opamp: combine two gain controls

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francesco81rs

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Joined 2011
#1
Hello everyone, first of all sorry for my bad English ...
The need that I would have is to combine a volume control (the first potentiometer) with an adjustable gain control (second trimmer).
This is because the volume control is shared by 6 channels, while I would like an independent gain control of the second stage.
As you can see the first opamp deals with receiving a balanced signal and turn it into unbalanced.
I would ask if, in principle, this scheme might work?
I am not very convinced of the configuration of the second opamp ....
It would perhaps be appropriate to isolate the first potentiometer (I was thinking to 100k) interposing a opamp configured as a voltage follower before the second opamp in this drawing?
 

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#2
The basic scheme is OK.

Make both the pots much lower than 100k. Something like 10k would be much better. You would need to calculate the ratio of R5 and R6 compared to the pot in order to get the range of control you need in the second stage, or just do it by trial and error. For example a 10k pot and 1k resistors would give a lot of gain alteration, a 1k pot and 10k resistors little gain change.

The second stage inverts... not a problem as long as you are aware. Simply swap the markings on the line inputs over to compensate.

LM393 is a comparator. No good. You need to use a FET opamp here because all the stages are directly coupled and so you want to minimise any small DC current flowing in the wiper of the pots. A TL072 or OPA2132 or OPA2134 would be fine.
 
#3
Mooly said:
LM393 is a comparator. No good. You need to use a FET opamp here because all the stages are directly coupled and so you want to minimise any small DC current flowing in the wiper of the pots. A TL072 or OPA2132 or OPA2134 would be fine.
Click to expand...

It is clear the LM393 is not right in this situation, it was only the first opamp that I had found in the drawing program, excuse me!
I had experiment on bread-board the first stage using a LM4562 with resistors 0.1% and the dc offset seemed ok (I also added a small adjustment of the dc circuit around the first opamp).

There would be a way to use the potentiometer 100k without incurring the problems described?
I ask this because the potentiometer is connected to an IR remote control board and if possible I would use it.
Just to clarify the first potentiometer I would like to use it as the volume control (Pot) for all channels, while the second (Trim) should be able to permit a delta +/- 6db completely independent for each channel.
 
#5
No, the first potentiometer is logarithmic, and in fact I mean to use it as a volume control for all channels, in fact, is a 6-channel potentiometer!
From what I understand with this type of scheme I create a certain dependence between the level of the first potentiometer and the gain of the second opamp ... if the second would be configured as a non-inverting, this interdependence should no longer exist, right? or how it would be better to do to make the gain of the second opamap completely independent?
 
#6
100k (and the following high impedance circuitry) will be more prone to allow stray noise pickup that a 10k but if you really want to use it then its workable. With 100k my suggestion of FET opamps may not be ideal, at least not for the second device. The 4562 is as good as any if you need to use a bjt device.

To overcome the interdependence involves either adding a third opamp as a buffer or adding a second volume control daisy chained to the first... which doesn't sound a good option tbh.
 
#7
Not clear from your schematic:
a) a single 100k pot drives 6 gain stages (that´s what your schematic seems to imply), is fed from a single balanced stage.
b) a 6 section pot drives 6 gain stages , one stage per section, all "hot" pot legs are in parallel (your schematic might still apply, as example of 1 section) is still fed from a single balanced stage
c) you have 6 balanced to unbalanced sections, 6 pot sections, 6 gain stages.

So, which one will it be?
 
#8
JMFahey said:
Not clear from your schematic:
a) a single 100k pot drives 6 gain stages (that´s what your schematic seems to imply), is fed from a single balanced stage.
b) a 6 section pot drives 6 gain stages , one stage per section, all "hot" pot legs are in parallel (your schematic might still apply, as example of 1 section) is still fed from a single balanced stage
c) you have 6 balanced to unbalanced sections, 6 pot sections, 6 gain stages.

So, which one will it be?
Click to expand...

c) I have 6 balanced to unbalanced sections to control together (6 channel ganged pot), each channel then should have its own indipendent gain control.
 
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