Raising bias on Marantz SR4200 AV Receiver

Andersonix · 15th November 2012, 10:54 PM

I have had success with new op amps (OPA1642), and it's sounding better as a DAC and preamp.

As this receiver uses the power amp section to drive headphones, I was hoping to raise the bias to help it sound better. How is the bias set, and how do I raise it? I'm guessing it's to do with the ratio of R611, R619, and R691? If so, maybe I can insert a pot here?

Any other low-hanging fruit to knock down in the power amp section while I'm in there? (C501 has already been swapped for an Elna Silmic...)

Thanks in advance!

Andersonix

nigelwright7557 · 15th November 2012, 11:13 PM

I would apply a sine wave and look on the scope for cross over distortion.
If there is then maybe adjust the bias if not I would leave well alone.

jaycee · 16th November 2012, 12:47 AM

Biasing is fixed but you would decrease R611. I wouldnt advise it. The heatsink is likely inadequate for this.

Andersonix · 19th November 2012, 10:15 PM

Thanks, jaycee! Decrease R611 only? Nothing with R616, R691? Maybe I will put a 1k Ohm pot there...

Every amp I've ever tried it on sounded better with more bias. I'll keep a close eye on the temp rise. I don't use the power amp section, so it's just for headphones.

15th November 2012, 11:13 PM	#2
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	I would apply a sine wave and look on the scope for cross over distortion. If there is then maybe adjust the bias if not I would leave well alone. __________________ http://www.murtonpikesystems.co.uk PCBCAD50 pcb design software.

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16th November 2012, 12:47 AM	#3
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	Biasing is fixed but you would decrease R611. I wouldnt advise it. The heatsink is likely inadequate for this.

19th November 2012, 10:15 PM	#4
Andersonix diyAudio Member Join Date: Jan 2004 Location: Silicon Valley	Thanks, jaycee! Decrease R611 only? Nothing with R616, R691? Maybe I will put a 1k Ohm pot there... Every amp I've ever tried it on sounded better with more bias. I'll keep a close eye on the temp rise. I don't use the power amp section, so it's just for headphones.