Controlling Gain of OP AMP with Expression Pedal

[MarshallPlexi]

Is it possible to control the gain of an op amp using an expression pedal?

I know if I vary the resistance in the feedback loop I can vary the gain but my concern is if I run the signal from the op amp out to an expression pedal that I'll pick up all kinds of noise etc and feed it back into the op amp.

Basically I'm trying to vary the level of my signal by using an op amp that varies from unity to off using external control and a 5V control voltage.

Perhaps I'm making it difficult. I thought about using a VCA but that seem complicated by comparison and I tried using Vactrols and they were inconsistent at best.

Thoughts?

[Rob Strand]

What you have said is pretty much how it is.
I can suggest a couple of things:

With the vactrols, if you are unhappy with the relationship between the control setting and the vactrol resistance then you can tweak things:
For example with additional circuitry you can make the LED current a linear or logarithmic function of the pot position, or, control a control voltage.   

Another trick is to use two vactrols and one or two opamps so one vactrol tracks the resistance of the remote pot. It relies on the vactrols being matched.

Going beyond that you could use a switched conductance type VCA.

[MarshallPlexi]

What you have said is pretty much how it is.
I can suggest a couple of things:

With the vactrols, if you are unhappy with the relationship between the control setting and the vactrol resistance then you can tweak things:
For example with additional circuitry you can make the LED current a linear or logarithmic function of the pot position, or, control a control voltage.   

Another trick is to use two vactrols and one or two opamps so one vactrol tracks the resistance of the remote pot. It relies on the vactrols being matched.

Going beyond that you could use a switched conductance type VCA.

I would be interested in either of those solutions if you can provide a reference to study.

[armdnrdy]

http://www.excelitas.com/downloads/app_analogopticalisolatorsaudioapps.pdf

[thehallofshields]

Has anyone tried anything like this:



http://hackmeopen.com/2011/04/voltage-controlled-amplifier/

I'm curious if it has an acceptable taper with a exp. pedal.

[Eddododo]

I feel like VCA is the simplest of the really GOOD solutions. Maybe something like the H11f1m photo FET fits the bill

[thehallofshields]

Here is something interesting I came across; a passive LDR Attenuator. It might not be Voltage Controlled, but it could yield fruit.



http://diyaudioprojects.com/Solid/DIY-Lightspeed-Passive-Attenuator/