# DIY iPod digital out



## joneeboi

I'm dreaming of a white Christmas where DIY-ing iPod owners can enjoy digital I2S output from their iPods. From the little research I did, the Opus DAC can input I2S, but I'm wondering how to carry this signal to the DAC. I've searched and searched but couldn't find anything on DIY I2S cables anywhere. I gather that the wires can't be too long and need some sort of shielding. Should I just open up an old CD player and yank that I2S cable instead? My plan is to pull the signal from the Wolfson (funnest soldering job ever) and send them to the unused pins on the dock (funnest soldering job ever), which are pins 7, 14, and 17. I'll need a 4th one, so I'm guessing I can steal one from, say, pins, 8, 9, or 10. Apple iPod dock interface pinout and signals @ pinouts.ru tells me that these are used only on iPod photo, so I guess these are fair game too, right? From one of the many iPod-implemented Wolfson datasheets (see diyMod post 1), the data accepted are master clock, bit clock, DAC data, and DAC left/right audio interface. I think usually I2S has only 3 lines, but for some reason the iPod has 4. What does the Opus DAC take in for its I2S module? Essentially we're swapping the iPod's (4G in this case) Wolfson for the Opus Dac's. Has anyone else looked into the matter?

 Any help is appreciated.


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## OverlordXenu

Cat5e cable.

 You could also make a breakout box that makes it SPDIF or optical or something, if you wanted to use like an iBasso D1 on the go.

Here's my attempt at creating a digital out for an X5.


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## Jambo

I2S in my experience is four wires, but *I think* I2S links are sometimes three when your MCLK is a system clock coming from somewhere else, but I could well be wrong.

 In this situation however you want all four. As the Opus uses Wolfson 8740/1s then I'm pretty sure it'll want those four.


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## jinp6301

I know that Audio Alchemy used basically S-video cables (or it seems) for its IS2 connection.


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## stevenkelby

You could put the I2S signal into a Pico. Would need more battery though. Unless you share the ipod battery.

 No technical help here, sorry.

 Good luck with it!


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## jsmithepa

Since THESE GUYS sell it for 2 grands, would u DIY it if it costs u say 1/2 as much?


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## 00940

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Should I just open up an old CD player and yank that I2S cable instead? .... From one of the many iPod-implemented Wolfson datasheets (see diyMod post 1), the data accepted are master clock, bit clock, DAC data, and DAC left/right audio interface. I think usually I2S has only 3 lines, but for some reason the iPod has 4._

 

Don't open an old cdp... all you would find is some poor ribbon wire. I2S wire have to be kept short (less than 20cm is good) and if you're going outside of the box, I'd go shielded. CAT5 could be ok I guess.

 Traditionnal I2S was 3 wires, for old DAC chips like the tda1543 (bck, data, lrck). Now that the DAC integrates digital filters and the like, you have to add the mck (ou sck) line. The WM8740 used in the opus certainly needs the 4 lines.


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## MusicallySilent

I guess Wadia does it too

http://www.head-fi.org/forums/f7/wad...-ipods-281975/

 This would be interesting to have, I imagine many users with portable DAC/AMP combos though it seems a limited amount have optical/coax in would appreciate this..

 I might have to follow this thread


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## joneeboi

I must have closed Firefox before posting my response. =T

 We could go the Wadia way, but as far as DIY goes, that's selling out.  It's pricier than DIY, plus it isn't as personalized as we can make it. This guy from OverlordXenu's thread seems to have gotten it right. Serves our purpose, small footprint. One drawback is that it would have to draw power from the iPod. Then again, while we are fabricating our own iPod docks, we might also want to add charging capability to the iPod while the transceiver steals power from it. I can't imagine this kind of setup being portable, but some have carried T-amps and K1Ks around too. The biggest hurdle would be taking the signal from the iPod's Wolfson and sending it to the 30-pin dock. Further, internal layouts may restrict generation-compatibility. For instance, the 5G/5.5G iPods have that metal frame that screws into the front plate, for those of us who have seen the innards, so routing the wires to the dock would suck. I find it'd be easiest with 3G and 4G iPods, as it seems Apple hadn't shrunken that layout just then. There's plenty of room indoors for those iPods, so signal retrieval wouldn't be so bad. For future reference, I soldered some navships stranded SPC to the iPod dock before but only used 2 strands (documented in diyMod thread). After you get the hang of it, you feel like you can solder 100 Alien DAC PCM2702 chips with a 10W soldering gun in the dark. Anyone who'll try routing wire-wrap through that 5G/5.5G frame to the dock can report on the fit. Also, I don't suppose making this fit the Alien DAC for the ultimate in portability is viable either, is it?


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## OverlordXenu

Quote:


  Originally Posted by *jsmithepa* /img/forum/go_quote.gif 
_Since THESE GUYS sell it for 2 grands, would u DIY it if it costs u say 1/2 as much?_

 

The $2,000 is just for their silly dock. The iPod modification costs $200, so they probably just pull the I2S to the dock connector.


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## joneeboi

Quote:


  Originally Posted by *jsmithepa* /img/forum/go_quote.gif 
_Since THESE GUYS sell it for 2 grands, would u DIY it if it costs u say 1/2 as much?_

 

Sure, I'll do the mod for you at half the price. Just for you.


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## joneeboi

Now, I'm not a design expert of any kind just yet, but perhaps we can design our own portable, full on DAC/amp solution using some of Wolfson's better DACs. I realize there are already solutions available along these lines, but none fitting the I2S input requirement. stevenkelby, you mentioned using the Headamp Pico for its I2S input, but that DAC/amp converts USB to I2s. Unless we open it up and route some wires, we can't do it without voiding the warranty. I'm not sure how we'd go about doing this, but is it a viable solution having a portable DAC/amp running off a 9V? That's quite a huge energy loss converting down to 3.3V, so maybe running off some AAs would be wiser. Before we even go there, we should probably pick out a DAC that'll work. I'm thinking something along the lines of a portable full-on DAC, like a portable TPA Opus DAC (okay, maybe not that far). I'm looking at some Wolfson DACs over at Mouser, and a couple of the models jump out immediately. They all seem to be designed for DVD players or digital set-top boxes, but if somehow we can plow through with the design, I think the head-fi community would be better for it. I'm no Alf, cetoole, Jambo, justin w., Ray Samuels, etc., but I can offer my limited knowledge and unbridled passion and enthusiasm to this kind of project. Please, someone with actual knowledge help me out here. 
	

	
	
		
		

		
		
	


	




 I've had a class in digital logic design, but I'm not sure how much of it actually applies here.

 Some of the DACs that I think would work here.

http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8521.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8956.pdf
http://www.wolfsonmicro.com/uploads/...728_Rev4.4.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf


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## FS2

Heh, I was wondering if anyone here was thinking of doing this from the interest the "iMod" thread is getting... 
	

	
	
		
		

		
		
	


	




 I've been thinking about it ever since I got water inside my iPod a few short months back. The most obvious problem is the 0.5mm pitch QFN the Wolfson DAC comes in.

 Beyond that, while searching around, I came across this (scroll down to 12-23-07) where someone took apart an iPod video and found that it had a non-standard I2S interface. That does sound like something Apple would do... It's possible that not all iPods are like that though, especially since MSB seems to be able to do it.

 Myself, I was thinking of tapping the 4 I2S signals out to the dock connector, and using an external portable DAC+AMP. I have two ideas for tapping into the I2S lines, but I don't know how much of a problem signal integrity would be. Perhaps a ASRC(?) would be needed somewhere along the line.

 joneeboi, you might find this interesting...


