125 Theremin

The isolation transformer will be a blessing and a curse. You do need to isolate a circuit like that because that old AC/DC design that basically connects "ground" to one side of the AC line and that is very dangerous! However, isolating all the grounds is going to make the thing much more susceptible to ground loop problems.

The did those sort of things back then because it saved the cost of a power transformer. That I know of, no one ever made use of the ability to run those sorts of things off of DC ... not too many 115V batteries available!

In retrospect, you could probably make this amp work with an isolation transformer, but you will need to do a few things regarding grounding.

1. Use insulated jacks on the input/output.
2. Connect the electrical ground lead (usually a green wire) to the metal chassis.
3. Connect back-to-back diodes in parallel with a 0.1uF, 250V capacitor, in parallel with a 10 Ohm, 5W resistor. Connect one end of this to the metal chassis and the other to the ground connection of all the rest of the circuit.

But you will need to make sure that anything that directly connects to the original chassis is isolated from the metal chassis of your cabinet.

Doing all this will yield an amp that will have minimum hum, but that will provide sufficient grounding to work with your theremin.

Don

Why not skip the middleman and contact Art directly?theremin1@att.net

Hi everyone,  I know this is an ancient thread, but anyway, please note that AT&T email is pretty much defunct, having gone through a zillion corporate acquisitions and subsequently becoming massively unreliable.

My current email address is arthur@harrisoninstruments.com, and I'd be happy to discuss the 125 design.