# PCM2704 USB DAC



## ble0t

Inspired by Alf and his recent efforts with the PCM2702 chip, I've taken a stab at a USB DAC that will be used with my computer (which I use as my transport quite a bit). I've designed it to provide a decent headphone section while giving me SPDIF out to run to my larger system as well. I have been using the SPDIF directly out of the computer thus far, but it does not use an output transformer and perhaps the TI chip might be better than the current CODEC. Also, I've decided not to step up the voltage (as in the Rev B PCM2702 DAC) due to the measurements Alf had taken. Instead, I decided to use a LDO linear regulator that has good high frequency PSRR and run the DAC in self-powered mode.

 Here's the schematic:







 Comments and suggestions please


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## Nisbeth

Absolutely fantastic idea! 
	

	
	
		
		

		
		
	


	





 /U.


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## ble0t

OK, here's the initial board layout as well. I've tried to include many of the optimizations that were included in the PCM2702 Rev B such as ferrites on all supply lines, single ground plane (with as few interuptions as possible), etc.

 It is designed to fit exactly into a Hammond 1455C801 enclosure (3.15 x 2.125 x 0.91) with the USB, BNC and 3.5mm all PCB mounted. The LED can be soldered in and the leads bent so as to mount 'into' the panel as well.






 Again, comments and suggestions please


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## halman

Here are something you can look at. Its sports a pcm2902 and it does not have any audio outputs but it should give you some layout suggestions.

 ...Works extremely well...

 It its made to fit the same case


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## dsavitsk

The resistors between the usb jack and the chip: if you make the 22R resistors larger, you can route the trace from the 1.5K down the middle of them which keeps it from blocking the other connections to the pcm2704 allowing those to move to the top.

 Similarly, make the cap between the poles of the crystal larger, and route the trace from the "digi-trans" down the middle so the traces to the crystal can come up to the top.

 Actually, it seems that the ground plane is the top? make it the bottom and I think you can have no cuts in it other than the vias.


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## saab

I remember reading that 90 degree angles are not desireable. You might consider making them all two 45 degree angles.


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## ble0t

dsavitsk: I'd prefer to keep the ground plane on top with the DAC chip, although maneuvering some of those traces around a bit might not be a bad idea

 saab: this is true, but from what I have read it really is only applicable to very high frequency signals


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## robzy

Quote:


  Originally Posted by *ble0t* 
_dsavitsk: I'd prefer to keep the ground plane on top with the DAC chip, although maneuvering some of those traces around a bit might not be a bad idea_

 

Just make sure you have a solder mask, else soldering will be very very hard.

 Rob.


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## DaKi][er

Don’t just slap on the transformer for the spdif output, that’s a multi-MHz and very sensitive signal there

http://www.diyaudio.com/forums/showt...threadid=67247 for a thread that was designing a spdif output circuit for a cd player, just about the same as what you are doing, might need to change a few values (depending on output impedance of your chip)

 Just listen to Jocko Homo, he's done it all before

 And make sure you pick up a *real* 75 ohm BNC connector, you do know how to tell the difference?


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## ble0t

Yep...saw that thread a while back and totally forgot how easy (and non-space consuming) it is to implement. Thanks for the reminder!


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## ble0t

OK...added Jocko's output stage mods and cleaned up a few things on the board. Any other comments/suggestions?

 Schematic:






 Layout:


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## Nisbeth

I'm sure 45deg. angles on the traces wouldn't hurt 
	

	
	
		
		

		
		
	


	





 /U.


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## ble0t

True...I'll probably do it at some point (since I think that if nothing else it looks better). I'm just looking for feedback to make sure the design seems solid and the layout is good. Then I can make it look pretty


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## dsavitsk

Quote:


  Originally Posted by *ble0t* 
_I'd prefer to keep the ground plane on top with the DAC chip_

 

Is there a benefit to this I am unaware of?