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## Jambo

The DVD player type DACs (generally) have the plus points of better performance and hardware control, but the down sides are their power consumption. They're all around the 70-80mW though from the datasheets I have looked at, which I guess is tolerable. I noticed you haven't mentioned the WM8740/1, they're Wolfson's best DACs which are worth looking at if you want best performance possible, and the power consumption for the WM8740 is ~80mW at 3.3V. Also, the WM8740 has differential output, so you don't need any caps on the output.

 I'm willing help out where I can, although I'm afraid my ipod is staying in one piece! And I also have my own project doing my head in at the moment as you can see in the Jambo DAC thread lol.


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## jsmithepa

Wadia Digital intros iTransport: the audiophile's iPod dock - Engadget HD

 Now when it becomes available, AND get a few reviews, then I may believe it.


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## error401

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Now, I'm not a design expert of any kind just yet, but perhaps we can design our own portable, full on DAC/amp solution using some of Wolfson's better DACs. I realize there are already solutions available along these lines, but none fitting the I2S input requirement. stevenkelby, you mentioned using the Headamp Pico for its I2S input, but that DAC/amp converts USB to I2s. Unless we open it up and route some wires, we can't do it without voiding the warranty. I'm not sure how we'd go about doing this, but is it a viable solution having a portable DAC/amp running off a 9V? That's quite a huge energy loss converting down to 3.3V, so maybe running off some AAs would be wiser. Before we even go there, we should probably pick out a DAC that'll work. I'm thinking something along the lines of a portable full-on DAC, like a portable TPA Opus DAC (okay, maybe not that far). I'm looking at some Wolfson DACs over at Mouser, and a couple of the models jump out immediately. They all seem to be designed for DVD players or digital set-top boxes, but if somehow we can plow through with the design, I think the head-fi community would be better for it. I'm no Alf, cetoole, Jambo, justin w., Ray Samuels, etc., but I can offer my limited knowledge and unbridled passion and enthusiasm to this kind of project. Please, someone with actual knowledge help me out here. 
	

	
	
		
		

		
			





 I've had a class in digital logic design, but I'm not sure how much of it actually applies here.

 Some of the DACs that I think would work here.

http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8521.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8956.pdf
http://www.wolfsonmicro.com/uploads/...728_Rev4.4.pdf
http://www.wolfsonmicro.com/uploads/.../en/WM8761.pdf_

 

I too would choose WM8740. It uses about 24mA at 3.3V, which isn't too bad if you're using AA cells (typically 2500mAh+), whatever amp topology you choose, it will likely use a lot more. Choose opamps carefully and you should get good battery life (I'd guess about 16h is possible).

 It's probably possible to fit a WM8740 DAC and (SMD) opamp-based amp into the smallest Hammond with two AAs, if you're very careful with your layout. I don't think you'd manage cramming a charger in there too though. If you choose a larger case, it should be easy.

 Be warned that I might get bored and do a layout for you 
	

	
	
		
		

		
		
	


	




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## joneeboi

Actually, error, I thought of listing you in that "group of people that I am not," but I already submitted the post.

 How did I miss the WM8741? It's listed in Mouser under "Stereo DACs, High End." *rolls eyes*

 Part of the rub becomes whether or not we should include the amp, much like what justin w. is doing with his Pico. The initial amp that comes to mind is the PPAS. In fact, they're so small we could probably fit two for ultimate in balanced configuration. I'll bookmark the WM8741 and read endlessly. I'll also try to grab one of the professors at school and see if they can guide me a little.

 How does USB power sound? I want to regulate it, but it would have to be cut down a bit. I vaguely recall a similar problem in the Alien DAC. But fear not, even in the tiny Hammond 1445C801(BK) we can fit some sort of DC-DC converter.

 Power supply solutions. The Fairchild Semi seems quasi-viable as it can supply up to Vin. I wonder what Team Alien DAC pored over before deciding on the REG101s.
http://focus.ti.com/lit/ds/symlink/tl2575-05.pdf
http://focus.ti.com/lit/ds/symlink/dcr011203.pdf
http://www.fairchildsemi.com/ds/FA/FAN6520A.pdf

 Brain...power...leaving...


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## error401

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Part of the rub becomes whether or not we should include the amp, much like what justin w. is doing with his Pico. The initial amp that comes to mind is the PPAS. In fact, they're so small we could probably fit two for ultimate in balanced configuration. I'll bookmark the WM8741 and read endlessly. I'll also try to grab one of the professors at school and see if they can guide me a little._

 

I think you should be able to fit a decent amp in a small case without making any major compromises. It's your project, but I don't see a reason to exclude it. The other thing is that you need either a) output caps or b) 4 amp channels (balanced) - if you include the amp, you can do a balanced solution, otherwise you probably need to include the caps since not many portable amps are balanced.

  Quote:


 How does USB power sound? I want to regulate it, but it would have to be cut down a bit. I vaguely recall a similar problem in the Alien DAC. But fear not, even in the tiny Hammond 1445C801(BK) we can fit some sort of DC-DC converter.

 Power supply solutions. The Fairchild Semi seems quasi-viable as it can supply up to Vin. I wonder what Team Alien DAC pored over before deciding on the REG101s.
http://focus.ti.com/lit/ds/symlink/tl2575-05.pdf
http://focus.ti.com/lit/ds/symlink/dcr011203.pdf
http://www.fairchildsemi.com/ds/FA/FAN6520A.pdf 
 

So you're planning on stealing power from the iPod, or are you talking about having it plugged to a computer all the time? I think you should include batteries for your DAC/amp myself, the iPod batteries are small and you'll drain the whole shebang quickly. A couple hours of use doesn't seem enough for anyone, and then you can't even use it unamped!

 You'll definitely need a DC-DC of some kind or you'll have no overhead whatsoever to drive your cans - or you have to use small and inappropriate 9Vs. I'd say 2AAs is about optimal - battery life should be similar to an iPod and it's small enough. You'll need quite a few regulators for an 'audiophile' approach though, I count a minimum of 4 (step up, linear for amp, linear for avdd, linear for dvdd), 5 would be better for battery life (extra dc-dc to 5.5V or so for the low voltage lines). Check out how dsavitsk did his HPDAC supply, you could probably poach most of that design, just respec it for 3V input.

 I'm really not sure why the AlienDAC chose the REG101 _in SO8 package_. The regulator itself is quite impressive; few match its specifications - but very few low noise regs are made in SO8 package so you cant' substitute anything! Plus it's a larger package than the alternative SOT23 that has lots of similar regulators with the same pinout. Neither has great availability right now though, so I wouldn't recommend them for a new project since TI can't seem to keep them on the shelves. There are tons of good switching chips out there, but I'd probably use the TPS61040 dsavitsk chose. I would probably use LP2985s for the Vdds. Not sure what would be good for the amp, would have to do some looking.

 That Fairchild device is a switching step-down converter - not sure why you'd need one of those - you should probably be using linear regulators for all your final regulation, and you need a boost converter from the batteries. Everything you listed is also physically rather large. Look for regulators in SOT23 or SO8 - you don't need to handle a lot of power dissipation, and you need as much space as you can get. Since you've got the DAC to deal with, no point in trying to do a non-SMD board or anything 
	

	
	
		
		

		
		
	


	




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## joneeboi

After doing a bit of looking around, TPS61040 looks good for DC boosting, 3.3V regulation can be done by the in-stock-at-Digikey Diodes Inc. AP131, with the LM7805 covering the 5V bit. I'm thinking of powering the DAC with USB power, and if possible we can stuff the thing in the Hammond 1445C801. That way it would operate like the Alien DAC, where we could stack the iPod on the DAC. Further, unlike the HPDAC, we could choose our own amplifiers. Even with this theoretical design, we could make a portable and full-on version. But before we go on, I need to do some more learning.

