Goals for a TW Theremin

Another wild a$$ed idea.

I've been playing with a simple, op amp based tuning indicator that shows when the VFO is at antenna resonance.

Maybe make both oscillators tunable. Set the VFO at resonance per the tuning indicator, and zero beat with the reference oscillator. One could also set the VFO slightly above or below resonance to change the linearity if desired.

On the other hand, an adjustable antenna is quite simple, and uses zero extra electronics.

Just thinking out loud......

"I've been playing with a simple, op amp based tuning indicator that shows when the VFO is at antenna resonance" - w0ttm

However it is used, this would certainly be a useful addition - either fitted in the theremin, or as a tool for constructors.

I have a simple circuit which takes input from both oscillators, and outputs a logic state dependent on thich oscillator has higher frequency.

For a more complex theremin, both of these circuits could be combined and provide the means to automatically tune the theremin for optimum linearity with the player holding their hand where they want the null point positioned.

However - The above are extra complexities - As soon as one has any kind of manual tuning using electronic components, one introduces the real-word temperature dependancies of these components .. Devices like varicaps can be a real pain, but even simple components like potentiometers or tuning capacitors vary with temperature - so even if one compensates the active components using temperature monitoring or ovening, one is still left with thermal errors from the controls.

The idea of tuning the VFO to the correct operating point to provide best linearity is a good one - It might even be possible to have both VFO and Ref adjusted using the same control (a variation on the automated method).. Player puts their hand at null position, and tunes for resonance - Ref is tuned electronically until it is at same frequency as VFO..

When I explored automated tuning, I had a MCU in control of the process - player pushed a button to start the process, red LED blinked for a few seconds to allow player to position themselves with their hand at the null point, LED went red while calibration was performed, and went green when process was completed - I had the same for the volume antenna, where the hand was placed at max and min positions and each was calibrated.

But - Like almost everything I do, it was over complicated - and ends up in my box of brilliant prototype boards which are going nowhere.

Fred.

Thanks Fred. This idea occurred to me while tuning my Kustom. I put a loosely coupled one turn coil on the VFO inductor, and watched it with my scope while I tuned for a a dip. The only place I found on the Termen oscillator with enough voltage differential at resonance was the source resistor, and even that is small. Fortunately, that's a low impedance point and anything connected to it won't have much effect.

In fact, this is so solid, I don't think a hand grenade would stall it.

Another way would be a couple of turns at the bottom of the antenna inductor, like the original RCA volume circuit. The more current in the antenna coil, the more voltage on the sampling loop.

Even without a built in tuning indicator, this looks like a place to plant a test point.

"Another way would be a couple of turns at the bottom of the antenna inductor, like the original RCA volume circuit. The more current in the antenna coil, the more voltage on the sampling loop." - w0ttm

"problems" like this make one wish you could turn a spice primitive into a real component, dont they ?  - Imagine being able to buy a CCVS, 4 terminals, put a current through two, get a proportional voltage from the other two.. set the multiplier to whatever you want, or put a equasion in to get a non linear relationship - Bingo! ;-)

Talk about crazy ideas though - two npn opto isolators wired with both LEDs turned 'bipolar' as in, D1:A <-> D2:K, D2:A <-> D1:K  .. Put the diode pair in the current path (it will be << 20mA) and the transistors C-C, E-E, 0V to E's, C's to +V via ( a probably quite big - depending on CTR) resistor, measure DC on Collectors.. No rectification required, just a smoothing C.

Wonder if the Vf of the LED's will cause a distortion problem, whether it will affect the Q etc..

???

Fred. 

- this might just work for volume..

They might cause a bit of quirkiness on the pitch side, but I believe it is a workable volume detector, if the LED's are fast enough.

I just had another thought :)

A pair of FET opto's , or LED's glued to LDR's and you have a crude, but complete volume circuit, bearing a great deal of similarity to the RCA design.

Looking at the split inductor configuration got me wondering..

Off the top of my head, I cannot see a reason why this couldnt be done with a parallel LC tank circuit - even perhaps with an EW..

Ok - Some sums would probably need to be done, and some changes made (perhaps) - but if the antenna connection (to the VFO) was moved to the junction of L5 and L12, then the antenna inductance would (I think) only be acting across L6, reducing the number of playable octaves, but improving stability and possibly linearity.

