket.com/albums/v3..._names_bot.gif


----------



## ezkcdude

I'm still wondering about the transformer. Is it S22230 (digi-key #470-1005-ND)?


----------



## steinchen

I got digikey #470-1004-ND TRANSFORMER FOR CS 8401,2:8411


----------



## ezkcdude

Close enough. Same footprint. Did you notice the other one has a slightly lower interwinding capacitance (5 pF compared to 15 pF)?


----------



## Radfahrer1

I can't wait to see the first finished DACs and see how they sound.
 Will there be a group buy for the PCB after the testing is successful, because i am defenitly interested in this project.


----------



## Clutz

Received my boards yesterday - thanks ble0t and 00940. It looks great. I'll try to keep it that way. I think it's time to replace my $8 40 watt soldering iron with something a lil better. Maybe a $25 25 watt soldering iron? That's an improvement of $0.20 / watt to $1 / watt. 
	

	
	
		
		

		
			





 Does that mean it'll improve my soldering skills by 500%?


----------



## ble0t

Just a couple of things...

 Has anyone made any progress on their boards? Just curious if the layout is good and the construction is fairly straight forward.

 Also, the PIC code had finally begun in earnest. The main issue is that the person who is writing the code for me does not have a board to test with, so I'll need to get off my arse and get one done for him!

 Sorry for a lack of progress/updates here...I just moved into my new (and first) house about 2 weeks ago and it's been hectic...


----------



## Clutz

I am planning on taking some vacation time on the 3rd or 4th week of September, but until then I am just too busy to do anything (counting about 30000 microscopic worms per day), unfortunately. 

 That plate has about 150 worms on it or so. The largest worms there are about 1mm long, but some are closer to 0.2mm. 

 I'm going crazy.


----------



## psilosome

Handling nematodes makes soldering SSOP look easy


----------



## ezkcdude

Quote:


  Originally Posted by *Clutz* 
_I am planning on taking some vacation time on the 3rd or 4th week of September, but until then I am just too busy to do anything (counting about 30000 microscopic worms per day), unfortunately. 

 That plate has about 150 worms on it or so. The largest worms there are about 1mm long, but some are closer to 0.2mm. 

 I'm going crazy._

 

Just a piece of advice from a fellow scientist: Get someone to write an image recognition algorithm for you, or better yet, DIY 
	

	
	
		
		

		
		
	


	




 .


----------



## Clutz

Quote:


  Originally Posted by *ezkcdude* 
_Just a piece of advice from a fellow scientist: Get someone to write an image recognition algorithm for you, or better yet, DIY 
	

	
	
		
		

		
		
	


	




 ._

 

The problem isn't the image recognition - the problem is the image acquisition at a sufficiently high magnification, without spending a lot of money on new equipment. Since this is a one (well, now two) time project, we're not going to spend major $$$$ on buying new equipment just to save me some time. In order to be able to identify all the worms, I need to be at 40-60x magnification, but anything greater than about 10x, and the plate falls out of view - hence, back to counting manually.


----------



## Clutz

Hi ble0t and 00940;

 Re: Digikey BOM: 

 There is a listing for 8 * CAP .001UF 63V METAL POLY - but I can't find this same part in the more recent BOM, which connects particular parts to locations..? Can you tell us what they are for?


----------



## ble0t

I think I may have goofed that one up...I'd say just go with the most recent BOM for any parts/qtys.


----------



## steinchen

Quote:


  Originally Posted by *ble0t* 
_Has anyone made any progress on their boards? Just curious if the layout is good and the construction is fairly straight forward._

 

took me some time to source all needed parts and got them about 2 weeks ago. I've been little swamped the last time and I started populating the board today. Hopefully I can report about the first run the next days.


----------



## 00940

as a reminder : don't forget you have to cut or lift the enable pin of the opa1632 or you won't get any sound.


----------



## ble0t

Ahh...thanks 00940...I totally forgot about that one.


----------



## Clutz

Hey guys,

 RE: AD1896: Why did you choose togo with a 3.3V PSU for both VDD_CORE and VDD_IO, instead of 3.3V for VDD_CORE and 5.0V for VDD_IO? Was it just to simply the power supply situation?


----------



## 00940

I couldn't find anything in the datasheet suggesting better performances for a 5V supply. This bit "The AD1896 is a 3.3 V, 5 V input tolerant part .... The AD1896 is 5 V input-tolerant only when the VDD_IO supply pin is supplied with 5 V" seems to suggest that a VDD_IO of 5VDC is only useful for higher inputs.

 So, yes, we went with what was easiest (and cheaper) to lay out.


