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Old 20th November 2009
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JohnRoberts's Avatar

Originally Posted by upinflames View Post
Thanks for the advice John. Since I removed R9 and R10 based on Jim suggestion for using the THAT 2180, do you suggest I put them back in since I intend to use them primarily in stereo? Or if I replace the other op-amps should I be safe? I re-read your post a few times and tried to understand it the best I could, but it's a little over my head.
R9 and r10 are symmetry trim adjustments... The symmetry trim in newer parts is different, >>100k for r10, and R9 is a smaller value and inside the VCA so not needed.

The modern parts are quite good without trimming, and unless you have bench equipment you can't adjust the trim anyhow, so just leave them out.
If the AD797 a drop in replacement for the 4458? I believe the opa2134 is, but mouser and digikey do not have them in stock.
I do not make specific opamp recommendations, there are others here more than happy to steer you to the latest opamp du jour. I only mentioned the 797 because that is what THAT used in their AES paper. I suspect there are cheaper parts that are significantly better than 4558.

My comments about stereo/slave mode were regarding different resistors and I am reluctant to suggest you get that deeply into a modification without doing more homework and a better understanding of the circuit.

I am a little unclear about how the "strapping" jack is used. If the "save" switch is actually a "slave" switch that makes some sense. When switched the VCA is controlled by remote CV while rms detector outputs from both combine. If "save: switch not toggled, the two opamp outputs 1/2 OA4 from each, may be shorted together. Presumably the 100 ohm buildouts inside the feedback loop are to help prevent opamp problems if shorted. Replacing that 4558 with a higher current opamp may have other consequences.

For mono operation I would suggest one thing, for strapped operation together, another.. I would have designed this part of the circuit differently, but what else is new, and that doesn't do you any good..

So for now, I would lose r9 and r10 as Jim suggests, and keep all other Rs (despite my earlier suggestion) . When upgrading OA4 perhaps avoid changing to an opamp with too much drive capability, if when strapped together neither unit is in slave mode and the opamps are fighting each other.

Have fun but do no harm.