Why does my Etherwave make funny noises when I touch the volume loop

Posted: 5/19/2010 10:42:59 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as WaveCrafter.com . ...................................

Joined: 12/7/2007

[i]"And yes, to physically touch the loop greatly impacts the pitch." - Kevin[/i]

I suspect the same mechanism is responsible.. The loop will be capacitively coupled to the Theremins grounding - so touching it adds this coupling to any other ground coupling.

If my above hypotheses are correct (and apply to your Theremin) you should find that playing the volume antenna with your hand as far from the Theremin (i.e. - as far left, assuming volume antenna is on the left) as possible, will reduce the effect on pitch..

In fact (this might be an easier way to solve my problem) if one could extend the 'arms' of the loop, using non-conductive material, and have a thin wire between conductive loop and its connection point (so that this contributes little to the loops capacitance) the extra ground loading caused by the hand being close to the Theremin could be significantly reduced.

For example:

The other 'fix' would be to increase your body's ground coupling so that changes caused by the hand coupling to the Theremins ground become insignificant..

An earthing strap connected to (say) ones RIGHT leg should keep you fully grounded and eliminate any interactions - but be sure not to touch anything with 100V or more on it! Right Arm to Left leg is the most dangerous conduction path through the body.. and it only takes a few mA to kill you.. So best to put a high voltage 10nF capacitor between you and ground (at 50Hz, 240VRMS 1k body resistance this would limit current to +/- 1mA.. Body resistance is normally much higher than 1k, so current would be more like 100uA [1/10th mA] and you would probably survive anything less than 10mA) - this should greatly reduce any risk.. OR.. You could use the ground on your earpiece (again, put 10n in series with any connection to body).. modify it so that, perhaps, there is a conductive clip for your ear lobe.. I think one can buy such clips from medical suppliers.

If you think I am being pedantic about safety, you are probably right! - But I have seen too much badly built equipment to presume that one can touch something and be safe.. If one touches something live, you will feel a shock but usually survive.. If one touches something live, and have a part of your body connected to ground, there is a good chance you wont survive!

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