PCM1704 Help needed with higher dc offset from one channel - diyAudio
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Old 9th October 2012, 12:03 AM   #1
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Default PCM1704 Help needed with higher dc offset from one channel

Hope someone has an answer to this. I have a Cary 303/200 cdp it has a PCM1704 per channel and OPA627 I/V stage and AD825 buffer both direct coupled.
The problem I have is that I have 17mV dc offset on the left channel but 50mV dc offset on the right channel, measured at the rca outputs. I have traced this back to the right PCM1704 being the culprit of the 50mV dc offset As you can imagine the right channel give a bit of a click through the system if the cdp is switched on or off, the left is not noticable. Both channels sound fine, I'm just wondering if this is normal for the dc to be so high, or if it can be nulled out or has a leaky cap maybe on it's pin 12 (BPO DC) or something similar?

Thanks George
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Old 9th October 2012, 01:19 AM   #2
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Hi George - the datasheet for PCM1704 says that the bipolar offset worst case is 1% so if your signal output is 2VRMS then the worst DC offset due to the DAC chip will be +/-28mV. The typical will be 14mV.
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Old 9th October 2012, 02:41 AM   #3
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Hi Abraxalito thanks for answering. I just double checked. I pulled the opa627 I/V opamps and then measured pin 2 of the socket which is the I output (pin 12) of the PCM1704 K's and I got 2mV left and 7mV right so they look ok. It's the I/V (opa627's) opamps that's giving the highish offset. I changed them left for right but still the right has 50mV on it so it's not the opamps them selves but what's around them. And I don't have a circuit for this. It's multi layered board and all makes it very hard to see what's what. I tried to get it from Cary, but they say they don't have it???? Unless someone here can get it out of them.

Cheers George
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Old 9th October 2012, 02:53 AM   #4
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I'd bodge up a bipolar reference circuit driving a pot, or you could take the reference from the analog supplies then filter heavily with an electrolytic and X7R ceramic in parallel. A 20k pot between the analog supplies, the caps to agnd from the wiper then a 220k resistor into the inverting input of the 627 ought to work.
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Old 9th October 2012, 03:01 AM   #5
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Could you maybe free hand draw that up and post it as I'm having trouble picturing it, in the mean time I have sent an email off to Cary again to get the schematic of the analogue stage, as this is so weird that it's something around the opa627 that be causing it.

Cheers George
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Old 9th October 2012, 03:32 AM   #6
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Here's the idea
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Old 9th October 2012, 05:12 AM   #7
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Does this way you've drawn have limitations as the data sheet way of nulling dc offset (attached) it says you should not try to null out more than +or-10mV.

Cheers George
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Old 9th October 2012, 05:16 AM   #8
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No, the datasheet suggestion is strictly for nulling out offsets within the opamp - note it explicitly says its not to be used for external sources of offset error. My circuit has no such limitations - its correcting external errors externally
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Old 9th October 2012, 06:08 AM   #9
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Good thank's for clearing that up, should I get no response again from Cary, I'll give your circuit a try.
Is the 22uf 6.3v a bi-polar, it would have to be I think?
Just a another question, is it detrimental to the sound to have 50mV of dc offset way back at the I/V stage all the way through to the output rca's beside the obvious dc clicks through the speakers? My dc sevo in the amps nulls it back out again after the initial click.
Cheers George
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Last edited by georgehifi; 9th October 2012 at 06:28 AM.
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Old 9th October 2012, 07:38 AM   #10
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Are these opamps the original manufacter fitment ? Have you scoped the output of these to make sure they are not oscillating as HF oscillation can give a DC shift.

50mv offset is a lot for what is probably a unity gain output stage. Can you lift the DAC output pin and see if the opamps settle to nearer zero ?
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