Toggling to a 2nd Output Volume Pot in Overdrives?

[cthilley1]

I'm building a Mach One overdrive and had a thought. I would like to toggle between two volume pots at the end of the circuit to achieve two different output volume "settings." This is what the output section looks like, pretty simple:


Could swapping between two pots be achieved with an SPDT switch if it were wired in this way? Or will this cause problems?



Would at least a DPDT switch be the only good way to do this, to break the flow both before and after the pot while keeping it connected to ground? I didn't know if that might be redundant for some reason:
Extra info: I am already adding a 3PDT switch, one pole/throw is dedicated to adding hard clipping diodes by connecting to VREF, and i was hoping I could use the second pole/throw to achieve toggling between two output volume pots to compensate for volume loss of adding the hard clipping diodes. I'd like to (ideally) use the third pole/throw set for an LED indicator, but can forego that if I need to.

Some education here is appreciated!

 

[giovanni]

You can’t use an SPST here. In your schematic above you can see that the pot that’s supposed to be disconnected still makes a connection from out to ground. So the two settings would always interact with each other which I don’t think is what you want.

 

[DGWVI]

You really only need to swap which wiper is connected to the output. So, that'll leave you one pole for clipping diodes, and another for LED indication if you use a 3pdt

 

[cdwillis]

You really only need to swap which wiper is connected to the output. So, that'll leave you one pole for clipping diodes, and another for LED indication if you use a 3pdt

The other pot will still be connected and will be affecting the output impedance and messing with the output to the other pot.

 

[cthilley1]

The other pot will still be connected and will be affecting the output impedance and messing with the output to the other pot.

Both of these responses make sense to me, but I guess my question is, if I go with just swapping the wipers that are connected to output (for instance) will the amount of interaction render the two volume pots effectively unusable? I planned to mostly set and forget this pedal, and the second volume pot is just to have that second preset to compensate for volume drop with hard clipping diodes.

I can live with some interaction between the two pots, but would it just mean each pot would need to be turned up “louder” to reach the volume it would otherwise normally reach? Or would something else happen?

 

[giovanni]

I think you may find that you set pot A to a volume that works, then switch to pot B, set it to a volume that works. Then when you switch back to A your initial setting no longer works for you, so you have to adjust. And then you have to adjust B again. And so on. It may work after several iterations but it’s a special kind of hell I wouldn’t recommend to my worst enemy.

 

[DGWVI]

The other pot will still be connected and will be affecting the output impedance and messing with the output to the other pot.

Yes, but if only the wipers are switched, impedance will not be effected in any meaningful way. The pots' value will essentially be halved, but that's not a terribly important parameter for a passive output volume

I think you may find that you set pot A to a volume that works, then switch to pot B, set it to a volume that works. Then when you switch back to A your initial setting no longer works for you, so you have to adjust. And then you have to adjust B again. And so on. It may work after several iterations but it’s a special kind of hell I wouldn’t recommend to my worst enemy.

This will not be the case.

Whipped this up to demonstrate. Lugs 1&3 of both pots soldered to board, only wiper is switched. No interactivity

 

[giovanni]

I was still thinking about the OP schematic with the switch disconnecting the pot. Now that I think about it, if you switch the wipers instead, there will be no interaction between the two pots because electrically the switch doesn’t change the circuit (it did in the OP). I would only recommend doubling the resistance on the pots since they are in parallel.

 

[CR0SSBL4DE]

1. Switch both pins 2 and 3 to have the exact same circuit.
2. Switch only pin 2 to have about the same output impedance when the volume is not at max. But circuit will see the volume pot as half the actual pot resistance.
3. Switch the pin 2 and double the pot value to have the same resistance seen from the circuit, but higher output impedance when the volume is not at max.

Seeing the whole output stage, I would do option 2:

• Super small tone control change: "biggest" difference will be with tone pot at minimum, cutoff freq will move from 226Hz to 233Hz (about a quarter tone), that's imperceptibly small.
• Super small volume drop: "biggest" difference with volume pot at maximum, volume will drop 3.1% (that's -0.27dB, also imperceptible).