 =T


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## 00940

We can probably explain the reg101 thing as an heritage of the "Guzzler USB DAC" on which the alien dac is originally based. I'm pretty much responsible for it 
	

	
	
		
		

		
		
	


	




 When we (Guzzler and I) designed a battery powered USB dac in February 2004 (!), the reg101 was readily available from digikey. When I started the layout, I was a real newbie when it came to smd parts. The reg101 was available, easy to solder and probably the "best" integrated LDO regulator out there. Why not pick it ? 

 When Guzzler later designed a small usb-to-spdif converter, the reg101 was considered too expensive and was also getting harder to source. He then switched to the TI TPS793* ldo regulators. Those are still available by the thousands at digikey. You need to get the adjustable version to go up to 5V though.

 The so8 reg101 was still the regulator we picked for our tentative "all-out" transportable USB dac with Ble0t.


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## dsavitsk

Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_Check out how dsavitsk did his HPDAC supply, you could probably poach most of that design, just respec it for 3V input._

 

I believe that it is happy with input DC as low as 1.8V -- look at the datasheet though.

  Quote:


 There are tons of good switching chips out there, but I'd probably use the TPS61040 dsavitsk chose. 
 

I don't know that it is anything special, other than being cheap and easy to find. I does need that ferrite after it as otherwise it is pretty noisy. But, I found it by calling TI. If you get the right person on the phone, they are pretty helpful.


  Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Further, unlike the HPDAC, we could choose our own amplifiers._

 

Why couldn't you do that there? You were welcome to choose anything you wanted, it is just that most of those choices, other than the LM6172, were poor ones. 
	

	
	
		
		

		
		
	


	




  Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_The so8 reg101 was still the regulator we picked for our tentative "all-out" transportable USB dac with Ble0t._

 

What ever became of that?


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## FS2

How about volume control? The I2S data isn't affected by the volume setting from the scrollwheel. It'll either be a matter of using the DACs on-board volume control using a microcontroller, or doing it on the analog output. Unless there are DACs that allow easy hardware control of the volume settings.

 Powerwise, I was looking at an inverting switcher to generate a negative voltage for the op-amp. Any advantage to simply using a rail splitter and a higher boosted positive rail?

 And a general question for you guys who have better ears than I do: Is there any difference in the "stock" iPod's sound when charging it?


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## error401

Quote:


  Originally Posted by *FS2* /img/forum/go_quote.gif 
_Powerwise, I was looking at an inverting switcher to generate a negative voltage for the op-amp. Any advantage to simply using a rail splitter and a higher boosted positive rail?_

 

I don't think that's necessary if you do a balanced configuration. There will be DC offset on each output node, but the net offset will always be 0V (or very close to it). So since the DAC operates from 0-5V, your opamps can be single ended as well.


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## FS2

I thought it was... If, say, L+ and L- both have the same DC offset, and one is applied to the non-inverting op-amp terminal and the other to the inverting, the DC offset cancels out. Unless I've missed something...


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## error401

Quote:


  Originally Posted by *FS2* /img/forum/go_quote.gif 
_I thought it was... If, say, L+ and L- both have the same DC offset, and one is applied to the non-inverting op-amp terminal and the other to the inverting, the DC offset cancels out. Unless I've missed something...
	

	
	
		
		

		
		
	


	


_

 

I'm talking about going balanced all the way to the headphones. In essence you have two amplified lines that are 180 deg out of phase for each side, instead of a single one and ground. The DAC itself provides these inverted outputs, and you simply need to use four opamps instead of two for your output stage.

 To make things a bit more clear:

 The DAC can only output a voltage between 0-AVdd. Since this single ended signal needs to represent an audio signal, it needs to be biased to exactly 0.5AVdd, which will be the output voltage of both the + and - outputs when outputting silence. If you follow both + and - with a non-inverting amplifier, they will both output Gain x 0.5AVdd - exactly the same voltage. Each output goes to the + or - input of a headphone driver. Since there's no ground reference fed to the headphones, the ground-referenced voltage is meaningless at the headphones - only the difference matters. They see 0V since both + and - are exactly equal.

 Problem is that normal headphone connectors don't allow this (it requires 4 wires instead of the usual 3), and the convention is to use XLR connectors which are rather massive. You could rewire your headphones yourself and use a less insane connector like a mini-DIN or 4-pin 3.5mm.

 Alternative options are to only use DC blocking caps with only one DAC output, or a proper balanced to SE converter (conventional designs usually need 3 opamps/channel). Or a DC servo I guess, but the converter usually makes more sense.


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## FS2

Right, I'd thought that you were outputing to a common 3 terminal headphone, not going balanced all the way.

 Been going over the design I have, trying to flesh it out. It'll need a DAC, micro for volume control, 6 op-amps for the output, a +/- voltage for the op-amps and a lower +ve voltage for the DAC.


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## joneeboi

I just ran through the Opus DAC thread, and I like the jumpering idea. My great idea is to use tiny switches instead, but progress is super slow. I was looking at the "Recommended External Components" section in the WM8740 datasheet and I wonder if we could open a discussion on the output stage in between the chip and the amplifier. Trying to fit this bad boy into the 1455C8 really limits coupling caps, as film and tants I've seen are usually well over 100V. If we slip the DAC board in the bottom slot, we can fit those giant caps lengthwise above the board. That still limits cap choices, but it still accommodates most popular boutique caps. We could also leave the option of opamp balanced-to-SE DC-eliminating output onboard too. Was there another option? DC servo, IIRC. I actually don't understand what that means, so maybe you guys can enlighten me.


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## error401

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_I just ran through the Opus DAC thread, and I like the jumpering idea. My great idea is to use tiny switches instead, but progress is super slow. I was looking at the "Recommended External Components" section in the WM8740 datasheet and I wonder if we could open a discussion on the output stage in between the chip and the amplifier. Trying to fit this bad boy into the 1455C8 really limits coupling caps, as film and tants I've seen are usually well over 100V. If we slip the DAC board in the bottom slot, we can fit those giant caps lengthwise above the board. That still limits cap choices, but it still accommodates most popular boutique caps. We could also leave the option of opamp balanced-to-SE DC-eliminating output onboard too. Was there another option? DC servo, IIRC. I actually don't understand what that means, so maybe you guys can enlighten me._

 

A DC servo is basically a circuit connected with a very slow low-pass filter. It ignores all the audio data and essentially only reacts to DC. It is configured to 'want' 0VDC into the output. Properly connected so it can control the output circuit, it will bias the output to 0V, thanks to the slow low pass, even if there is an intrinsic offset. If you wanted to use one, I believe you'd need two opamps: the servo and an output stage after the DAC. Since you can't inject feedback into the DAC, I don't believe you could connect a servo directly to the output (though I might be wrong...).

 If you do bal->SE with a separate output stage, you need 6 opamps for the bal->SE and another 2 (or some other type of amp) for output. If you can fit it, that's what I would do. Use low-Iq opamps as much as possible and you can probably keep the Iq fairly low. It's going to hurt battery life though. You'll also need a negative-side DC-DC as well I believe, if you want to avoid coupling caps. You *might* get away without one by creating a buffered 1/2Vpos and using it as ground for the DAC IC. Not sure if it'd work though, I'd have to think about it more than I have.

