USB DAC project with PCM2707 and PCM1794, some feedback wanted - diyAudio
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Old 24th June 2013, 10:51 AM   #1
JanErik is offline JanErik  Finland
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Default JEDAC: USB DAC project with PCM2707 and PCM1794, some feedback wanted

I am about to start designing a USB DAC as my vacation project.

This will be based on PCM2707 and PCM1794, and I was thinking to make it totally USB powered. Have TI's charge pump switchers for 5V->3.3V and 5V->-5V. Do you think it is OK to power it that way?

Opamps used will be some of TI's high-end SMD chips, r2r I think and OK from around +-2.7V. But will +-5V be enough headroom?

I also want several identical line outputs (e.g. for desktop speakers, larger livingroom speakers and subwoofer), to get rid of splitter cables. Do you think they need separate buffers or would a series resistor for each be enough separation?

Last edited by JanErik; 15th July 2013 at 07:49 PM.
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Old 10th July 2013, 03:47 AM   #2
Mull3t is offline Mull3t  United States
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Check this out... I think it fits the bill...

PupDAC Overview

I was one of the prototype builders and use the DAC hooked up to two powered speakers at work. Then I do digital attenuation with Decibel on my Mac.
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Old 11th July 2013, 09:42 AM   #3
JanErik is offline JanErik  Finland
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That looks quite like what I will be building, with some differences. I think I will use a normal crystal. Will also only use hole-mount components. Will use single-side board.
Good to have a reference for hooking up the chips anyway.

Seems that one is running the opamps at +-2.5V, is that really enough?
What about the ferrites/inductors, there is no other value than 600-800R... are they necessary?
The reset chip for the PCM1794, according to the datasheet it has power-on reset built-in.

Last edited by JanErik; 11th July 2013 at 10:01 AM.
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Old 15th July 2013, 04:24 PM   #4
JanErik is offline JanErik  Finland
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The analog part of PupDAC differs quite a bit from the reference design; http://www.diyforums.org/PupDAC/sche...chematic-3.gif as compared to http://www.ti.com/lit/ds/symlink/pcm1794.pdf page 20.
What do you think about that, which is better?
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Old 15th July 2013, 06:16 PM   #5
JanErik is offline JanErik  Finland
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OK, I see this one using the same output stage as PupDAC: PCM1794A audio DAC

This one is using pull-up resistor on the DAC reset, is there any reason to do so?

I see one might put a jumper on pin 2 (CHSL) to be able to select sharp/slow rolloff on filter.

What about this De-emphasis filter, what is that?

Last edited by JanErik; 15th July 2013 at 06:25 PM.
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Old 16th July 2013, 05:42 AM   #6
Mull3t is offline Mull3t  United States
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The Pup can drive headphones. People do it all the time. I prefer to use an amp after it. You'd have to ask the designer why he chose +-2.5v.

Most of your technical questions would be better suited to the designer because I'm a mere noob when it comes to designing this stuff. For now I just build it.

Here is a thread where you can ask questions of the designer...
pupDAC Step-by-Step Build Thread
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Old 16th July 2013, 07:15 AM   #7
JanErik is offline JanErik  Finland
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I dont really use headphones but since I have only quad opamps I might put a second buffered output.
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Old 18th July 2013, 03:00 PM   #8
MWP is offline MWP  Australia
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I made something similar some years back using: PCM2707, PCM1798, OPA4228 and a TPA6120.
The TPA6120 is supplied by -12V, 12V using TI DC-DC converters.

It was a quick cheap (almost free, all ICs were TI samples) project that worked surprisingly well!
I built it back in 2005 to use with my Laptop for OS travel.
I am still using to this day (gets used every day).

The odd PC/USB-hub doesn't like it due to the large in-rush current on pluggin it in, but these days almost all are ok with it.

Front side:
Click the image to open in full size.

Backside:
Click the image to open in full size.

In its case with 3.5mm jack:
Click the image to open in full size.

Last edited by MWP; 18th July 2013 at 03:04 PM.
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