[sdiy] Op-amp versus noise

[Dave Brown]

Correct if you added the amp before the capacitor.  This is an emitter
follower output so there is a DC component to the signal.  When you moved it
past the attenuator you moved it past the DC blocking capacitor.

I'm curious about needing gain.  With the controls flat I measured a gain of
1.25X through the mixer.
http://modularsynthesis.com/moog/984/984.htm

Dave

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
Sent: Sunday, March 23, 2014 11:56 AM
To: David Ingebretsen
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Op-amp versus noise

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