 Using one side of the DAC and amplifying it in the conventional manner only requires 2 opamps, so uses less space and power. If you have a battery life budget you might be able to use 'nicer' opamps as a result. But you need coupling caps, and boutiques aren't going to fit. If you're one of those that believe that boutique caps make a difference, that might be a showstopper.


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## joneeboi

Thanks for the help. It sounds like we might have to copy Twisted Pear more than I'd prefer. IIRC, they have the option for all three, right? I'm thinking of regulating USB and forgoing the whole battery mess.

 With the boutique caps issue, I understand you can block DC with any appropriately spec'ed cap; the boutique bit would only be useful for preventing the use of poorer spec caps. Their sound difference is doubtless overemphasized and almost deified/idolized in some sense, and I can understand wanting to shy away from them. Nevertheless, I'd still argue they make a difference when placed directly in the signal path just like switching in lower-quality parts, audible or not. Is there something I'm missing? I'm sure the layout can be implemented such that their inclusion doesn't interfere with conventional non-boutique options.

 I merely suggested the boutique option for those who want a simpler output stage as opposed to using six (expensive?) surface mount opamps. One lead to the positive leg, other lead to output jack. Six opamps in that casing along with switches/jumpers for hardware control on that tiny board is going to be tricky for this EAGLE-newbie.


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## error401

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Thanks for the help. It sounds like we might have to copy Twisted Pear more than I'd prefer. IIRC, they have the option for all three, right? I'm thinking of regulating USB and forgoing the whole battery mess._

 

That makes things easier. You should still watch your current consumption though, the iPod will suck some and really you shouldn't draw more than 500mA from USB. Since you'll need a boost converter, 50mA on a dual-opamp probably converts to 100mA or more on the USB supply.

 If you're willing to make this non-portable (by depending on USB), why not use a higher voltage external supply. Then you don't need the onboard switcher and don't have to worry so much about current.

  Quote:


 With the boutique caps issue, I understand you can block DC with any appropriately spec'ed cap; the boutique bit would only be useful for preventing the use of poorer spec caps. Their sound difference is doubtless overemphasized and almost deified/idolized in some sense, and I can understand wanting to shy away from them. Nevertheless, I'd still argue they make a difference when placed directly in the signal path just like switching in lower-quality parts, audible or not. Is there something I'm missing? I'm sure the layout can be implemented such that their inclusion doesn't interfere with conventional non-boutique options. 
 

Well, no reason to say 'NO BOUTIQUE CAPS ALLOWED', but it's not going to be easy to do a layout to accommodate them, since they're so long. I think you're pretty much going to have to put pads for a small poly film cap and if you want to use boutiques, you'll need to airwire them. Twisted pear uses OSCONs for output; not exactly boutique 
	

	
	
		
		

		
		
	


	




.

  Quote:


 I merely suggested the boutique option for those who want a simpler output stage as opposed to using six (expensive?) surface mount opamps. One lead to the positive leg, other lead to output jack. Six opamps in that casing along with switches/jumpers for hardware control on that tiny board is going to be tricky for this EAGLE-newbie. 
 

Good opamps don't have to be expensive. OPA134 is fairly cheap, and sounds good. There are lots of others out there that aren't prohibitively expensive, but sound good. And you don't need 6 parts, you need 6 amps. You could use a quad amp in SO-14 and a dual amp in SO-8 for all the amps you need. Just two packages to route 
	

	
	
		
		

		
		
	


	




. If you go this route you'll probably still need a separate output stage though, so volume control can be done.

 I can't really think of an easy way to switch between balanced and SE output stages. I guess it could be done by substituting some resistors around the positive side of the bal->SE and taking the output directly. How are you going to do volume control here? Maybe you can come up with something, but with the space considerations, it's probably better to just choose an output topology and go with it.


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## joneeboi

For the boutiques, I was thinking of airwiring them since they make everyone's life so difficult and deserve it as punishment. =/ Making board room for them would be a huge no-no, surely.

 And by going with USB power, it makes the project a bit more portable. If I went with the external power supply, I'd pretty much be redesigning the Opus. My proposed design would somewhat be a niche product, I suppose, as I don't imagine most people wanting to grab the power from USB and going with the digital source of the iPod over ASIO over USB. I guess that really defeats the purpose of portable. Shoot. I guess we could go with something like a 1455J12 and throw in some batteries with leftover room. Is that portable? Transportable. Going with an integrated amp seems to be the smartest thing, as nobody really wants to carry three things around like that. PPAS? I was hoping to control the volume on the external amp side of things, but if we throw it all in one case, it would still be viable with the PPAS. Mini^3? 3ch CMoy? We'd have to keep things high quality in the small footprint to make good use of the high quality DAC.

 It feels like I'm trying to force something that's not going to fit (easily). Can someone talk some sense into me here? Should I just go with a USB-powered Opus? Is that even an option?


----------



## FS2

Um, USB power? Thought this was meant to be a portable amp that took I2S from the iPod? But if it's a desktop amp, is there any reason to limit it to USB power instead of a wall wart? I wouldn't have thought USB power was that clean..

  Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_I can't really think of an easy way to switch between balanced and SE output stages. I guess it could be done by substituting some resistors around the positive side of the bal->SE and taking the output directly. How are you going to do volume control here? Maybe you can come up with something, but with the space considerations, it's probably better to just choose an output topology and go with it._

 

Tap off the balanced output from the I/V converter op-amps before they go to the balanced to single ended converter?


----------



## error401

Quote:


  Originally Posted by *FS2* /img/forum/go_quote.gif 
_Um, USB power? Thought this was meant to be a portable amp that took I2S from the iPod? But if it's a desktop amp, is there any reason to limit it to USB power instead of a wall wart? I wouldn't have thought USB power was that clean.._

 

If you use USB power you need to use a switcher onboard anyway. IME USB power isn't all that bad anyway. But yea, I don't really get that either.

  Quote:


 Tap off the balanced output from the I/V converter op-amps before they go to the balanced to single ended converter? 
 

It's a voltage-out DAC so there is no I/V, I'm just thinking that routing the board so that you can easily pass either a balanced or unbalanced signal to a balanced or unbalanced (onboard) amp might be difficult. I haven't tried though...


----------



## FS2

I was wondering about that. I've only skimmed through the datasheet and didn't see it explicitly stated whether it was voltage or current out.

 Powerwise, I was thinking I'd need a switcher anyhow, since I'd be obtaining a boosted and a negative voltage of my batteries. I'll, have to look at filtering for that.


----------



## joneeboi

I was thinking USB power because I was planning on making this my transportable, laptop, doing homework at school rig, though I can see the redundancy in such a design. Having an external DAC/amp for iPods would mean double battery charging, less space on board for whatever, simpler circuits due to space restriction unless we're really good. Perhaps there can be a charging circuit that will charge both the DAC's and iPod's batteries. The iPod dock connector would have to be either hardwired into the DAC/amp casing or use something like a D-Sub connector. I fear the project will inevitably become some sort of portable Opus, basically redesigned to fit into a Hammond or other case. I can live with that, I guess. Battery powered DAC...


----------



## error401

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_I was thinking USB power because I was planning on making this my transportable, laptop, doing homework at school rig, though I can see the redundancy in such a design._

 

Design what you need, not what you think others will need (unless you're trying to create a salable product). Do that first, and if you can, make it flexible so others can use the design. Don't concede your needs because of our comments 
	

	
	
		
		

		
			





. It's your project, do it the way that's best for your needs.

