Variable voltage from batteries

Site reader 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 could change the voltage from one setting to the next.  In this case, you 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.


Wiring

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 time:




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.





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