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usb dac

post #1 of 519
Thread Starter 
Browsing on diyaudio, I found this :

it's using pcm2902 as usb receiver, dir1703 as spdif receiver, pcm1730 as dac and opa627's (6 of them!) as output stage.

it looks really interesting, main difficulties being you have to etch your own boards and solder those little chips.
post #2 of 519
6xOPA627?? ouch, thats £90 straight away! but it does look very interesting. Just for interest, HeadRoom are about to release something similar based on the TAH headphone output, no pics or specs as of yet though...

post #3 of 519
It is an interesting idea. However, having worked with a few computer audio devices in the pro arena I have noticed a general lack in quality USB devices, which can be attributed to the fact that they are USB devices. USB is not quite suited for the transmission of data for audio. It might be useful to do a search for pro audio input and output devices, the ones made by Digidesigns and MOTU are almost exclusively either Firewire or PCI. The one USB digidesigns device that I know of, the MBOX has had problems.

post #4 of 519
Thread Starter 
well, m-audio has decent usb soundcards (the sonica to begin with). Furthermore, zzz used such a chip to get spdif from an usb out and reported good results.

Regarding the opa627, i guess cheaper opamps could perhaps be used, the opa134 perhaps ?
post #5 of 519
Hm, so how would one go about getting high quality SPDIF from USB?
post #6 of 519
Thread Starter 
post #7 of 519

firewire equlient chip?

aos is coming up with a dac daugtherboard for the ppa. i found this thread, and then a thought came up in my head! is there a firewire chip that does the same thing as this usb chip? puts out normal stereo audio, plug&play? if so, culdn't you place this in the stage before the dac chip in aos dac, and get a bitppa?
post #8 of 519
The issue is drivers as well.

I use a Sonica, and it works well. But I need to figure out how to wire it up for a coaxial output (or add an optical input to my ART DI/O). I'm thinking the prior might be easier, but who's to say since optical stays TTY and coaxial isn't.

Anyone know of a simple circuit to add a coaxial digital output to the Sonica chipset?
post #9 of 519
There are firewire chips, but they're much more complicated to implement. Also, the firewire audio interface supports copy protection, and if you want to be able to play back copy protected streams, you have to sign a license with the vendor (at least Burr-Brown makes you do this) before you can get the chips.

As for the Sonica question, can't you just remove the optical transmitter, add a 75 ohm resistor to ground, and a coaxial jack?
post #10 of 519
Wodgy is right, licensing makes Firewire as good as dead. And Sonica definitely has driver issues - it is certainly not plug and play. For example you don't have linux drivers - because what actually happens is firmware upload every time device is plugged in the USB port! That is also why you must install softwre first, before plugging in Sonica. There are more and more devices like this, completely defeating the plug-and-play concept.
post #11 of 519
To be fair, the firmware "upload" is from the onboard ROM to the PCM3200 on the Sonica -- nothing is transferred to/from the computer on power up/when you plug in the device. Linux drivers are possible but no one has gotten around to writing them yet. (It would have been better to support the USB Audio standard, but a big part of their marketing is the SRS processor implemented in the driver, making a direct USB Audio implementation impossible.) If you're running Windows, of course, none of this is an issue, and I have tested the S/PDIF output to verify that it is bit-perfect. It's almost impossible to DIY a PCM290x solution for the retail price of the Sonica unless you can make your own PCBs or feel like soldering directly to the pins of a SSOP-28 device (sounds crazy, but there's one guy over at DIYaudio who did this by bending every second pin upwards).
post #12 of 519
I was attempting to build a HTPC (home theater pc) back when I took a week of vacation in November, and I ended up wasting a fair bit of money and precious time as I tried to build a linux one. Sonica was one of the things I bought and that's how I found out about drivers and firmware upload. I attempted all patches on the web at the time but even though I got it pass error messages so it all appeared to work, sound drivers still wouldn't connect to the card, so I just gave up. I haven't returned Sonica to the store, I have it still, but according to RMAA, DAC inside is nothing to write home about. However, since that time I am unable to get my Revolution 7.1 to work in 24 bit or anything over 48kHz.

I kept it because of optical audio - so if you have your own DAC you can use Sonica with a laptop to get excellent sound. I don't have a laptop yet but maybe one day...

But if you get a chip that works as standard USB audio, and outputs bit-perfect digital stream, then it'd be cool to add it to a good DAC.
post #13 of 519
Ahh, Linux driver issues. I feel your pain.

If anyone's interested in measurements of the Sonica's analog output, go here:
Very good for the price, but not excellent. Most of the noise is >100db down, which is better than almost all sound cards at this price.

The analog stage of the Sonica is a 10uF coupling cap, followed by a minor resistor divider, then a ceramic cap to ground (presumably to filter digital noise, though neither the data sheet and eval board for the AK4353 recommend one). Modders can bypass the entire output stage with coupling caps of their choice to RCA jacks. I've done this with my Sonica (2 uF AudioCaps bypassed with 100uF Elna Silmics) and the sound improved noticeably, especially the highs which were a bit grainy before. Definitely a recommended tweak for modders. It's a very listenable device.
post #14 of 519
That's far better than I got - I got noise flor at 80 something dB, distortion, non-flat frequency response etc. I did of course disable the DSP processing - or at least I think I did. I guess installing Sonica drivers screwed up my Revo setup. Now everything gets resampled to 44 or 48kHz. It makes sense as I didn't belive Sonica would be quite that bad.
post #15 of 519
Does anybody have an opinion on which usb audio chip performs better? Or is the performance pretty similar?
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