----------



## Clutz

I'm thinking of going with the following

 PCM2707 (requires 3.3V supply, in addition to the 5.0Vbus provided by USB) -> AD1896 (3.3V and 5.0V), Df1706 (3.3V part), AD1862 (+/-12V for the analog side, and +/- 5 to 12V for the digital side). 


 a) The 3.3V PSU for the digital poarts (PCM2707, AD1896, DF1706) would be derived from the +12V PSU for the digital portion of the AD1862, and then a separate +/-12V for the analog portion of the AD1862/I/V stage

 OR

 b) I could go with a single 3.3V supply to power the PCM2707, AD1896, and DF1706, and have a 5.0V PSU for the AD1896 and the digital side of the AD1862, and then a +/-12V PSU for the analog side of the AD1896. 

 c) I could go with a 3.3V PSU for the PCM2707, DF1706, and AD1896, plus a 5.0V PSU also for the AD1896, derived from the +/-12V PSU for the AD1862 (ultimately 3.3V derived from the 5.0V, derived from the +12V) for the digital portion; and a separate +/-12V PSU in the for the analog portion.

 So for a) I could create a +3.3V PSU from the +/- 5-12V PSU required for the AD1862 (e.g. one transformer), and one +/-12V PSU for the analog side of the AD1862 and the I/V stage (which will be an opamp - probably ths4631). 

 or for b) I could create a +3.3V needed for the PCM2707, AD1896, and DF1706 from the +/-5V PSU required for the digital portion of the portion of the AD1862; and then a single +/-12V power supply for the analog portion of the AD1862 and the ths4631 opamps used for i/v conversion. 

 What do you think?


----------



## Clutz

IC6: USBUFxx? Where did you get the Eagle library for this particular part?

 Thanks,
 Clutz


----------



## 00940

Bleot made the part I think.

  Quote:


 or for b) I could create a +3.3V needed for the PCM2707, AD1896, and DF1706 from the +/-5V PSU required for the digital portion of the portion of the AD1862; and then a single +/-12V power supply for the analog portion of the AD1862 and the ths4631 opamps used for i/v conversion. 
 

I'd go that way, with some RLC networks to isolate the channels.


----------



## ble0t

Yea...that was a custom part. I've gotten fairly good at those in EAGLE 
	

	
	
		
		

		
			





 Remember...datasheets are your friends


----------



## Clutz

My analog power supply will consist of two: 20VA 20VAC with center tapped secondaries. The first will be for the digital portion, and the second will be for the digital portion. One power supply for digital, one for analog. Each of the power supplies will be identical


 Mains :115VAC -> Center Tapped Transformer 14VAC -> RCLCAC -> SoftRecovery Diodes (or Diode Bridge) + snubbers -> Voltage Regulators

 Digital Power Supplies

 12V PSU's:
 Voltage Regulator (LM7812, 12V OUT): 12V PSU for the DIGITAL AD1862
 Voltage Regulators(LM7912): -12V PSU for the DIGITAL AD1862

 5V and 3V PSUs
 Voltage Regulator (LT1763, 5V OUT): 5V PSU for the AD1896
 Voltage Regulator (LT1763, 3.3V OUT): 3.3V PSU for the PCM2707, AD1896, DF1706

 Analog Power Supply:

 Separate Transformer: Center Tapped Transformer: 14VAC-> RCLCRC -> Bridge + snubbers> LM7812/LM7912

 I'd actually make this on two separate boards.

 How does that seem?


----------



## Clutz

On the updated BOM:
 Re: R7, R10, R17, R19. I cannot find any 600R resistors anywhere - is 560 close enough? Additionally, Idon't know what KK1, and KK2 are supposed to be - and I can't find anything for the part L1. 

 There seems to be a large number of parts that need to be ordered from Mouesr. Is thjere any reason to use parts from mouser (e.g. vishay dale) over equivalent parts from Digikey (e.g. Panasonic, Yageo), simply so I can simplify any resulting order?

 Thanks,
 Clutz


----------



## Pars

Quote:


  Originally Posted by *Clutz* 
_On the updated BOM:
 Re: R7, R10, R17, R19. I cannot find any 600R resistors anywhere - is 560 close enough? Additionally, Idon't know what KK1, and KK2 are supposed to be - and I can't find anything for the part L1._

 

Would 604 ohms be close enough? Or 590? Check PPC604YCT-ND at Digikey... all kinds of BC and PPC (basically the same) there. I like the BC resistors (SRC25 IIRC). 1% 0.4W metal film.


----------



## ezkcdude

I second the vote (if that was a vote) for going with digikey parts. The only resistors I like better at Mouser, because digikey doesn't carry them, are the Vishay 0.1% VTF.


