[sdiy] Op-amp versus noise

David Ingebretsen dingebre at 3dphysics.net
Sun Mar 23 18:50:54 CET 2014


In goofing around with some of the old Moog circuits, in particular the Moog
984 matrix mixer, I thought I'd try adding an op-amp to the Moog original
output to buffer and add a little gain to the typically low Moog levels. In
the 984 Mixer, the final output goes to a 5k attenuation pot. I tried adding
an op-amp between the output stage and pot, and also after the output pot.

In the former configuration, I was getting a very loud "scratching" whenever
I turned the pot. By passing the op-amp, no scratching at all.

In the latter configuration, I get major hiss regardless the gain of the
opamp stage.

Please enlighten me. 

I just used a basic non-inverting configuration with 22k in series with a
100k trimmer in the feedback and a 30k from "-" to ground. 22p cap in
parallel with the feedback "resistor". The opamp is an NE5532.

David




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