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rrrremus 24th September 2010 07:30 AM

Help with DIY DAC and m-audio delta192 SPDIF problem
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Hello all. I find myself in a puzzle.
I am using a DIY DAC built arround SRC4382 and PCM1792A.
I have used the DAC since 2 years without problems. I recently swapped my Audigy PC sound card for an M-audio delta 192.
The problem is that my DAC does not detect the SPDIF of the delta 192.
The DAC works with every other SPDIF output i can find at home ( old Audigy sound card, motherboard sound card, CD player ).
Also, the Delta192 outputs SPDIF correctly, because i connected it to a commercial build receiver and everything is in order.
I took a look on the SPDIF signal with the oscilloscope and everything seems to be in order, both on the SPDIF cable and on the SRC4382's input.
Between the Audigy's spdif output and the Delta's output i can only see a slighty lower p2p voltage on the delta 0,4V compared to a 0,5 V on the audigy.

I attached the schematic of the SPDIF input in my DAC

Bill Fuss 25th September 2010 09:17 AM

Did you try both inputs? I don't think the opt receiver can handle 192 sample rates.

SY 25th September 2010 09:51 AM

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Other issue could be the slightly lower level. When I ran into this, I threw together a little box that took things to AES/EBU levels and the problem went away. You could certainly modify this for 72 ohm output if you want. Total parts cost was well under $10.

Bill Fuss 25th September 2010 07:04 PM

After looking at your diagram again, you are completely defeating the pulse trafo. The cable shield should not be grounded and the windings should not be tied together with a resistor. I don't know if it's related to your problem though.

rrrremus 28th September 2010 06:28 AM

@Bill Fuss. The trafo is not defeated. R101 and R102 are not fitted. But the output windings are tied together by 75ohms. This is the connection example from TI's SRC4382 datasheet. I will try to remove this resistor when i get home and see what happens. You might have something here.

@SY. The level is more than 50% lower, but the thing is that the SRC4382 datasheet states that the minimum acceptable voltage ( unless i am mistaking ) between the RX+ / RX- input pairs is of 150mV. I can measure at least 400mV here. A level boosting unit would be my last resort.

@Bill Fuss : The optical input works fine and should support 192k

Bill Fuss 28th September 2010 11:06 AM

R103 is the proper termination for a 75 ohm line, unless you used other than a 1-1 pulse trafo.

So you are sure your torx receiver can handle 192, where does that leave you as far as solving your problem.
All your inputs and outputs work correctly but the system does not work. Are you inadvertantly trying to feed compressed 7.1 from your new card to a dac that can not handle it? A receiver input would be capable of handling it because it uses DSP.

rrrremus 28th September 2010 11:23 AM

Thank you for your interest Bill and for your help.
Let me try to make things a little bit clearer.
The torx input works well and is not a part of the problem.
The problem is located on the Coaxial input. The coaxial input is working with other signal sources. It is only not working with the m-audio sound card ( at any sample rate ).
It is impossible for me to feed >2 channel data because the m-audio sound card is only a 2 channel sound card. I will fiddle with the device when i get home, if i still won't solve the problem i will post here pictures with exact measurements and parts used.

rrrremus 29th September 2010 06:20 AM

I got the thing going. There was a grounding issue. The SPDIF ground was connected to the DAC analog ground trought an analog cable coming from the computer. However it only works up to 96khz for now.
Thank you all.

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