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## Francis_Vaughan

The logic of having the groundplane on top with the 2702 design was that the chip is a one chip wonder and keeping the loop areas absolutely as small as possible for that chip was going to be quite important. Essentially the entire digital design was just the 2702. The same appplies here, since the 2704 is the same chip, but with digital out as well. So long as we are using the internal DAC in the 2704 we should try as hard as possible to preserve the integrity of the ground returns etc, and that means a ground plane right under it. 

 As we move to multi-chip solutions it matters less, as the change in loop area is proportionatly much smaller, so the other issues in layout will quickly become more important.


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## ble0t

OK, here's (hopefully) my final revision before attempting a proto. I've tried to clean things up more and I also mitered all the corners and made sure the silkscreen is good to go. As always, comments/suggestions appreciated 
	

	
	
		
		

		
			





 Schematic:






 Layout:


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## guzzler

Your C15 should be going to ground, not in series with the Vcc trace. Won't work as it stands! Inductors on the USB data lines aren't a terribly good idea as they will blunt the pulses, possibly leading to errors. Resistors are sufficient to isolate the PCM. I would recommend not having the Dout trace crossing the crystal trace as well; the crystal trace is the most sensitive on the board


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## ble0t

Thanks for the reply!

 The C15 label actually applies to the large electrolytic cap, not the bottom layer. Sorry for the confusion there...that bottom layer is actually a ferrite. As for the ferrites in the data lines, I guess I could try it with them jumpered and with them in and see what happens for now. Finally, I think you're right about the DOUT line...I never liked crossing the clock signal with that for fear of contamination on either of them. I'll see what I can do to rework that section. Thanks!

 EDIT: I found these for possible use on the data lines. Sourcing could be difficult if they don't sell in small quantities (they only sell direct). I also found these which would not only provide me with the 22R resistors, but also with the pullup for D+ and some filtering.


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## ble0t

OK...here's the final edit before I submit today for a proto. I added the small USB termination/filter IC that I linked to in my previous post and also tried to improve some of the routing.

 Schematic:







 Layout:


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## DaKi][er

There is plenty of room to have the digital output go up the left hand side of the chip, then put the resistor across the crystal there on the bottom layer and move the crystal in as close as you can


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## halman

You might also want to tie the ´negative´ end of the transformers output to ground via a capacitor, to help reduce emi.


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## ble0t

Ahh...both good ideas. I put in the order for the proto before implementing either of them, but I will most likely make the changes for a future version. 

 Oh and DaKi][er...thanks for that link (not sure when/where it was posted) for BatchPCB. It is hands down the best place for this type of thing


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## ble0t

Just a quick update/FYI for anyone interested...

 I contacted TI tech support concerning the output impedance of the DOUT pin on the PCM2704 and also asked about the PCM290* as well. Here's what they had to say...

  Quote:


 We do not have maximum value for this spec, but the estimated maximum values are +/-30% of 50ohms. This is the case for the PCM27* and PCM29*. 
 

Doesn't narrow it down too much, but it's better than nothing I suppose...


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## halman

You should buffer it and use something with a known impedance anyhow... 

 imho


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## ble0t

Good point. I guess I was wrong to think that TI would have some more concrete specs than that


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## halman

It might be so that the impedance is not equal for transitions from 0->1 vs 1->0. In wich case you _really_ should use a buffer for it.


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## ble0t

Got the board today along with the MAX4410's I ordered from Maxim. I'll try and get some shots posted later tonight. Hopefully, the rest of the parts will be here tomorrow and I can get it built by the weekend. Woohoo


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## nleahcim

Quote:


  Originally Posted by *ble0t* 
_Ahh...both good ideas. I put in the order for the proto before implementing either of them, but I will most likely make the changes for a future version. 

 Oh and DaKi][er...thanks for that link (not sure when/where it was posted) for BatchPCB. It is hands down the best place for this type of thing 
	

	
	
		
		

		
		
	


	


_

 

batchpcb is very very slow - often taking more than a month. 4pcb.com does much better boards of pretty much any size in a week for $33 if you're a student. (and they actually know what customer service is too)

 Anyways - one thing that bothers me about your design: where are the thermals? I expect you'll have some trouble soldering some of those parts that are connected to the groundplane - it'll just act like one big heatsink.