In fact - If one had a tapped inductor and a switch (or a switch to select either the mormal connection point or the "mid" connection point), one might even be able to select the number of playable octaves...

The above is ONLY a "Perhaps"..

Fred.

"A pair of FET opto's , or LED's glued to LDR's and you have a crude, but complete volume circuit, bearing a great deal of similarity to the RCA design" - w0ttm

Or a single H11F1, a bridge rectifier made from low Vf signal diodes (or 1n4148's may even work) connect 'AC terminals' of BR in series with antenna inductors, connect 'DC terminals' of BR across LED, stick a big (22uf?) tantilum across LED perhaps ..

Provided the amplitude from the oscillator waveform is sufficient to easily overcome the combined Vf of the diodes + LED (two diodes on per 1/2 cycle = say 1.4V, plus LED Vf at say 2V, total Vdrop = 3.4V.. Tank waveform +10V / -10V - it may just work..)

One then has possibly the simplest volume control combined with VCA..

Extend this simplicity by bufferring the reference oscillator and using this as a fixed frequency volume oscillator, with the current being determined by this reference frequency into a variable resonant volume antenna circuit (This works beautifully, and bypasses all the problems of volume-pitch oscillator interactions.. I do it slightly more complex - I multiply the reference oscillator *2 or *4, as my volume resonant circuit has some other things to do as well... ;-)

Fred.

Ps - I am not really into the kind of simplicity of the H11F1 idea - it is actually quite limited - I like to have control over such things as the impedances of my signal paths, and with the idea described above one is at the mercy of whatever currents you can get from the antenna circuit - low currents mean the H11F1 resistance will be high even when at max volume position (in series or pass mode) or not particularly low when used in shunt mode - High resistance means greater noise.. Also, to avoid distortion signal levels across the H11F1 must be low - this means gain stage/s must follow this VCA.. The combination of low signal levels and high impedances means that even if this works, its not going to be good.

Agree on all points. That's why I used the word "crude".

But......ya' know I'm gonna' have to try it just to see what happens :)

This is just too much fun...Rob.

"But......ya' know I'm gonna' have to try it just to see what happens :)" - w0ttm

Yeah, I know that force! ;-) .. I have yielded to it far too often... For me though, those days have probably gone - I think I get my fun now from coming up with ideas, and not so much from weilding a soldering iron to create yet another breadboard to join the others in my junk box.

Fred.

I think it is probably time to re-visit this topic..

Particularly as a TW member is now advertising that they may be selling the "TW Theremin" next year.

To quote from their web site: "The Complete Theremin "enigma" This may be re-introduced as the $125 TW Theremin"

I do not believe any person has the right to use the "TW" reputation to promotes something they have developed on their own (or perhaps copied from stuff published openly on TW).. The TW project, as I see it, is OPEN - As in, anything used in any "TW Theremin" should be fully published at TW and fully shared with this community, and should be approved by this community and / or the owners of "Theremin World" first.

I also object to this individual berating those WHO HAVE CONTRIBUTED MOST technical material related to theremins, or more particularly the TW Theremin, here at TW (us "arrogant European Engineers"). The TW theremin is a collective effort, to which this individual has contributed NOTHING.

I believe that this individual should apologise here at TW, And that those who (by default) own the copyright to "Theremin World" should prevent ANYONE from producing a "TW Theremin" which is not at least fully published on TW

Fred. (An arrogant EUROPEAN Engineer).

(I also find reference to nationality somewhat offensive - I am British, Thierry is French, and there are other great theremin developers in the EU. So What ? As for "Arrogant Engineers" - I would sooner buy an electronic product from an arrogant engineer than from an ignorant hobbyist.. Regardless of whether this was an ignorant European hobbyist or an ignorant American hobbyist.)

ps - Regarding the words "arrogant" and "ignorant"..

I would say that anyone promoting their simplistic $125 hobby design as superior to the EW-Pro, is delusionally arrogant.. The inferrence is that they are vastly more knowlegable and competent than Bob Moog, something only the most deluded arrogant wanna-be theremin designer would assert.  And anyone declaring that a theremin would not work in a vaccum (because the vaccum will short effectively short circuit the "field") is laughably ignorant about capacitance and the fundamental operation of the theremin.

IMO, Such an individual must not be allowed to decieve anyone into believing that TW endorses him in any way.