Variable voltage from
Bill Carnes came to me for
help in designing the switching for this as applied to his active EMG
pickups (which have a preamp built in). The idea is that you
change the voltage from one setting to the next. In this case,
have your choice of 9V, 18V or 27V, using three 9V batteries.
Bill already had a
4-pole, 3-position rotary switch on-hand, so I simply designed the
circuit to use that. You could do similar with two SPDT switches
as well (or three switches if you want to have 0V). This is
easier because you can't mess this up and try to turn on the third
battery without having the second one in the circuit.
As for what it does? Not much,
really. As I understand it, additional voltage in a preamp
doesn't really boost the output, just the headroom for the
signal. In other words, higher voltage to the preamp means you're
less likely to get clipping distortion if your input is too high.
Here's how it's wired (Professional-looking diagram courtesy of
Bill). Bill treated the series of batteries like a three-coil
humbucker that the switch then taps one coil (battery, actually) at a
Here's my diagram (not so
professional-looking, but better than the paper and pencil version I
sent Bill), which is wired a bit differently than what Bill ultimately
did. I treated the batteries like single coils and wired them
together one at a time with the switch, sort of like the wiring in
Brian May's guitar.
Pick which works better for you and let me
know if you found this useful in any way.