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## chaostic

Bit off topic, but did you get asked by ti to verify that you are american when you asked about the 290*? I did when I asked about the test circuit they had.


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## ble0t

nleahcim: I ordered from BatchPCB on Jan 15th and got my board yesterday. Two weeks for double sided soldermask and silkscreen isn't bad turn around if you ask me 
	

	
	
		
		

		
		
	


	




 As for the thermals, I didn't have any issue soldering on Alf's board and I wouldn't expect much here. It's a question of 'local' heat, and the groundplane isn't thick enough and/or big enough to dissapate enough heat to prevent soldering. 

 chaostic: No, they never asked me anything like that. When I submitted my request, they asked me (in a web form) for my name, address, etc.

 Lastly, here are links to some scans of my board if anyone is interested...I hope to get some soldering done tonight 
	

	
	
		
		

		
		
	


	




Top
Bottom


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## Nisbeth

I see another project for my ever-expanding collection of half-made PCBs 
	

	
	
		
		

		
			





 Keep us posted on developments! 
	

	
	
		
		

		
		
	


	





 /U.


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## ble0t

Well, I have it all built aside form the LT1763 (those should be here Monday). Being that it is a prototype, there were a few goofs, but nothing that should prevent operation. It seems I misaligned the BNC jack, so that needs to be fixed. Also, I didn't assign the pins quite right for the tranformer (and should have additional ones since there are always 6 legs regardless if they are used or not). This brings me to a question...

 If you look at the datasheet, it shows pins 1-4 and 3-6 as corresponding. With the current layout of my board, I actually have pin 3 on the primary side as the side taking the signal input and pin 1 connected to ground. On the secondary side, I can just switch which side of the resistor network to use so as to correspond with this (i.e. use the side connected to pin 6 as the 'signal' side). Am I wrong to assume this should still work out?


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## ble0t

Well, after some minor trouble shooting, it's up and running and it sounds quite good (although I might be a bit biased 
	

	
	
		
		

		
		
	


	




)

 I forgot to have a resistor that pulls up the DT pin to VDD, so I had to solder a resistor between the pin and the output of the LT1763 to get things working properly. Doh!

 As for the MAX4410, it sounds quite good. The detail and seperation is very nice, and with my Senn 497, it gets plenty loud enough. I can't hear any audible background noise coming in from the computer even at maximum volume.

 The SPDIF is working and with the transformer and Jocko's mods, it sounds way better than the SPDIF directly from my computer. The increase in detail and the tightness of the bass is much better.

 I'll post an updated version of the schematic/layout and try to get some pics of it before I case it up.


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## Clutz

Congrats ble0t! I'm excitedly awaiting the last few parts I need to finish my two Alf DACs One is going to be USB powered, and the other is powered by > 10 volts (two 9 volt batteries). 

 I'm becoming a DAC ADCict (ha ha ha).


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## dsavitsk

Quote:


  Originally Posted by *Clutz* 
_I'm excitedly awaiting the last few parts I need to finish my two Alf DACs_

 

They are on the way. Depending upon the speed of the mail, you should have them tomorrow -- or this week anyway.


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## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_They are on the way. Depending upon the speed of the mail, you should have them tomorrow -- or this week anyway. 
	

	
	
		
		

		
		
	


	


_

 

Thanks very much for your help. It's Canada Post that worries me the most though. I sent Christmas cards to my friends in Toronto last year. I mailed the cards the week before December 9th- and they didn't turn up until the week following Christmas. Hell, I almost didn't get accepted into graduate school because Canada Post took a really long time mailing my acceptance package back to UBC.


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## ble0t

OK, here's the updated stuff concerning the board. I managed to get some pics last night as well, but I couldn't find my data cable and I don't have a card reader 
	

	
	
		
		

		
		
	


	




 The changes I made from my proto order are as follows:
 -Added resistor to pull up pin 3 (DT) to VDD
 -Changed the headphone jack model (newer one will sit taller and more centered in end panel)
 -Corrected BNC jack layout
 -Corrected pulse transformer layout
 -Modified pads of the USB filter (better fit)
 -Moved some caps closer to their respective ICs where possible

Schematic

Layout

Layout (No Silkscreen)


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## Clutz

Quote:


  Originally Posted by *dsavitsk* 
_They are on the way. Depending upon the speed of the mail, you should have them tomorrow -- or this week anyway. 
	

