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Chipomatic balanced input RIAA
Chipomatic balanced input RIAA
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Old 29th January 2019, 07:04 AM   #81
Dimdim is offline Dimdim  Greece
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Chipomatic balanced input RIAA
I finally managed to de-bodge my prototype PCB and do some testing.

Everything looks fine, except that I'm getting a small imbalance between the two channels in the DC offset that I am getting in the outputs.

One channel is OK, outputting close to 0mV. The other channel has about 1mV offset on the OUT-DB header which gets amplified to about 9mV on the output. It's like there's something wrong with the DC servo.

All DC offset testing was done with the input shorted to GND.

Any ideas?
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Old 29th January 2019, 01:59 PM   #82
Salas is offline Salas  Greece
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I would think its about the intrinsic offset of IC2 per sample and section. Have you got another to replace on the higher offset channel?
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Old 29th January 2019, 02:01 PM   #83
Salas is offline Salas  Greece
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To verify my theory just swap IC2 between channels. If the offset will migrate its verified.
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Old 29th January 2019, 02:20 PM   #84
Dimdim is offline Dimdim  Greece
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I will do that and get back to you.
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Old 29th January 2019, 02:25 PM   #85
Salas is offline Salas  Greece
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If my guess holds water, then I had a lucky strike 0.000V offset on the proto as you had on one of your channels.
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Old 30th January 2019, 06:41 AM   #86
Dimdim is offline Dimdim  Greece
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Chipomatic balanced input RIAA
You were absolutely right. I swapped the op-amps (IC2) between the channels and the elevated offset also switched channels. So it seems to be the intrinsic offset of the op-amps.

I can definitely live with ~8mV of DC offset, since my DCG3's DC servo will easily take care of that.. but the temptation to go parts hunting for the perfect 2134 is strong..
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Old 30th January 2019, 07:57 AM   #87
Salas is offline Salas  Greece
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Or for equivalents. There is a DIP-8 socket option we put on the board for a reason. Unashamed op-amp rolling that is. Just imagine for best offset average between samples and as subjective sound winner emerges the TL-082 in the end! LOL.
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Old 30th January 2019, 08:09 AM   #88
Dimdim is offline Dimdim  Greece
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That would be an epic plot twist.. Like a fellow around here says, "More distortion to the people"
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Old 30th January 2019, 08:12 AM   #89
Salas is offline Salas  Greece
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And noise... don't forget to add a dose of good old epic noise

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Old 30th January 2019, 08:26 AM   #90
vgeorge is online now vgeorge  Greece
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In my pcb though offset is very low, 0.15 mV on one channel and 0.50 mV on the other.
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