Balanced Line volume control (Leslie 122)

[Rhodesplyr]

Over time, I've noticed that the volume control on balanced line Leslie 122 amps tends to be very abrupt--and this is with audio taper pots. You don't get much useful control range between silent and full volume.

I'm assuming this happens because the way it works is to blend the out-of-phase balanced audio signals. It also presents a varying load to the organ preamp's output transformer as you move the pot. In contrast, the 147-type unbalanced Leslies do have a user-friendly range of volume adjustment.

This is mainly a theoretical consideration, but I was wondering if something jumps out at one of the circuit experts on here in terms of why the volume control setup in this amp has such an abrupt taper. I'm thinking of experimenting with a lower value audio taper pot to see if that works better.

[Steve Conner]

It's because the volume control is used as a variable resistor, shorting out a signal whose effective impedance is only 20k.

I'd change the control for a 10k audio taper one. Or better still, replace all of R1, R2, R3 with a 100k dual-gang pot. The wiring will be similar to the so-called Post-PI MV for guitar amps - except you can't ground the bottom end of the pot, because the Leslie uses the common-mode voltage to switch rotor speeds.