Hotrodded Gakken Mini

“Force sensing resistors...pencil lead volume sliders...solar cells...boy, y'all got no problem taking a simple problem and coming up with complex solutions!”

I hate to make this more difficult than it already is but trying one sensing resistor, one pencil or one solar cell could be a nice learning curve for your future projects.

As mentioned by Fred, using a sensing resistor is a good approach. You might try a CdS photo resistor removed from a bathroom night light. You could mount it in the handle of the ground grip you mentioned and control the volume and staccato by raising and lowering one finger. This approach is also "clean and distortion free" but you are going to have to use two parts on this one. ;-) You might get away with using just the CdS photo cell by itself if you don't need maximum volume cut-off.

A Finger Volume Control (http://www.oldtemecula.com/theremin/volume_cds/index.htm)

Christopher
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“Force sensing resistors...pencil lead volume sliders...solar cells...boy, y'all got no problem taking a simple problem and coming up with complex solutions!”

LOL! - My year of working on Theremin designs has led to some surprising discoveries.. "simple" ideas evolve into monster circuits, simulations which work superbly simply dont work at all on the bench.. then there is the damn bench - packed full of test equipment and loads of radiated signals and large grounded chunks of equipment / racks etc... you need to view waveforms and measure signals in the prototype - but cannot use a real antenna because of location - so use a variable capacitor instead - get it running, move it to a quiet low-background-capacitance zone, connect the antenna.. and it doesnt work (or requires adjustments you need your bench equipment for)..

I dont think the word 'simple' applies to any 'real' Theremin - And surprisingly, the 'simplest' good theremin design takes one right back to Lev's original ..

But you are right - it is a fantastic learning process.

However..... I do not think that turning the audio output on / off is at the high end of the "Theremin complexity spectrum" ;-)

Resistor didn't help.
Really I don't think it's an issue with the wiring as much as the antenna and/or circuit board itself throwing out a signal that the cable is picking up.

No, stop, ironic mode is on! The procedure below is dangerous and will kill you and your theremin!

You may perhaps disconnect the amp and feed 58V AC into the audio output in order to clean it up from noise...

"Resistor didn't help.
Really I don't think it's an issue with the wiring as much as the antenna and/or circuit board itself throwing out a signal that the cable is picking up."
There are, as I see it, 2 possible causes - The first is unlikely.. but IF you have no wiring / grounding problems.. well .. perhaps.. IF Modulated RF is being transmitted from the board, and IF this is being demodulated in the cable / connector / input circuitry (any non-linear component, or even oxide on the connector, might do this) then any attempt to break the signal with a switch, and loading the input with a resistor, may not work / may have unexpected results... Disprove this by trying a different amplifier and cable..

Otherwise, it is a less esoteric problem..
Ok - lets go back to basics.. In order for a "cable to pick up" signal, that signal must, somehow, be coupled to the cable...

Mechanisms for signal coupling are: Resistive, capacitive, inductive.. more than one of these can (and usually are) be acting at the same time. These are defined by their impedence (Z)

Now.. The amplitude of the coupling will depend on the source Z (sZ), coupling Z (cZ), and the load Z (lZ). The Output level (at the cable / input to amplifier.. ie, signal seen across lZ)will be = 1 / (Output Level * (sZ+cZ)/lZ)..

So, lets take an example.. 1V from a low Z output (say 100 ohms) via 100k coupling (cZ=100k)into a 10k load (lZ=10k) gives an input of 1/(100/10) = 1/10th of the signal from the source..

Into a 10k load resistor, at audio frequencies, extremely large capacitive coupling would be required to inject signal into the cable.. Inductive coupling is even less likely..

There is something basic wrong with what you are doing... I am starting to think that you have a grounding problem... If your cable grounding is bust, then connecting a resistor between signal and ground will have no effect.. There must be some problem of this kind... Audio frequencies simply do not fly through the Aether and impose themselves on low impedence inputs - and for audio, 10k is too low for capacitive coupling to enable enough signal to be coupled that it could be heard unless wired to a fully open Pink Floyd PA rig!

{sorry double post}

"IF Modulated RF is being transmitted from the board, and IF...[snip]... Disprove this by trying a different amplifier and cable.."

I've been trying it on two different setups - my bench-testing amp, which is a battery-powered Pignose with a cheap cable running into it (admittedly a potential source of all kinds of misdiagnosis), as well as my nice home guitar setup, which is a not-inexpensive tube amp that's functioned reliably for many years, and much nicer cabling. And all along I've been getting the same results on both.

The reason I brought up the (unlikely) scenario that the circuit itself is radiating signal, is because when the cable is not even plugged into the Theremin - just sitting on the table near it, nothing at all plugged into the Theremin and only the cable into the amp with nothing attached - I can STILL hear the background noise coming through the amp. Since both the amp and Theremin are running on batteries and sitting on a wooden desk completely isolated from each other, I don't see how it could be anything BUT the signal being picked up through the air.

But I do appreciate your "lets go back to basics" as I have never been "to basics" in the first place, let alone back to them. I have no theory and are just learning on the fly and I thank you for your patience and kindness of bothering to write that kind of knowledge up...I can take it in these small doses without getting confused (and bored).