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
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
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
Cheers George
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.
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
Cheers George
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
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
Last edited:
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 ?
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 ?
They are original and socketed, I swaped left for right but still had the 50mV on the same channel, it could be the layout of the right is making that side oscillate. Next time I have my scope out and hooked up I will check for oscillations.
PS thanks for the reminder of opamp stability
Cheers George
PS thanks for the reminder of opamp stability
Cheers George
Hi abraxalito, thank you very much for the dac dc offset nulling circuit it worked a treat, instead of 17mV left and 50mV right comming out of the OPA627 I/V's, now I have +- .1mV I don't know why but the sound has improved very much overhaul esspecially in the bass. Is this possible, that the OPA627 as an I/V does not like to see any dc comming into it from the dac/s???
Cheers George
Cheers George
Hi George - since you asked for my input I can only say scrap the opamp I/V if you're really concerned about SQ. Its really a waste of a decent R2R DAC chip. Go for something with more immunity to glitches as a post amplifier like a discrete JFET design, or use passive I/V and fierce passive post-filtering to remove all the RF. I've posted up a suitable filter schematic on my thread called 'Digital, but not by the numbers' in Digital Line.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
- Home
- Source & Line
- Digital Source
- PCM1704 Help needed with higher dc offset from one channel