Goals for a TW Theremin

In this thread, @dewster proposed the idea of us collaboratively designing a "TW Theremin" that would meet the following goals:

1. Be simple to build with common, inexpensive, readily available components.

2. Require little or no fiddling in order to get it operating correctly (no scope).

3. Give satisfactory results in terms of linearity, sound, stability, etc.

I love the idea.  Let's use this thread to flush out additional goals and non-goals so we can scope out the project.

"I would be very forth coming in my research but to be trashed by TW engineers who have openly trashed every theremin out there gives no constructive benefit to revealing anything." - RS

Chris, There is "constructive benefit to revealing" even if what you reveal is "trashed".. We dont get better because people pat us on the back and say how wonderful we (or our designs) are - we get better when we listen, learn, and try to do things better if we got them wrong.

"If I humbly include myself we have four active people I know of at TW that design."

The majority of my designs are published here and at Element 14 .. I probably will not be contributing much to this project.

Also - I advise anyone wanting to keep copy of my designs, to download them now.. It might not happen, but I might be required to remove some of my stuff from public access.. all depends how my negotiations with a manufacturer go, and how strictly they wish to enforce our agreement/s.

Fred.

Thanks for your thoughts guys. Going with dewster's original idea, I think we should try to keep it relatively simple and not load it up with radical new deisgns, differentiating features, or anything that looks like it should be patented.  That said, I'd still like to include the goal:

4. It makes use of heterodyning to help builders grasp the principles behind the original theremin.

Perhaps we could also embark on a "TW Optical Theremin" project at some point too, but let's keep it basic for our first attempt.

I'd like to propose a modular design.

Identical, independent oscillator boards that can be configured as pitch, reference, or volume with only parts changes.

A pair of those with a simple diode detector would yield a pitch only instrument that could be upgraded later with linearizing coils, a better detector and a volume section.

Later still, add timber control, pitch preview, etc.

"4. It makes use of heterodyning to help builders grasp the principles behind the original theremin."  - Jason

That's a good project-focusing addition to the list.

I don't have any specific plan for an analog Theremin, though the idea has been floating around in the back of my head for a while. 

One of the design goals might be to use non-adjustable inductors.  People have trouble finding the specific ones needed or trouble finding the proper cores / forms / wire for winding their own.  The adjustability of several of these in my EWS leads to a difficult and non-intuitive calibration procedure that is probably beyond the average experimenter without a scope (I'm kind of surprised they offer the EWS as a kit for this reason alone).  FredM's capacitive tuning via antenna length might be useful, and might get around the need for adjustable capacitors (also somewhat problematic components).

Not to go on and on about CMOS, but I get a bit of a rush every time I see a schematic that uses them because the potential is obviously there to build something quite simple yet effective.  And I favor these high gain oscillators because they don't stall easily.

I also wonder if a higher frequency design might be able to obviate the need for large linearizing inductors, or even an inductor there at all?

For playability a lower sensitivity design is probably called for.

I don't think we can avoid adjustable inductors due to variable component tolerances, but they are available from digikey and are cheap. Adjustable capacitors should not be needed. Diodes or transistors can and do serve this purpose. The EW uses transistors as electrically controlled capacitors to fine tune the pitch and volume oscillators.

For pitch tuning, the EW adjusts the reference oscillator, and this is not the best place to do this. Fred's idea tunes the antenna circuit, which is better.

Keeping adjustments to a minimum has always been preferable.

The important adjustments are setting the pitch and volume oscillators to the resonant frequency of their antennas and can be done with an RF probe and volt meter.  Very easy to build and use.

One of the goals of this project is learning how a heterodyning instrument works. I think documentation is at least as important as circuit design. If the builder understands how the antenna circuit interacts with the oscillator, tuning will be much more intuitive.

High gain in an oscillator is both a blessing and a curse. The currents in a resonant circuit are much higher than in the bits driving it. This can lead to instability due to component heating surprisingly fast, even in low powered circuits. The goal is just enough gain for reliable operation.

Uncle Bob's EW oscillator has a lot going for it, but obviously we don't want to rip it off. There are plenty of other proven designs, although I think it would be best to avoid taped inductors.

Higher frequency's do require smaller linearizing inductors. There are compromises. The linearizing inductor forms a series resonant circuit with the antenna and hand capacitance. As we move up in frequency, the inductance needed goes down, but the antenna capacitance says the same, therefore the pitch range goes up because the ratio between L and C is greater.

Higher frequency = more octaves.

If we go too high, we run into the AM broadcast band. A theremin operating on the same frequency as a 10,000 watt transmitter would not be a happy thing.