  Quote:


 Having an external DAC/amp for iPods would mean double battery charging, less space on board for whatever, simpler circuits due to space restriction unless we're really good. 
 

If you get creative with routing I think it can probably be done without much compromise. Put the battery pack and any tall parts ('lytic caps, i/o connectors) on one side of the board and the majority of the stuff on the other side.
  Quote:


 Perhaps there can be a charging circuit that will charge both the DAC's and iPod's batteries. 
 

Wouldn't be too hard. You can just pass the USB power lines directly to the iPod. Fitting a charger in the DAC might be possible, or it might not.
  Quote:


 The iPod dock connector would have to be either hardwired into the DAC/amp casing or use something like a D-Sub connector. 
 

I'd probably use mini-DIN or 1394 (or even HDMI or eSata). You get 6 conductors with each and a much nicer and smaller connector than D-Sub.

  Quote:


 I fear the project will inevitably become some sort of portable Opus, basically redesigned to fit into a Hammond or other case. I can live with that, I guess. Battery powered DAC... 
 

Opus is basically a datasheet implementation of the DAC, it doesn't do anything else. Ending up with a similar circuit isn't a bad thing. Besides, you're adding the bal->SE stage and an amplifier onboard, there's still room for something new.


----------



## joneeboi

Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_Design what you need, not what you think others will need (unless you're trying to create a salable product). Do that first, and if you can, make it flexible so others can use the design. Don't concede your needs because of our comments 
	

	
	
		
		

		
		
	


	




. It's your project, do it the way that's best for your needs.

 Opus is basically a datasheet implementation of the DAC, it doesn't do anything else. Ending up with a similar circuit isn't a bad thing. Besides, you're adding the bal->SE stage and an amplifier onboard, there's still room for something new._

 

I needed to hear that. I didn't realize what I was doing till I thought about it today. *shakes head*

 Okay, onto MY needs. 
	

	
	
		
		

		
		
	


	




 Thinking out loud here, USB power, boosted and regulated for linear power, iPod charging ftw, PPAS if possible or SMD PIMETA since I've recently fallen in love with the AD8610/AD8620 combo I have. I talked to my professor just now and he referred me to one of his employees who works at the University and is very busy, but I'm hoping he can give me some of his expert time. How so very exciting. BUF634s are out of stock forever, so I'll probably go with one of Sijosae's simpler discrete buffers. I'm thinking top side for amp, bottom side for DAC. Slip the board into the bottom slot of the 1455C8, making some kind of room for the PIMETA's power 220uF/470uF 16V rails. Maybe I'll get two boosters, one for 7VDC on the DAC side and one for 12VDC for the amp section. I may be pushing the USB's current capabilities here. Maybe I can shoot for a 9V compromise to power both sections, though heat may become an issue.

 What have we got here? 3.3VDC for DVDD, 5VDC for AVDD, 9VDC for amp section, 9VDC for iPod charging. Yikes. If the WM8740's DVDD is 5V instead of 3.3V, its output current will be a third more, though it would only sit at 19mA. Looking at the TPS793* line, input is limited to 6V and output is max 4.75V. REG101 looks like our candidate, and it seems it's in stock but only in SO-8 and not SOT-23. One can safely input up to 12V, so that works out. Summing things up, REG101 for 5V, TPS61040 for boosting DC to 9V, and I'm unsure for 3.3V regulation. Using Digikey's fine search function, a whole bunch are suitable for the application, but sticking with the brand names, I clicked on Fairchild Semi's FAN2500. From TI, we have both the TPS76033 and TPS76033 at 50mA. We have a total power consumption of 32mA max from the Wolfson, so 50mA should be enough I think. Is there any preference for any of the above regulators or am I just being absurd?

 More head-scratching to ensue.


----------



## 00940

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Maybe I'll get two boosters, one for 7VDC on the DAC side_

 

Why 7V ? You're wasting power in linear regulation. 5.5V is more than enough with modern LDOs. You could even not use a booster for the DAC, as the WM8740 will very happily run with both supplies at 3.3V (and it eats less power that way too). 

  Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Looking at the TPS793* line, input is limited to 6V and output is max 4.75V._

 

Wrong I'm affraid... max output is 5.5V (variable output) TPS79301DBVR. And even if you insist to use the wm8740 with 5V supplies, 4.75V is close enough. Noone forces you to use either 3.3V or 5V. It must be in between those two values and the analog supply equal or higher than the digital. That's it.


----------



## Jambo

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Is there any preference for any of the above regulators or am I just being absurd?

 More head-scratching to ensue._

 

I haven't looked at the ones that you suggested, but something you should consider is staying away from LDO regulators if you can. Because of the way their feedback works their stability depends on the load and you need to be careful what you load them with (output caps need to have their ESR within a given range, lower is not always best). Not getting at anything in particular here, just food for thought.


----------



## joneeboi

Quote:


  Originally Posted by *00940* /img/forum/go_quote.gif 
_Why 7V ? You're wasting power in linear regulation. 5.5V is more than enough with modern LDOs. You could even not use a booster for the DAC, as the WM8740 will very happily run with both supplies at 3.3V (and it eats less power that way too)._

 

The DC boosting would be for the amp section, though your point is well taken. I'm just assuming that as input voltage increases for the DAC, the better the efficiency and performance. Does my assumption correspond to reality? Then I could just run both supplies off of a REG101's 4.75V output, could I not? It'll run a little hotter and draw more power, but if the USB can take it, I'm down. If it doesn't improve performance in any significant way, going all the way down to 3.3V is cool too. Thanks for that.

 Jambo:

 If I followed the datasheet recommendations, wouldn't that make your point moot? I don't know, I'm just wondering. TI seem to have compensated for that in the REG101 because they say in the first couple paragraphs of the datasheet that an output cap is unnecessary for stability.


----------



## Jambo

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_Jambo:

 If I followed the datasheet recommendations, wouldn't that make your point moot? I don't know, I'm just wondering. TI seem to have compensated for that in the REG101 because they say in the first couple paragraphs of the datasheet that an output cap is unnecessary for stability._

 

Absolutely... But I don't read every word of datasheets and I don't know anyone who does :d 

 While an output cap may not be necessary for stability it still helps for load regulation and an unwise choice can *cause* instability.


----------



## error401

Try your layout only on the topside first. It might not be that hard to route since there are few signals that cross stages and their path is very linear. I think you can probably fit it all on one side.

 For analog, especially if you're driving low impedance phones, I'd keep the voltage relatively low. This will make it easier to ensure that the necessary transient current is available on a small board. 9V seems a good compromise to me. So I'd boost to about 10V and use a good linear reg to get 9V. A ~4.75V linear reg for AVDD (directly from USB power) and 3.3V for DVDD.

 Use REG101 for both analog supplies if you can get it. I've had a hard time finding anything with similar specs. LP2985 is about as good, but it's fixed voltage only. If you really wanted to use a 'quality' reg for the digital supply you could use one. Otherwise I'd choose the TI from the ones you linked, it's specs are a lot better and it's not much pricier. Plus I like TI more than Fairchild 

 Read the datasheets carefully. Jambo is right, LDO regs can be finicky, so make sure you're loading them properly. If they recommend particular part numbers for capacitors, use those parts.

 Since the Wolfson is spec'd only at 5V, I'd hazard a guess that performance does degrade with AVDD at <5V. Whether it's worth wasting the current regulating your high voltage rail down to 5V is up to you. Personally I'd probably go with the 4.8V solution.


