How can I change the gain of this IC stage?

[John Lyons]

Showing my weak skills here. 

[PRR]

This is a minimalist phase switchable inverter/noninverter.

The non-inverting mode is ONLY unity gain.

You seem to have modded for the inverting mode to be gain of 10. But that is a huge jump from - mode to + mode, not what we usually want in audio.

Go back to the basic switchable inverter/noninverter and add a gain stage after it.

There may be a more clever way, but a TL07x and two resistors is 42 cents, any brain-pain is billable as more than a buck, KISS.

[John Lyons]

Thanks Paul!
So would the gain in inverting
mode be set by R7 and R8 or just R8?

EDIT:
Ah! Found my mistake.
R8 should be 100k not 1M.

[Rob Strand]

Instrumentation designers tend to use this circuit,



The input is switched other V1 or V2 while the other input, V2 or V1, is switched to ground.  So the switching is more complicated (2x SPDT's).  The reason they do this isn't perhaps so obvious.   The opamps have a DC offset.  When you switch the input like this the DC offset at the output doesn't change.

In many other configurations the DC offset at the output changes in the positive phase and negative phase modes.   So when you switch the phase switch at some frequency the DC offset is modulated and creates a new AC artifact signal at the output.

For measurement circuits we often DC couple the output to the next stage.   But the thing is the artifact is AC and once it gets in you cannot easily remove it.

For audio we are often only interested in the AC.  However, we still don't want an artifact AC signal getting through.   There's a few configurations where AC coupling can avoid switching the DC offset and creating an artifact.  For example,

  Vin --->  Non-inverting gain stage  (output V1) --->  inverting-stage unity gain (output V2)
  If you take outputs V1 and V2 and AC couple them to a SPDT type selector switch
  the AC coupling removes the DC offset at the new outputs so when the switch switches there's
  no AC artifact created.
  If however you switch between the DC couple outputs V1 and V2 then you will get the artifact.

For some applications the phase switch is set once and left for a long period, in this case the artifact is a small click when you select the phase and after that no artifact.   However if the phase inversion is used for signal processing then the switching can occur at a high frequency and you will hear the AC artifact create by the varying DC offset.

For an octave it's a bit debatable how important it is to remove the glitch/artifact.   If you look at the Boss OC-2 schematic I think they do phase switching with a gain less than 1.  IIRC, as shown it's not quite right but if you swap the positions of the 100k and 47k on the opamps + inputs it kind of works.   I don't recall a gain greater than unity being possible (with the one opamp ckt).

https://www.hobby-hour.com/electronics/s/oc2-octave.php

[John Lyons]

Thanks Rob.
I had seen the dual switch version
but wasn't sure what was going on there.