I don't think we can eliminate linearizing inductors. To be playable a series resonant antenna circuit is needed.

Rob.

w0ttm, good points.

I'm hoping we could think outside the box a bit for this project.  Is there really no way to avoid adjustable inductors and capacitors?  The EWS has multiple interacting adjustments inside and outside, too many really.  I'd like to just turn it on and have the pitch and volume sections in the same state they were last and not have to adjust it at all (other than for brightness / tone / etc.).

Some retreading of past designs is unavoidable, but borrow too much and one finds oneself on the same roads, making pretty much the same decisions that everyone else has.  That may or may not produce a simple, robust, responsive, and easy to build design, but it is a rather uninspiring process IMO.

If we tried higher frequencies we might run into whatever the heck is going on in RS Theremin's designs!

I have no problem with higher frequency's :) I've bumped the frequency up a bit on my Kustom with good results, and my Kep will also run higher than originally designed.

And I'd love to push the envelope a bit. It's just those dang laws of physics that keep wrecking my fun. Affordable inductors are usually rated at 10% tolerance, and active devices, be they tube, FET, MOS, or Kryptonite have different capacitance from one device to the next, even in the same batch.

I even ran into that wall when working with (very) high priced RF transistors.

"The box" is a tireless and unspeakably evil thing.

I'm thinking on this a great deal. Between us all, maybe a solution will present itself.

I've even considered crystal control.

"For pitch tuning, the EW adjusts the reference oscillator, and this is not the best place to do this. Fred's idea tunes the antenna circuit, which is better." - w0ttm

Yes, I believe it is better..

But - It is also a lot more "difficult" .. The principle is that one has a fixed reference frequency, and varies the equalizing inductance to perform tuning (varying the resonant frequency of the VFO tank doesnt do the job).. It is also possible to achieve tuning by varying the antenna capacitance (using a varicap diode circuit) but this also has major headaches.

My circuit uses a specially made module with custom manufactured saturable core reactors that electronically adjust the equalizing inductance - Development of a stable design took me several months, and involved a lot of coil winding - There are no off-the-shelf parts suitable (small signal magnetic amplifiers and saturable core reactors is a technology which was in use before tubes, tubes - with their simpler amplification - stopped further use and development of this technology in all but high power applications).

So, much as I think my scheme is the best (LOL) I do not think it is ideally suited to the TW theremin project.. My design, for example, monitors the temperature in each reactor and compensates for drift - calibration of this is only feasible if one has a 'oven/fridge' (peltier cooling / warmin box) into which one can place the circuit, set a high temperature , calibrate at this high temperature, set a low temperature calibrate at this temperature... One then has a circuit which never drifts - otherwise it tends to behave more like an audio thermometer than a theremin! ;-)

The simpler varicap method does work - effectively changing the capacitance seen at the antenna, thus tuning (via the antenna circuit) the VFO.. But there are problems with this which IMO make this idea unsuitable for for the TW project.. The varicap circuit sits at the most sensitive point in the circuit, so any problems are difficult to debug.. Also, the voltages at the antenna can (should) be high - this causes difficulty in terms of the control voltages one can apply to the varicaps.

A simple adjustable (tuning) capacitor on the antenna is the simplest (and the method used on the Ether Vox) but this capacitor must be extremely high quality, as any drift in its capacitance due to plate expansion etc directly translates into change in pitch.. Good quality variable capacitors are difficult to get, and not cheap. (or at least I have not found a supplier of these - If anyone knows of a source of good variable capacitors I would be most interested).

Bottom line ? - Me thinks that for this project you are probably better off tuning the reference oscillator!

Fred.

There is another way which I will only hint at.. It is the method my latest theremin uses.

Dont have a tuned antenna circuit at all.. no antenna inductance. Make the VFO linearise the response .. take the VFO frequency into a tuned circuit (much like the circuits used in some volume antennas) and use the voltage from this to adjust the VFO frequency. Doing this one can get exactly the same response using a tiny inductance (in the filter) as one does using a large EQ inductance combined with the tiny antenna capacitance.

Its so simple and bloody obvious it makes me laugh - I wasted months in developing my saturable reactors so that I could change the large equalizing inductance - when all I needed was one IFT to make a filter! ;-)

But more than this I cannot say - And I will probably delete this posting in a few days.

Fred

One other tip - you dont need any seperate oscillator for the volume antenna - simply buffer the reference oscillator and drive this into a tuned circuit formed by a large L in combination with the volume antenna capacitance..

As the volume antenna will be pumping out the reference frequency, there is no problem of interaction.. no possibility of 'birdies' or 'ghost tones' .. and tuning is a whole lot simpler!