Relay switching 2 Master Volumes. Best practice?

[adew]

Wondering which of these methods is better for switching between two independent master volumes in a Marshall style build. My so-called "Plan A" uses the DPDT relay to switch in/out the MV pots' wipers and grounds. "Plan B" switches in/out the MV pots' inputs and wipers. Is one way better than the other, in terms of likelihood of pops or other issues?

Plan A:


Plan B:


Tone stack on left, output to PI, in case it isn't obvious. "XX" is ground.

Any other options? Anyway to do it with a SPDT relay?

Thanks!

[adew]

No opinions?

One possible advantage of the Plan A option, from a layout perspective, is that the Treble to MV wiring can be kept on the pots themselves. Therefore, I would only need shielded wire from the MV wipers to the relay. Not sure there's any need for shielded wire to Ground in this setup. Plan B would have more complicated wiring from pots to relay and back again.

Thoughts?

[pdf64]

Plan A leaves a lot of unused signal circuit hanging around, which will act as an antenna; high signal voltage, so a transmitter, but high impedance, so also a receiver.

An SPDT could be used to just switch the wipers, leaving both the pot tracks permanently in place. I think some commercial designs do that. If the fx loop is fitted, then that would definitely be the best option, as it should have no problem driving 2 pots in parallel, could bring the pot values down too, eg maybe 100k would be ok, thereby improving parasitic characteristics.

[adew]

Thanks pdf64, hadn't thought of that disadvantage of Plan A. Good point!

I had been thinking about only switching the wipers using only one half of the relay, but I had been wondering about possible negative effects of leaving the pot tracks always connected. Doesn't the inactive pot act as a resistance in parallel with the active pot and therefore has some effect on the apparent taper of the active pot? If yes, I suppose the question I'm really asking is whether this effect would be noticeable in use.

For info, yes, there will be a Metro style FX loop fitted.

The more I think about it, the more I would like to go the SPDT route, as that will free up the other half of the relay for something else.

[pdf64]

Doesn't the inactive pot act as a resistance in parallel with the active pot and therefore has some effect on the apparent taper of the active pot?

No, the downside is purely that it halves the load resistance on the preceding circuit. Which would be a concern in the case of a high impedance tone stack, not so much with the return stage of a loop.
I suggest to check what minimum load the loop’s return stage will happily drive; 250k or 500k pots will have less tendency to a fizzy timbre (due to the lower RC of the parasitic capacitance between terminals etc) at low settings than 1M.

[adew]

Thanks for the clarification on that question. Very useful. Cheers!

[Traynor]

Could you fill us in on what you ended up doing. I'm looking to do the same thing.
Thanks

[FUCHSAUDIO]

Doesn't the inactive pot act as a resistance in parallel with the active pot and therefore has some effect on the apparent taper of the active pot?

No, the downside is purely that it halves the load resistance on the preceding circuit. Which would be a concern in the case of a high impedance tone stack, not so much with the return stage of a loop.
I suggest to check what minimum load the loop’s return stage will happily drive; 250k or 500k pots will have less tendency to a fizzy timbre (due to the lower RC of the parasitic capacitance between terminals etc) at low settings than 1M.

You may lose a little gain, and you may alter the way the tone stack works, loading it with 500-K instead of the 1-Meg it was designed for.
Use 2 2-Meg pots in parallel, and select between the wipers, put some high value resistors on the relay contacts (10 or 20-Meg), and you should be fine.