GT Theremin

Thank you for that explanation. Very understandable.

In my not-quite-a-theremin-yet breadboard, I can change the osc frequency by wrapping my hand around either oscillator coil. Curiously, until I had the pitch and local oscillator frequencies fairly close together, the external antenna did not affect the frequency of the pitch oscillator.

I will take that 200kHz frequency suggestion posted up a ways in this thread under advisement ... looks like I got some more winding to do :(

Don

This is great. I built a Keppinger theremin and let me tell you that the tube jobs have a unique sound all their own. Having rod antennas sized about the size of an Etherwave or RCA may improve sense distance. Also, as other have said, the frequencies may be important as well. My Keppinger operates at roughly 190 KHz for the pitch oscillators and around 400 KHz for the volume oscillator.

Congratulations on your project. Please endeavor to play it and get better. As you gain in experience you will likely modify it along the way.

Regards

Philip Neidlinger

"I will take that 200kHz frequency suggestion posted up a ways in this thread under advisement ... looks like I got some more winding to do :("

Please dont take what I said about 200kHz as any kind of 'gospel' - it is simply what I have found on my (somewhat unconventional) circuits!

I think Matt probably got close to the reason though - Imagine one has an oscillator where the output frequency is a function of LC, and C varies by 10% due to object detection.. At 500kHz this will give a frequency variance of 50kHz, whereas at 200kHz the variance will be 20kHz - the usable audio output frequency / range / linearity will all therefore be affected by the COMBINATION of the oscillator frequency (at null) AND the variance of the sensed capacitance.
This does not explain everything I have observed - but perhaps does go some way to giving a clue about the best frequency for the oscillator.

Matt.. I have found you postings a delight! You have clear understanding of the principles and the rare ability to communicate these clearly and simply.. Your posting re wiring to antennas is an example of the kind of clarity and simplicity I envy! Great stuff!

> the usable audio output frequency / range / linearity will all therefore be affected by the COMBINATION of the oscillator frequency (at null) AND the variance of the sensed capacitance.

Also realize that the hand capacitance is probably not the same over all frequencies. Some time I'd like to construct an elaborate test for estimating the hand capacitance with various antenna geometries and at various frequencies. I did some impromptu testing at 1 kHz, but I don't suspect the results I got would hold very well at 500 kHz.

It's actually quite a difficult thing to measure simply (I used an impedance bridge, which is flawed methodology for many reasons), and I think the best way of doing it would probably be to build the actual Theremin oscillator you want to use, hook it up to an accurate frequency counter, do some calibrations to record an effective baseline impedance for the feedback network, and try your darndest to hold your hand steady at various measured distances while you record data.

> Matt.. I have found you postings a delight!

You are too kind!

I created an EXCEL spreadsheet to get a better idea of how many turns I would have to add to my coils to drop the freq. to 200kHz ... about 650T!

Not too practical given how hard it was to get the 100T with 2 taps wound. But if I add another 100T and raise the capacitance a bit, I can at least hit 500kHz.

I just had to be different (ARGH!) ... I chose 1" O.D. lucite tubing as my coil form. Cut it into 3" lengths and wound 100T tapped at 25T and 50T of AWG30 magnet wire on them. Unfortunately, with 100pF capacitors, the oscillators are sitting at about 1.4MHz.

There was another post somewhere about the "guesstimate" of 15pF for hand capacitance, so that sets the limit on how big of a parallel capacitance can be used ... 68pF - 220pF is probably reasonable. Above that the hand capacitance might not be enough to affect tuning.

I am still at a loss for understanding how to select a value for the large antenna coils people are using. If you are basically adding or subtracting capacitance from the tank circuit, why would you need or want a series-resonant circuit in your way. It obviously works well and gives better linearity from what everyone says. So is it there for impedance matching? Does anyone have some empirical design data for the antenna coil?

Also... a question for the tube theremin designers ... why doesn't anyone use the 6BE6 pentagrid converter tube (http://www.nj7p.org/Tube4.php?tube=6be6) for their mixers? These were used in every "5 tube superhet" AM radio ever built, I think. Since none of us guitar player types have had any use for them, there ought to be plenty of them to be had.

Don

[i]"I am still at a loss for understanding how to select a value for the large antenna coils people are using. If you are basically adding or subtracting capacitance from the tank circuit, why would you need or want a series-resonant circuit in your way. It obviously works well and gives better linearity from what everyone says. So is it there for impedance matching? Does anyone have some empirical design data for the antenna coil?"[/i]

The series resonant circuit formed by the coil and the antenna has to be tuned a bit below the oscillators default frequency. This allows the antenna capacitance to be more important, the more the hand approaches the antenna. So you come nearer to the exponential tuning behaviour that thereminists call "linearity".

This is definitely an Uncle Howie question. Get on Levnet.

Actually I am on Levnet and I will try asking "Uncle Howie" my questions there as well.

Don

Thank you for ignoring my explanation :-(