[sdiy] Op-amp versus noise

[Tom Wiltshire]

Hi David,

It sounds like you've got  a DC voltage across the pot. I'd suspect that the Moog circuit has the audio imbedded on some DC level, which the op-amp then amplifies along with the audio. When you move the pot, you get the scratching noise you mentioned. A DC blocking cap ahead of the op-amp should fix it, but you could probably remove the offset at source too. I've seen some Moog filter circuits that have a trimmer for that, so others have had the same thought.

HTH,
Tom

On 23 Mar 2014, at 17:50, David Ingebretsen <dingebre at 3dphysics.net> wrote:

> 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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