Theremin Pseudo Science

unobtainium = large sums of cash for use on fun projects. :)

There are some ways to obtain some of the above "unobtainium" - but you need to be willing to take some big risks!

Some famous musical instrument manufacturers started life in this way - They had incomes, but realised that a lot of this income (enough to fund their fun projects) was being paid to the tax man.. So they set up small companies doing "R+D" with no (real) intention of making money.. Simply of "losing" money on their fun projects - money which they could write off against tax.

In order to meet the legal requirements, they had to advertise product (actually be offering a product or service) - and much to their surprise, there wasa a big demand for their deliberately over-priced kits..

I believe that one of the best known American Synthesiser KIT manufacturers started life this way, and I worked for a synthesiser manufacturer in the U.K. (early 80's) who also started this way.

Some interesting science has come to light in my recent experiments.. These effects appear, at first, extremely mysterious..

My discovery was that shape and position of antenna can alter audio output tone..

I found that there is an audio feedback mechanism via the pitch antenna and (less noticably) the volume antenna.. It works like this.. If the antenna vibrates at the audio output frequency, or resonates at a particular frequency, this vibration / resonance modulates the frequency of the oscillator because the antenna is working like a capacitive microphone!

I previously thought that antenna shape could have no bearing on tonal quality - but have found that this is not true.. An antenna which is highly resonant at some audio frequency will apply some FM distortion to the Theremins output audio at that resonant frequency.

There are an infinite number of possible permutations on what the effect of antenna 'microphony' will be on the output audio - all sorts of factors such as phase angles etc - and in most cases the effects are neglidgable.. They only really become observable if antenna vibration is large (I noticed when I had a breadboard on top of a monitor, with the gain set too high) - Since then I taped a length of piezo cable to an antenna, and drove this hard from the audio output - this gave antenna vibration levels you would probably never see under normal playing conditions.

From personal experience, there is a lot of "art" at putting these things together and making them work well; antenna spacings, shapes, diameters, whether or not both ends of the loop are connected, lead lengths to the antennas to the coils, etc.

However, the generation of absolutely pure sine waves in the oscillators will tend to remove overtones, or "character", from the theremin's sound. This is why I hesitate to clean up my theremin's waveforms more than removing the 60 Hz riding over from the filament supply.

But once its all working well, and that tuning tool hits the sweet spot, and all three oscillator circuits work perfectly together, then the magic occurs, and your pride sings for you and almost has a soul of its own.

Such is the joy of building. I praised God when Gabby sang for the first time.

Philip

Speaking of pseudo-science and accidental microphony, there was one made the science periodicals a while ago. The theory goes like this, with some embellishment...

Ug, the caveman, is painting a stalking sabre-tooth on the cave wall with a brush. His assistant is speaking loudly, just next to him, and the sound vibrations cause the bristles of the brush to vibrate as he applies the paint, leaving a recording in the strokes of a caveman saying "it's getting closer" and "can't you paint from memory?" and "aaaaaaaaargh!"

RE: Removing 60Hz from filament lines ...

There is a very simple and effective way of reducing 60Hz noise from filament lines. At a point near the power transformer, solder 2 100 Ohm, 1/2W resistors in series across the filament leads. Solder the connection between the 2 resistors to the chassis or run a wire from them to GND.

This technique works like a charm! It has been used in nearly every tube guitar amp ever built.

Don

Hum in tube guitar amps ...

The remaining hum in older tube amps is not due to filament noise, but rather to the limited size of filter capacitors. In the better amps they used a LC pi filter (caps to GND from either side of a large iron choke). But large iron chokes were expensive.

Nowadays we have these great high value, high voltage caps we can use ... 100uF, 350V, for example! vs the 10uF - 20uF caps from "back in the day".

Don