PCM1704 NOS Strange Clipping - diyAudio
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Old 27th March 2013, 11:37 PM   #1
LuisMCP is offline LuisMCP  Spain
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Default PCM1704 NOS Strange Clipping

The DAC seems to work properly in the beginning a nice sinusoidal can be saw in scope, but 1 minute after, a clipping in negative zone is slowly growing up to a certain level.

Both channels L and R are affected, two different I/V converters was tried and same results was obtained.

Pin 17 (SERVO_DC) has different voltage in both chips. One at 0,00V and another at -2,40 volts.

Some idea what the matter?

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Old 28th March 2013, 01:38 AM   #2
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Are pins 11 and 20 at + and - 5vdc constant? Maybe one of your 5v regulators are on the fritz when it warms up.

Cheers George
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Old 28th March 2013, 11:56 AM   #3
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I agree with George, the first thing to do is check whether the power supply rails are stable and without significant ripple voltage, especially, the negative rail.
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Old 28th March 2013, 01:10 PM   #4
LuisMCP is offline LuisMCP  Spain
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I'm using a Twisted Pear Dual Power Supply (LCBPS).
I have checked +/-5V correctly, and I have change LM337 and electrolitics caps and all remains the same.

1-2 minutes of clean signal, then clipping in negative sinusoidal. Power off the DAC and exactly the same thing ever and ever.

Both channels fails equally, I have interchanged the PCM1704 (they are socketed to DIP 20 adapter), I have put only one PCM1704 in L, then in R channel, and allways get the same results.

I don't have another PSU for testing right now, but probably this afternoon I would get one.
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Old 28th March 2013, 02:50 PM   #5
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Have you checked with scope?
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Old 28th March 2013, 04:37 PM   #6
LuisMCP is offline LuisMCP  Spain
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Quote:
Originally Posted by merlin el mago View Post
Have you checked with scope?
Yes, as I have mentioned it in my fist post.
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Old 28th March 2013, 05:03 PM   #7
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Luis could you post a pic and schematic used?
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Old 28th March 2013, 05:52 PM   #8
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Drop the I/V, put some 20ohm resistor to ground instead, check with scope. This way we eliminate everything but the DAC and it's digital side.
If the clipping is still there, and the supplies are absolutly fine then... well, i hope you wont see that clipping thing again
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Old 28th March 2013, 06:03 PM   #9
LuisMCP is offline LuisMCP  Spain
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Quote:
Originally Posted by s3tup View Post
Drop the I/V, put some 20ohm resistor to ground instead, check with scope. This way we eliminate everything but the DAC and it's digital side.
If the clipping is still there, and the supplies are absolutly fine then... well, i hope you wont see that clipping thing again
interesting. I will try this

A question... I/V I use is a common opamp v to i converter, and of course, inverts the signal. Then, If I see negative side clipped in scope and i/v inverts, DAC output is with positive side clipped, and the problem would be in positive rail of psu, not the negative rail.
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Old 28th March 2013, 06:10 PM   #10
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Opamp's PS may fail too, so it won't supply the current required = give high virtual impedance = clip. What is the resistor value in opamp's feedback?

Anyway, lets check the DAC first
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