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Old 7th September 2012, 02:13 AM   #1
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Default Looking to design USB DAC

Hello, just found this place; for someone just starting my first design its been an awesome resource so far.

A little background by day I am an EE and design switching power supplies. I have been looking for a personal project to do in spare time and came up with idea of designing a USB DAC + headphone/speaker amplifier.

The idea is to connect via USB to my computer and output to speakers or to a pair of headphones. The device will output to the speakers if headphones are not plugged into the port, when headphones are plugged in it will mute the speakers.

I am in the early stages of planning but wanted to get some feedback on what i have so far. I plan to use PCM2707 to obtain the audio signal from the PC then transfer the data via I2S to either a DF1704 or directly to the DAC (thinking of PCM1795). Then the I/V converter and LPF, then have 2 Class D amps one for each channel for the speakers and 2 class A/B for the headphones.

My questions are, is the DAC overkill, what DAC would you recommend i would like at least 24bits and 96KHz sampling rate, also would my amplification scheme work. Any comments would be appreciated
Thanks!

Last edited by dmgash; 7th September 2012 at 02:26 AM.
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Old 7th September 2012, 02:40 AM   #2
wlowes is offline wlowes  Canada
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Default a couple of suggestions

your basic plan seems ok. You could look at Peter Daniel's nos usb dac for ideas and inspiration on implementation of a fine implementation of a pcm2706 to a tda1543.
I have to bring your attention to WaveIO for the best in class USB that will take you to 96k and beyond. For the output have you considered tubes?
I run WaveIO - I2S - tda1541a - I/V - 6np2 output stage (see lampizator site). Not difficult and a ton of resources here to make it best in class. Sound? Out of this world.
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Old 7th September 2012, 03:00 AM   #3
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Do you care how this thing sounds in the end or is it just for building/designing experience you're doing it?

I ask because as you only need 24/96k I can't see any point of going direct to USB - rather buy a cheap (<$30 USB-SPDIF box) and build an SPDIF DAC. This solves the biggest bug-bear in sound quality - isolation from PC SMPSU noise - because SPDIF can go through a transformer.
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Old 7th September 2012, 03:05 AM   #4
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Quote:
Originally Posted by abraxalito View Post
Do you care how this thing sounds in the end or is it just for building/designing experience you're doing it?
I would like it to sound good and be able to use it on a daily basis but I think I could deal with a small loss in quality if that means i can have one fully integrated box on my desk.
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Old 7th September 2012, 03:23 AM   #5
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I'd recommend using PCM1792A (PCM1794A HW control version) instead of PCM1795 if you want a TI DAC. There is no need for DF1704 with these parts, they have built in digital filters that are very similar if not better (in the case of PCM1792/4).

If you want something "better" I would use ESS ES9018, however the cost increase is significant.

I would recommend using an ASRC such as SRC4192 or AD1896 to reduce jitter if you are going to use PCM2707 which operates in the USB Audio Class isosynchronous mode. Also makes life simple because the DAC will then operate at a fixed rate which can be an odd frequency.

http://www.benchmarkmedia.com/discus...d-asrc-devices


If you plan to use PCM2702 which is a USB Audio Class 1.0 device (if I remember correctly), I would use the Analog Devices ADUM4160 USB Isolator. 12Mbps USB can carry 24/96 maximum I think so it should work. Alternatively you could use optocouplers or GMR isolators on the I2S but they will add significant jitter if not using an ASRC or a USB interface that supports asynchronous mode (XMOS, custom, etc) which allows you to use the USB receiver's I2S ports in slave mode.


FYI, Just in case you are new to audio and here, there is a group that will recommend a certain obsolete low-end Philips DAC to you and suggest you use them improperly to get the "best sound". In fact, I will probably catch hell for saying this .

Last edited by chris719; 7th September 2012 at 03:34 AM.
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Old 7th September 2012, 03:31 AM   #6
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Quote:
Originally Posted by chris719 View Post
FYI, Just in case you are new to audio and here, there is a group that will recommend a certain obsolete low-end Philips DAC to you and suggest you use them improperly to get the "best sound". In fact, I will probably catch hell for saying this .
As a card-carrying member of that group I'm careful to recommend such chips only to people who are passionate about SQ Hence my earlier question. Is 'improperly' a reference to passive I/V by any chance?
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Old 7th September 2012, 03:45 AM   #7
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I only mean TDA1543 mostly, TDA1541A at least has "good" specs. I'm not a fan of passive I/V but it's probably not as bad as non-OS with an inadequate analog filter on the output .

Last edited by chris719; 7th September 2012 at 03:48 AM.
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Old 7th September 2012, 03:53 AM   #8
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TDA1543 is a great beginner's DAC for SQ motivated (as opposed to meter motivated) DIYers. I agree its specs are amongst the lousiest out there.
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Old 7th September 2012, 04:08 AM   #9
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Quote:
Originally Posted by dmgash View Post
I would like it to sound good and be able to use it on a daily basis but I think I could deal with a small loss in quality if that means i can have one fully integrated box on my desk.
For many people in digital audio, the measurements they know their kit has has an influence on the sound they hear. I seem to recall one guy on this forum (he's not been around recently though) wouldn't even listen to a DAC which measured bad because he knew it would sound bad to him. Nocebo effect is very real - so could you live with a DAC which you knew had fairly poor measurements ? You're an EE so presumably the measurements are important to you ?

<edit> the loss in SQ due to not isolating your DAC you might not notice as a loss in SQ, rather you'll just find the DAC fatiguing to listen to over a longer period.
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Last edited by abraxalito; 7th September 2012 at 04:13 AM.
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Old 7th September 2012, 05:56 AM   #10
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Quote:
Originally Posted by dmgash View Post
I would like it to sound good and be able to use it on a daily basis but I think I could deal with a small loss in quality if that means i can have one fully integrated box on my desk.
A member here, gmarsh, had designed a very similar piece of equipment to what you may be looking for a few years ago. Uses SPDIF instead of USB but you can get a good idea from it. He posted the complete set of Eagle files and everything. It's nothing crazy, but fairly good quality parts and a compact desk unit.


My latest project - DAC + headphone amp
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