----------



## 00940

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_The DC boosting would be for the amp section, though your point is well taken. I'm just assuming that as input voltage increases for the DAC, the better the efficiency and performance. Does my assumption correspond to reality? Then I could just run both supplies off of a REG101's 4.75V output, could I not? It'll run a little hotter and draw more power, but if the USB can take it, I'm down. If it doesn't improve performance in any significant way, going all the way down to 3.3V is cool too. Thanks for that._

 

According to the datasheet the performance indeed increases with voltage but efficiency decreases. At 5V (both AVDD and DVDD), you have a 32ma current draw. At 3.3V, a 24ma draw. It's worth noting that the digital section is accounting for most of that change. So, having a first regulator at 3.3V for the DVDD and a second at 4.75V for the AVDD seems the best compromise between efficiency and performance. And it can run directly from USB power. I thus echo error401 on this.

 I don't think one should do without LDO in such a project. The 2 or 3V of headroom a conventionnal regulator needs are a luxury. Just follow the layout and part indications of the datasheet. To avoid problems with the reg101, it is often good practice to have after the regulator: a "not too low esr" electrolytic cap (like 10uF), followed by a ferrite and a ceramic cap (10-100nF) right at the supply pin of the powered device. 

 To layout your digital section (the whole wm8740), as well as the associated regulators, the best way to proceed on a two sided board is to have your chips sitting on a groundplane, with all (if possible) the signal traces running on the other side. It usually provides for the shortest ground loops. For the amp, scrap the groundplane and go for star ground.

 Good luck 
	

	
	
		
		

		
		
	


	





 edit: and don't forget the thermal relief option on the groundplane.


----------



## WindowsX

ok guys, how could iMod get better than squeezebox, transporter or olive music station? could something like this really worths a shot? squeezebox3 is around the same price of iPod.


----------



## FallenAngel

Quote:


  Originally Posted by *WindowsX* /img/forum/go_quote.gif 
_ok guys, how could iMod get better than squeezebox, transporter or olive music station? could something like this really worths a shot? squeezebox3 is around the same price of iPod._

 

The Squeezebox uses the BB PCM1748, iPod/iMod (best case scenario - 4G iPod Photo) uses the WM8975. Just compare the specs. It looks like the Squeezebos DAC performs better than the iMod one, but that's DAC specs alone. Best tests would be to actually listen to both and see which one sounds better using the same files. 
	

	
	
		
		

		
		
	


	




 There's an idea, how about listening 
	

	
	
		
		

		
		
	


	




 Not trying to be a jerk, but way too many times people recommend things they never heard themselves that it's just a good idea to get your hands on both of the things you're trying to compare and have a listen yourself.


----------



## WindowsX

No offense but I own these sources.....
 -iMod (G3 DIY, Photo for both DIY and RWA)
 -Heavily modded Linn Genki (can beat up Ayre CX-7e's ass)
 -DAC1

 I have listened to.....
 -cheap $400 Sony CD player
 -squeezebox v3 (no mods)

 Too bad, iMod couldn't beat up anything I have tried.....maybe just for my impressions.


----------



## error401

Quote:


  Originally Posted by *WindowsX* /img/forum/go_quote.gif 
_No offense but I own these sources.....
 -iMod (G3 DIY, Photo for both DIY and RWA)
 -Heavily modded Linn Genki (can beat up Ayre CX-7e's ass)
 -DAC1

 I have listened to.....
 -cheap $400 Sony CD player
 -squeezebox v3 (no mods)

 Too bad, iMod couldn't beat up anything I have tried.....maybe just for my impressions._

 

We're talking about a digital out so you can use whatever DAC implementation you want. Not sure why all this talk about Squeezebox vs. iMod is relevant...


----------



## joneeboi

While lying in bed and enjoying some good tunes, I realized that the Pico is pretty much what I'm trying to design. Portable DAC/amp, rechargeable battery, tiny, great chips that presumably give good sound. It's a pricey unit, but it's pretty much what I was shooting for. The only difference is that there is no portable iPod external DAC feature; the DAC is only for USB. Nevertheless, I'm sure some minor drilling or a one-off request would enable that, and all my problems would be solved. It's been fun trying to design this DAC/amp, but it seems that the niche has already been filled. I don't plan on buying a Pico anytime soon, but it seems it's on my horizons. Maybe I'll send Justin a quick email after he's done all the Pico pre-orders.


----------



## rippingdragon

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_While lying in bed and enjoying some good tunes, I realized that the Pico is pretty much what I'm trying to design. Portable DAC/amp, rechargeable battery, tiny, great chips that presumably give good sound. It's a pricey unit, but it's pretty much what I was shooting for. The only difference is that there is no portable iPod external DAC feature; the DAC is only for USB. Nevertheless, I'm sure some minor drilling or a one-off request would enable that, and all my problems would be solved. It's been fun trying to design this DAC/amp, but it seems that the niche has already been filled. I don't plan on buying a Pico anytime soon, but it seems it's on my horizons. Maybe I'll send Justin a quick email after he's done all the Pico pre-orders._

 

Or alternatively, is there a solution for all Pico, Predator, and other USB DAC owners out there to somehow extract the digital signal from an iPod (or other player) to deliver to the DAC in USB? I've got my fingers crossed that one of you brilliant DIY guys can come up with some custom soldering/cable job that will make this a possibility

 P.S. I would get on the list now for the Pico (wait time is about 10 weeks, and payment isn't due until shortly before shipping, plenty of time to save up).


----------



## joneeboi

The fun way of doing this is to open up whichever iPod you have, identify the Wolfson DAC you have, steal the I2S signal from those pins and send them to unused pins on the dock, make a dock connector to steal those signals, open up your Pico and solder to the I2S input of the Wolfson WM8740. That's a pretty tall order cause you could destroy both your iPod and Pico in one afternoon. Either that or you could pay MSB for their iPod transport. Non-video iPods have more unused pins than video-enabled iPods, but I think you could still work something out with the existing pins. Check out Apple iPod dock interface pinout and signals @ pinouts.ru.


----------



## rippingdragon

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_The fun way of doing this is to open up whichever iPod you have, identify the Wolfson DAC you have, steal the I2S signal from those pins and send them to unused pins on the dock, make a dock connector to steal those signals, open up your Pico and solder to the I2S input of the Wolfson WM8740. That's a pretty tall order cause you could destroy both your iPod and Pico in one afternoon. Either that or you could pay MSB for their iPod transport. Non-video iPods have more unused pins than video-enabled iPods, but I think you could still work something out with the existing pins. Check out Apple iPod dock interface pinout and signals @ pinouts.ru._

 

While difficult, it sounds like we're approaching the realms of possibility. Would this modification require the cable to be permanently affixed to the Pico? Would it disable the USB input? I plan on using mine both for portable and work (off of a laptop and would therefore still need the USB input to function) applications.

 Is there anyway to get a similar mod to work without modding the Pico?


----------



## FallenAngel

Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_While difficult, it sounds like we're approaching the realms of possibility._

 

Everything is possible, it's a matter of risk vs outcome.

  Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_Would this modification require the cable to be permanently affixed to the Pico?_

 

I would assume that you could make a jack on the Pico (if you find any room in there to use this), you generally need 4 wires for I2S, so you'll seriously need to consider your options. Perhaps having a small female connector _beside_ the Pico with wires going in it will be most practical as I doubt you can find a 4-contact jack small enough to fit inside the Pico.

 Then it will only be a matter of building an iPod connector with a matching plug.

  Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_Would it disable the USB input? I plan on using mine both for portable and work (off of a laptop and would therefore still need the USB input to function) applications._

 

Of course not, USB does not come into the picture at all, you just won't be able to use it at the same time.

  Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_Is there anyway to get a similar mod to work without modding the Pico?_

 

No


----------



## joneeboi

I would probably just hardwire the iPod connector into the Pico to save on the connector. It seems crazy to think about soldering straight to the pins of the dock, but I did it for another diyMod just the other day and it wasn't so bad with just two strands of SPC. Practice makes perfect. Anyway, you could probably remove the input connector for the Pico's amp portion and rewire things as needed. Throw the dock connector wire through there, send the signal to the DAC, let the amp do the rest.


----------



## error401

If you wanted to use a connector, there are lots of pretty tiny connectors. You could start with mini USB type B, they're pretty tiny. You'll need 4 conductors plus ground, which should be fine on the shield.


----------



## rippingdragon

Quote:


  Originally Posted by *joneeboi* /img/forum/go_quote.gif 
_I would probably just hardwire the iPod connector into the Pico to save on the connector. It seems crazy to think about soldering straight to the pins of the dock, but I did it for another diyMod just the other day and it wasn't so bad with just two strands of SPC. Practice makes perfect. Anyway, you could probably remove the input connector for the Pico's amp portion and rewire things as needed. Throw the dock connector wire through there, send the signal to the DAC, let the amp do the rest._

 

If anyone gives this a go (either hardwired or with a new connector), could they please post a guide with pictures. It would be most helpful to the less competent (i.e. myself)

 Cheers!


----------



## FallenAngel

Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_If anyone gives this a go (either hardwired or with a new connector), could they please post a guide with pictures. It would be most helpful to the less competent (i.e. myself)

 Cheers!_

 

Considering this involves soldering 4 wires to a the pins of a SSOP 28 chip (it's less than 1cm square), it's generally not recommended for beginners.


----------



## rippingdragon

To clarify one of my previous questions:
 Why isn't it possible to use the USB port as the connection if you perform the appropriate mod to the iPod? Is this port looking for a different signal form/format?


----------



## error401

Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_To clarify one of my previous questions:
 Why isn't it possible to use the USB port as the connection if you perform the appropriate mod to the iPod? Is this port looking for a different signal form/format?_

 

Yes. First of all, USB has 'host' and 'peripheral' devices. The host is responsible for mediating the bus and there must be exactly one on each bus (it's also responsible for providing power). Peripherals are attached to the host, and provide services to it. Peripherals can't communicate with each other, only host<->peripheral communication is allowed. Both the iPod and a USB sound device are USB peripherals. Connecting them to each other will do nothing since there isn't a host on the bus, and no way for them to communicate with each other.

 USB On The Go (OTG) changes this a bit so that it's possible for peripherals to sort of 'be both' and talk to each other directly. Unfortunately the iPod doesn't support USB OTG, and there's not really any reason for Apple to provide support, so that's a no-go too.

 Furthermore, the iPod presents itself on USB as a Storage Class device, which is simple block-based storage like a hard drive. There isn't any way to get or send the iPod real-time audio data over USB.

 It's probably possible for a USB host stack to be written for RockBox or iPod Linux (the PortalPlayer CPU in the iPod has a USB port that can be either a host or device) that could potentially allow connection of a USB audio class device for audio playback. You're welcome to try - but tapping the I2S lines is sure to be much simpler. This would be pretty cool though.


----------



## FallenAngel

Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_To clarify one of my previous questions:
 Why isn't it possible to use the USB port as the connection if you perform the appropriate mod to the iPod? Is this port looking for a different signal form/format?_

 

USB is USB (Master - Slave communication controlled by Master, which the iPod is not), I2S is I2S (simple audio digital transfer protocol, controlled by lots of chips - USB receivers, SPDIF receivers and of course DACs), no relation between the two.


----------



## joneeboi

Thanks for the short lesson in USB, error401. 
	

	
	
		
		

		
		
	


	




 I wonder about the space constraints involved with putting another mini-USB jack, because AFAIU there is already a mini-USB jack for the USB DAC.

 That'd be quite a job getting the wires into that kind of space. *whistle*


----------



## rippingdragon

Quote:


  Originally Posted by *error401* /img/forum/go_quote.gif 
_Yes. First of all, USB has 'host' and 'peripheral' devices. The host is responsible for mediating the bus and there must be exactly one on each bus (it's also responsible for providing power). 

 <Snip>

 It's probably possible for a USB host stack to be written for RockBox or iPod Linux (the PortalPlayer CPU in the iPod has a USB port that can be either a host or device) that could potentially allow connection of a USB audio class device for audio playback. You're welcome to try - but tapping the I2S lines is sure to be much simpler. This would be pretty cool though._

 

Thanks for all of the info. That was massively helpful and what I was looking for from the start. 
 Two things I did pick up from that are:

 1) Re: USB power - I think I found a flaw in the IS2 direct connection approach. The Pico DAC is powered by USB and not by the included battery (this powers the amp section only). Therefore to use the DAC you would need to have the Pico also plugged into a powered USB source/host. Other then cramping the portability, this may also impact functionality (unless it would still work with no audio signal being received from the USB host, just the power).

 2) If it is possible to write a hoststack for Rockbox this does sound like the preferable option. I don't think it would be any "harder", it would just require different skills. The advantage to this would also be that is infinitely replicable with minimal effort on all users part (all they have to do is install the new firmware). Also re: power, the Pico would then be drawing power from your iPod and as such this would reduce battery times.

 In conclusion I would love for someone to investigate the option of performing a firmware modification that would allow this kind of interaction


----------



## rippingdragon

This thread also happens to coincide with the annoucement of the iPhone SDK. Perhaps an application could also be developed to enable this function on the iPhone


----------



## joneeboi

Quote:


  Originally Posted by *rippingdragon* /img/forum/go_quote.gif 
_1) Re: USB power - I think I found a flaw in the IS2 direct connection approach. The Pico DAC is powered by USB and not by the included battery (this powers the amp section only). Therefore to use the DAC you would need to have the Pico also plugged into a powered USB source/host. Other then cramping the portability, this may also impact functionality (unless it would still work with no audio signal being received from the USB host, just the power)._

 

For that, you would just make a USB cord that doesn't connect D+ and D-. After that, you'd have to wonder why portability is such a hassle. You'd have to wonder if the chips chosen for the iPod are all that bad and if it's worth the trouble replacing them. Looking at the project from the end result, it seems like it's not all that bad; wire this here and there, wire that there and here. Make your own USB cable, get on what seems to be some kind of donor transplant list to get a Headamp Pico (donor transplant list because it's long, not because Justin isn't doing a great job with what he has to deal with), wire up your iPod's silly small chip to more silly small pads. Are we forcing a cube into a circular hole?


----------



## giuliaudio1

Hello, do you know how I can get an I2S signal from an Ipod Touch ?


----------



## Ynis Avalach

Quote:


  Originally Posted by *giuliaudio1* /img/forum/go_quote.gif 
_Hello, do you know how I can get an I2S signal from an Ipod Touch ?_

 

 You have to open up your iPod, then find your dac chip and solder some wires from the input of your dac chip (I2S input of the dac chip) to some unused dock pins, that's it.


