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

 

[Nisbeth]

Absolutely fantastic idea!

/U.

 

[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

 

[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

 

[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.

 

[saab]

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

 

[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

 

[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.

 

[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?

 

[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!

 

[ble0t]

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

Schematic:

Layout:

 

[Nisbeth]

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

/U.

 

[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

 

[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?

 

[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.