----------



## ble0t

L1 was in the Mouser BOM:

 434-01-101JFastron High Reliability Radial RF Chokes

 As for those other resistors, I think I missed those (oops), so I guess just try and get as close as you can 
	

	
	
		
		

		
		
	


	




 EDIT: The reason I went with Mouser for most of the resistors and capacitors is because they don't force an order in multiples or 5 or 10, thus saving some $$$ which is always a good thing


----------



## Clutz

ble0t: That's what I figured. I'll have to decided if it's worth waiting 2 weeks for a USPS shipping to Canada, or find out if the FedEx brokerage fees are ridiculous. I've got most of the BOM for this figured out for Digikey - just gotta figure out a BOM for Mouser, or convert it to digikey - and then add on some parts for other projects. Could be the single largest order yet.

 I can't seem to figure out how to make new parts in Eagle.. 
	

	
	
		
		

		
		
	


	




 Do I just draw the outline as I want it to appear in the schematic first and THEN layout the pads? Any advice?


----------



## steinchen

While populating the board I realized that I'm short of one reg101ua-5, Mouser is out of stock, may someone hook me up on his digikey order ?

  Quote:


  Originally Posted by *Clutz* 
_I can't seem to figure out how to make new parts in Eagle.. 
	

	
	
		
		

		
		
	


	




 Do I just draw the outline as I want it to appear in the schematic first and THEN layout the pads? Any advice?_

 

The easiest way is to copy existing devices / packages and alter them. The device editor is all but intuitive to me too, when creating new parts I always hold the manual in my left hand :X


----------



## MASantos

Quote:


  Originally Posted by *steinchen* 
_While populating the board I realized that I'm short of one reg101ua-5, Mouser is out of stock, may someone hook me up on his digikey order ?_

 

You need the 5volt fixed one right?

 I have two of those I don't need. Send me a Pm with your adress, I'll ship them tomorrow.


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## ble0t

Here's a pretty basic overview of how to add a new part to the library. The basic premise is that you need to design a symbol (schematic) and a package (layout/board) and then combine those into a device so that when you add a part to your schematic/board, it has an equivalent symbol/package to add to the other.

http://myhome.spu.edu/bolding/EE4211/EagleTutorial4.htm

 I hope that helps a bit...


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## DaKi][er

Quote:


  Originally Posted by *Clutz* 
_I can't seem to figure out how to make new parts in Eagle.. 
	

	
	
		
		

		
			





 Do I just draw the outline as I want it to appear in the schematic first and THEN layout the pads? Any advice?_

 

http://www6.head-fi.org/forums/showthread.php?t=200278


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## ezkcdude

I was looking at the schematic for this DAC again, and noticed that the Vcom caps on the PCM chip (C63,C67) have the polarity opposite to the data sheet. I was wondering if anyone noticed this earlier.


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## ble0t

Yep (link)...that (at least at this point) appears to be the only board error.

 It was also mentioned that you need to clip/lift the 'ENABLE' pin of the OPA1632's so don't forget that either.


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## Clutz

Oh ble0t: I forgot to mention: I know of at least one example where intertia won and gravity lost.


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## Clutz

Can someone check my .lbr file for the SOT323-6 package I've made for the USBUFW02? It's not pretty, but I think I've got the measurements right. I'll attach both the .lbr and a link to the appropriate pdf file. 


 And here is the link to the data file:
http://www.st.com/stonline/products/...7041/usbuf.pdf

 If anyone could take a look at it I'd appreciate it. I know I've mangled it - but it's my own mangling job.. and it makes me happy.


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## ezkcdude

So, any update this week? Has someone finished this board yet? Anybody? Bueller?


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## Clutz

I've still gotta finish my BOM at digikey.. 
	

	
	
		
		

		
		
	


	




... Work is taking up too much time.

 ble0t / 00940, any chance I can get the .sch files you used or the library for the USBUF and some of the other custom made parts/ I've made my own - but I lack confidence in them.

 Cheers,
 Clutz


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## Clutz

Question for you guys: What are you using for the C84 part? 0.1uF 50V capacitor size 1206. I can't find anything at digikey or mouser that matches it. Would 0.15 work okay?


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## ezkcdude

Search harder! (PCC104BCT-ND)

http://rocky.digikey.com/scripts/Pro...=ECJ-3VB1H104K

 That's not the only one either.


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## Clutz

Since they didn't specify that a ceramic cap here was a good idea, I assumed that I should be going with a film type.


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## ezkcdude

Well, I could be wrong but in that size, I was assuming it's for decoupling, and that's pretty much always going to be a ceramic. Correct me if I'm wrong.