	
	
		
		

		
		
	


	


_

 

Woot! They came in the mail today.. 
	

	
	
		
		

		
		
	


	




 I just gotta get my GF to let me have a few minutes to finish it up. Thanks a lot!

 Cheers,
 Clutz


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## ble0t

Finally got some pics of the board. You can see the resistor I had to add due to my error concerning pin 3. I also managed to catch the blue film cap with my soldering iron once...oops!

Overall

Side

Bottom

 I'm quite happy with the results thus far. The only thing I would say is that I need to up the gain on the MAX4410 a bit. I have to turn up the volume on my computer much further than I used to and would like to keep it more toward the lower portion of the 'slider' vs the upper portion. Other than that, I just need to get it cased up at some point, but it's hard to stop listening


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## Alf

Quote:


  Originally Posted by *ble0t* 
_I'm quite happy with the results thus far. The only thing I would say is that I need to up the gain on the MAX4410 a bit. I have to turn up the volume on my computer much further than I used to and would like to keep it more toward the lower portion of the 'slider' vs the upper portion. Other than that, I just need to get it cased up at some point, but it's hard to stop listening 
	

	
	
		
		

		
		
	


	


_

 

What do you think of your DAC vs the PCM2702 DAC?


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## ble0t

I haven't done a direct comparison as of yet. I did like the sound of the Rev B PCM2702, but I never truly got it hooked up to my loudspeaker system. Also, I'm not sure if it would be a fair comparison in that I am using the SPDIF out from my board to connect to that system. The DAC there is a Monica 2 without any input mods (which I need to do next). I don't yet have the capability to do any good testing (outside of listening) on either of them unfortunately. I will say again though that I am impressed with the MAX4410. At least for me, it sounds pretty dang good for only 3.3V


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## 00940

Quote:


  Originally Posted by *ble0t* 
_I'm quite happy with the results thus far. The only thing I would say is that I need to up the gain on the MAX4410 a bit. I have to turn up the volume on my computer much further than I used to and would like to keep it more toward the lower portion of the 'slider' vs the upper portion. Other than that, I just need to get it cased up at some point, but it's hard to stop listening 
	

	
	
		
		

		
		
	


	


_

 

Why ? The lower you are, the more you are killing dynamics by reducing resolution. The higher you are the better.


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## ble0t

Well, I don't mean the bottom of the volume control. I am currently only using the top 10% of the slider...it'd be nice to have a bit more play in it than that. I wasn't talking about taking the gain from 1 to 20 or something


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## ble0t

Well, I finished the updates to the layout and I figured I'd share the gerbers if anyone out there wants to make a board for themselves. I'll try and get the BOM for it put together sometime this weekend as well so I can post that too. Enjoy 
	

	
	
		
		

		
		
	


	




 The zip labeled 'nosilk' has everything but the silkscreen files and the zip labeled 'silk' has just the silkscreen files in it. I had to break them up so they would fit under the 25KB max file size.


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## philodox

Sorry if this is blatantly obvious in the schmatics/circuit boards, but I noticed that there is a SPDIF output via BNC. I take it this is so that you can use an external DAC. If you do not use an external DAC is the SPDIF conversion still there or are you taking I2S from the USB and passing that directly to the DAC?


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## ble0t

The PCM2704 only outputs SPDIF i.e. it cannot output I2S. The internal structure of the IC converts the USB data supplies both it's own internal DAC and the internal SPDIF transmitter. In the case of my design, both are always on i.e. you can have it outputting to an external DAC as well as listening on your headphones if you like.


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## philodox

Ah, I see. Looks like a cool design... I hope it sounds good.


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## Clutz

Anybody else going to put this together? PM me.


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