----------



## qusp

well then why not use a D10?? uses the same dac as the pico, but the dac is powered by the battery as well as the amp. so if its as simple as ^^ you say (which I guess it is in theory) then why dont you do it and show us how its done


----------



## Ynis Avalach

Quote:


  Originally Posted by *qusp* /img/forum/go_quote.gif 
_well then why not use a D10?? uses the same dac as the pico, but the dac is powered by the battery as well as the amp. so if its as simple as ^^ you say (which I guess it is in theory) then why dont you do it and show us how its done 
	

	
	
		
		

		
		
	


	


_

 

 Me? I haven't got a D10, nor have I got a D/A converter, well nothing standalone and I wouldn't want to risk my MDS-JB940. 
 But I just thought about my old portable MD, maybe I can figure out something tomorrow. I think though, that why should I be the one to achieve this, why hasn't someone else gotten this already, because my idea is soley based on theory. I'm definitly no pro, so trial and error
	

	
	
		
		

		
		
	


	



 Ava


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## FallenAngel

Makes me wonder why... but I guess people will always want to play with their damn iPods


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## Ynis Avalach

Guess so


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## Ynis Avalach

Now that I've found BCLK, DACLRC and DACDAT on my Mini 2G board I'll try to connect them to something (well some kind of dac which I still have to find, or I'll try to build a SPDIF out for my iPod). 
 Oh man how much fun...no it isn't! Damn this board is sooo filled with tiny components. Argghh


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## steven2992

Couldn't you wire the i2s to the dock in the ipod, and out through a dock cable to something like this: CS8406 connected to an optical transmitter. Kind of bantam cable dac style. A smal box with a dock cable on one side and an optical transmitter on the other.


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## boomy3555

I only touched on some of the posts here in this thread, but how do you control the play functions once you get the digital signal from the ipod to the device. AFAIK the digital pins only allow communication between the ipod and a device. It does not send out a digital signal during the playback process. In other words the ipod can only act as a hard drive. it can't play a song through it's digital pinouts When you press buttons and turn the touch wheel the Ipod plays songs through it's headphone out or analog pin outs. not trough it's digital out.

 I think that's why this thread was abandoned last year


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## qusp

no boomy, you are incorrect; what about the wadia then?? any device that reads PCM or digital audio (transport) has an i2s stream at some point along the way. directly before the dac would be one place to look a digital audio stream that travels along traces in a device is generally i2s; the problem is tapping that and keeping it intact; as i2s is very prone to interference and jitter. any signal that is able to be tapped is likely to need reclocking as well. its obviously doable, but just where in the signal path it is taken from is well guarded info. anyone got a wadia and the modded ipod willing to make a sacrifice for the greater good? ;p sounds easier than it is in reality, but it is most certainly doable, given the right equipment, skill and knowledge


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## boomy3555

Thanks.. I understand. I thought that people were trying to use the digital pinouts to get a signal. I understand how you can catch the signal before it gets to the DAC and re-routing it through a separate device such as the Wadia.


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## steven2992

look what i found: iLink Music Server 
 It looks a lot like what we are discussing here.


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## komi

Quote:


  Originally Posted by *steven2992* /img/forum/go_quote.gif 
_look what i found: iLink Music Server 
 It looks a lot like what we are discussing here._

 

Riiiiight !

 $2375 with iPod included
 add $199 (with upsampling)

 whats the point there ?


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## tbone57

Many posts here have referenced the Wadia Transport as a useful tool to retreive a digital signal from your IPOD.

 THERE IS A BETTER WAY!

 Thats not to say the Wadia isnt a great product. It does more than just supply a digital output from your IPOD.

 However, if you are like me and cautious when spending $$$ on audio, try this otpion.

 SendStation.com makes a product that I bot for $20, that now costs $30. This adapter plugs into the pure digital, bottom output of your IPOD. The adapter has two female jacks. One for the 3.5mm cable (same as the crappy headphone jack output) and another jack for USB.

 I bot a Kimber interconnect with a 3.5mm male connector on one end and dual gold plated RCA connectors on the other end. The result-I connect my IPOD dig out directly to my Cambridge Amp. The resolution is great.


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## m1abrams

Quote:


  Originally Posted by *tbone57* /img/forum/go_quote.gif 
_Many posts here have referenced the Wadia Transport as a useful tool to retreive a digital signal from your IPOD.

 THERE IS A BETTER WAY!

 Thats not to say the Wadia isnt a great product. It does more than just supply a digital output from your IPOD.

 However, if you are like me and cautious when spending $$$ on audio, try this otpion.

 SendStation.com makes a product that I bot for $20, that now costs $30. This adapter plugs into the pure digital, bottom output of your IPOD. The adapter has two female jacks. One for the 3.5mm cable (same as the crappy headphone jack output) and another jack for USB.

 I bot a Kimber interconnect with a 3.5mm male connector on one end and dual gold plated RCA connectors on the other end. The result-I connect my IPOD dig out directly to my Cambridge Amp. The resolution is great._

 

I am a little confused as to what you did to get digital out? What did you connect the Kimber interconnect to? If it was the 3.5mm jack on the SendStation then you did not get digital out from the ipod.


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## Lil' Knight

Quote:


  Originally Posted by *tbone57* /img/forum/go_quote.gif 
_
 SendStation.com makes a product that I bot for $20, that now costs $30. This adapter plugs into the pure digital, bottom output of your IPOD. The adapter has two female jacks. One for the 3.5mm cable (same as the crappy headphone jack output) and another jack for USB.

 I bot a Kimber interconnect with a 3.5mm male connector on one end and dual gold plated RCA connectors on the other end. The result-I connect my IPOD dig out directly to my Cambridge Amp. The resolution is great._

 

No idea what you're saying here.

 You are using an analog cable to take the digital out of your iPod?


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## FallenAngel

I think the poster was a bit confused and thought "digital out" was USB/WireFire, while the rest meant a streaming digital S/PDIF signal.


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## komi

Quote:


  Originally Posted by *tbone57* /img/forum/go_quote.gif 
_Many posts here have referenced the Wadia Transport as a useful tool to retreive a digital signal from your IPOD.

 THERE IS A BETTER WAY!

 Thats not to say the Wadia isnt a great product. It does more than just supply a digital output from your IPOD.

 However, if you are like me and cautious when spending $$$ on audio, try this otpion.

 SendStation.com makes a product that I bot for $20, that now costs $30. This adapter plugs into the pure digital, bottom output of your IPOD. The adapter has two female jacks. One for the 3.5mm cable (same as the crappy headphone jack output) and another jack for USB.

 I bot a Kimber interconnect with a 3.5mm male connector on one end and dual gold plated RCA connectors on the other end. The result-I connect my IPOD dig out directly to my Cambridge Amp. The resolution is great._

 

I got that thing too, and is nothing but what name says - POCKET DOCK [line out audio, sync and charge] ... Nothing digital in there ...


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## FallenAngel

Quote:


  Originally Posted by *komi* /img/forum/go_quote.gif 
_I got that thing too, and is nothing but what name says - POCKET DOCK [line out audio, sync and charge] ... Nothing digital in there ..._

 

Well... except the syncing


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## komi

Quote:


  Originally Posted by *FallenAngel* /img/forum/go_quote.gif 
_Well... except the syncing 
	

	
	
		
		

		
		
	


	


_

 

Actualy, syncing too ...


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## Billyk

Well, we should give him an A for enthusiasm. It's not spdif but the LOD is a step up! If he keeps it up he'll get there eventually.
 So welcome to Head-Fi and sorry about your wallet


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## qusp

anyhoo, this thread seems to be going around in circles. how about the cowon X5L? I have one on its way as well as all the parts for a PCM1794A version alex_Mod, if I can get the PCB from him, but even if I cant, the cowon's i2s is easily tapped, not a screw around like the ipod seems to be.


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