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## cetoole

Quote:


  Originally Posted by *Clutz* 
_Question for you guys: What are you using for the C84 part? 0.1uF 50V capacitor size 1206. I can't find anything at digikey or mouser that matches it. Would 0.15 work okay?_

 

I'll be buggered if I can figure out which of the schematic/boards you guys actually had manufactured. This is the latest schematic I have found, and this the latest board layout. Maybe its just me, but the two dont seem to match. What is labled as C84 in the schematic seems to be C35 on the board, so if that is the case, it sure doesnt look to be a 1206 part to me, seems like a 7.2x2.5mm radial leaded one. If this is the right board I am looking at, I would use either a Polypropylene film cap, such as Digikey BC2054-ND, but it is 6mm thick, so would have to be mounted a little creativly, PPS film, which, for 0.1uf can be had in 1210 package like digikey PCF1208CT-ND, and depending on the board, may fit, or NP0 or C0G ceramic, which can be had in 1206 as Digikey 490-1767-1-ND. Also, why did you pick 0.1uf? Unless I am looking at the wrong part, or missing something in the OPA1632 datasheet, I cant find anything to suggest that value.


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## ezkcdude

Yeah, if it's C84 from the schematic that cetoole showed, that cap is definitely not decoupling. In that case, I don't think 0.1uF can be the right value, though.


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## Clutz

Quote:


  Originally Posted by *cetoole* 
_I'll be buggered if I can figure out which of the schematic/boards you guys actually had manufactured. This is the latest schematic I have found, and this the latest board layout. Maybe its just me, but the two dont seem to match. What is labled as C84 in the schematic seems to be C35 on the board, so if that is the case, it sure doesnt look to be a 1206 part to me, seems like a 7.2x2.5mm radial leaded one. If this is the right board I am looking at, I would use either a Polypropylene film cap, such as Digikey BC2054-ND, but it is 6mm thick, so would have to be mounted a little creativly, PPS film, which, for 0.1uf can be had in 1210 package like digikey PCF1208CT-ND, and depending on the board, may fit, or NP0 or C0G ceramic, which can be had in 1206 as Digikey 490-1767-1-ND. Also, why did you pick 0.1uf? Unless I am looking at the wrong part, or missing something in the OPA1632 datasheet, I cant find anything to suggest that value._

 

The latest board layout seems to match my board. Why 0.1uF? It was listed in the BOM posted by ble0t - but I didn't put any thought into it before I posted the question. 
	

	
	
		
		

		
		
	


	




 I know, I know, I'm being lazy.


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## steinchen

I'va almost finished the board, just a few caps and resistors missing, I should have them by the end of the week top bottom 


 what are the values of R38 and R42 ? they are missing in the BOM

 the ground pads are a pita to solder, I cranked up my soldering iron to 300°C but the solder is still as dogged and sticky as tar. I think we should add thermals to the ground pads.

 The area around the opamps is totally crammed, it should be dispersed imo.

 How is the progress on programming the PIC ?

 How to set the jumpers to make a first run at USB only ?


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## 450

I read through some parts of this thread and I realize this is still in the planning stage but what kind of DACs would this USB DAC compare with?


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## steinchen

We've already entered the prototyping phase. It started as an USB DAC but meanwhile got an SPDIF input too.


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## ble0t

Howdy 
	

	
	
		
		

		
			





 R42 = 100R
 R38 = 75R

 Sorry about leaving those out.

 As for the PIC, I've bumped into some issues with it as of late (such as frying my development board accidentally). To be quite honest, I don't expect to have anything done until the holidays. I apologize for all the delay in that respect.

 To use the USB portion, JP4 (the one right next to the PCM2707 crystal) needs to be set so that the multiplexer has a high voltage input i.e. have the jumper set so that it is on the two pins closest to the PCM2707 crystal.

 Do all the voltages appear to be good as far as the power supply is concerned? Feel free to post as much commentary and results as possible. Thanks again for participating!


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## steinchen

just powered up the dac for the first time and the voltages does not seem right as far as I can judge this. to have a detailed look at the voltages I need a schematic where the IDs match the IDs of the silkscreen / BOM. 

 jumpers: what about / how to set the other jumpers (jp1 to jp3) ?


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## threepointone

I don't know how important this will be in practice, and i'm not exaclty sure what's what on the board, but shouldn't the digital and analog grounds be separate (except right at the power supply) to reduce noise being injected into the analog circuit? might want to consider this in the final design


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## Clutz

Quote:


  Originally Posted by *threepointone* 
_I don't know how important this will be in practice, and i'm not exaclty sure what's what on the board, but shouldn't the digital and analog grounds be separate (except right at the power supply) to reduce noise being injected into the analog circuit? might want to consider this in the final design_

 

No - it causes more problems than it solves.


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## ble0t

There are a couple 'stages' of the power supply (LM317, LT1206, etc.), so which voltages look off? 

 As far as JP1 and JP3 are concerned, those are there to allow external input if the builder wanted to try supplying the clock and/or PCM179* from a different power supply. A quick look at the traces should tell you how to set those in order to take advantage of the built in power supply.

 I'll try and get a fully updated BOM/Sch/Brd put together and post it as soon as possible.


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## Clutz

Hey ble0t - any updates?


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