Goals for a TW Theremin

Double ditto, Fred.

Varicaps would be a royal pain in the antenna.

RF Parts has some very good caps, but they are not cheap.

A good 2 to 10 pf is about $20.00 US

http://www.rfparts.com/caps_variabletuning.html

We posted at the same time again ;)

The above information is now tattooed onto my brain....Thanks a ton...Rob.

http://www.rfparts.com/caps_variabletuning.html - w0ttm

Thats a great link, thanks.. theres an 0.5 - 5 pf there for about $10, if its any good then this could be a solution. IMO, if one used this method (Bourns / Miller 630x-RC inductors for equalizing and a variable capacitor perhaps 'trimmed' with a NOP fixed capacitor in series) one could achieve good linearity in a similar way to how some of the best theremins (E-Vox for example) do. I think even a $20 capacitor is worth it - it certainly simplifies the electronics and set-up.

Fred.

RF Parts has always provided quality products. I've bought a fair bit of stuff from them, including one of their "store brand" 3-500Z tubes. It's been in my RF amp for a couple of years now and happily squirts 1000 watts to my antenna. 

There is one huge advantage to tuning the antenna circuit rather than the reference oscillator, which I have not yet mentioned..

With reference oscillator tuning, one is dependent on the antenna circuit charactaristics being roughly consistant - if antenna capacitance changes due to environmental factors, the linearity changes - this I have mentioned before - But.. With tuning at the antenna, not only can one compensate for environmental factors, you can also deliberately change the (actual)antenna capacitance to achieve useful results..

The sensitivity (number of octaves covered) is determined by a number of factors - a major one being the change in capacitance seen by the antenna.. If one shortens the antenna, and is able (as you can if you tune the antenna circuit) to bring the antenna resonance point back to optimum, then you reduce the number of octaves covered - and if you increase the antenna length (or diameter) you increase the number of octaves covered.

Everything becomes less critical at the antenna end - because you can adjust away any errors introduced by wiring or general construction. Antenna length becomes uncritical - you can start with a long antenna and cut it down to size until you get the desired range - you can even have several different length antennas you can plug in to get different coverages, or even use a telescopic antenna.

If you do extreme variation on the antenna regularly, you will have problems getting the tuning right using a single variable capacitor - even a 5pF variable capacitor requires only tiny adjustment to radically alter the pitch - you would need to have a series capacitor to reduce this - and for a system where one changes the antennas, a switch to select a few different series capacitors would make tuning easier.

Fred.

Linearization coils make a theremin more easy to play, at least in the highest register (>1.2kHz). But they make it also much more difficult to tune and the instrument will be more sensitive to the capacitive environment and the player's posture.

Several professional theremin designs achieve a decent but not perfect linearity without linearization coils : the tVox tour and (even better) the Henk theremins. In both cases, very low oscillator frequencies were chosen (135kHz for the tVox and 145kHz for the Henk). The rest is done by an intelligent choice of the oscillator's L/C ratio and (here comes the huge difference between both instruments) the ratio between the pitch rod's static capacitance and the hand capacitance. The tVox works with a big metal ground plate which greatly increases the static capacitance and it has an unusually thick pitch rod to "see" more hand capacitance while the Henk has an unusually long and thin rod to minimize both, the static and the hand capacitance.

I think (but that's my personal opinion and experience) that a design in which the pitch rod is part of the main tank circuit and thus without separate series resonant circuit would be more robust and easier to build and to tune. 

Less is more... ;-)

Perhaps what needs to be done first, is to draft a minimum specification for this new instrument, and to clearly define the objectives.

Some issues I can think about which 'should' be adressed are:

Power supply requirements - Single or split ? Battery option ? etc..

Playable range - As in, how far should the usable field extend ?

Pitch coverage - How many octaves should be required in the playable area (min and max) ?

Pitch Linearity - What is the minimum linearity target?

Volume range and linearity.

Waveforms and sound - How adjustable should these be ?

Bass stability - Design for good stability near the null point ? - Largely depends on oscillator syncing and locking (as in, the less oscillators pull on each other, the more stable the bass is ).. Isolating oscillators from each other with good construction, and buffering the oscillators pre-mixer achieves better bass stability, at the expense of the harmonics generated as oscillators pull each other.. Also, if equalizing inductance (or other equalizing scheme) is not employed, decoupling oscillators will make linearity worse.. This, IMO, is a big issue, as both playability and good bass stability are important (to me) - and I like the harmonics generated by oscillators pulling into sync.. :-(  not on the EW - its bass gets lost due to instability IMO - but from what I have heard, Thierrys isolation board sorts that problem out ).

Simplicity of construction / calibration - Is the TW theremin to be a "starter" theremin for complete electronics novices, or a theremin suitable for serious playing / learning / performing ? I do not believe one is likely to get anywhere near the quality of an EW without approaching the 'complexity' of the EW - The EW is an extremely simple design / construction, Bob Moog published it (the EM which is effectively an EW) as an article in an electronics hobbyist magazine - If the objective is to make a simpler theremin, dont expect too much from the TW theremin.

Why a TW theremin? Dont get me wrong, I think its a great idea.. But there are cheap simple kits for non-serious theremins on the market (SC / Jaycar being the main ones ) and, IMO, unless the TW theremin is A LOT better than these, there just isnt any point in spending time and money developing a new theremin.

Fred.

This is my opinion only - I think that using the EW oscillators ( I do not think this is "cheating" - the EM design was effectively put into the public domain by Bob when it was published, and IMO is there to be used for NON COMMERCIAL projects like this ) is the most reliable starting point for a good theremin - Use the EM variable and reference oscillators and the equalizing inductors - change everything else - Make the reference oscillator fixed frequency (trimmer adjustable) and stick a small $11 tuning capacitor (with series capacitor  so that perhaps  no more than 2pF adjustment is available) on the antenna. Buffer the variable and reference oscillators - One then has a solid foundation upon which to build - mixer, wave shaping, volume antenna circuits, VCA etc can all be designed and built on this foundation.

I think this calls for a shootout, as in build some, possibly pitch only, prototypes of different design schemes and let our great players beat the tar out of them.

Fred, we posted at the same time, again:)

My vote would be single supply, but that's a minor point for me.

I'd like it to be better than an EW.

A lot better. I'll gladly trade simplicity for a true, professional grade instrument.

There are tons of simple, relatively easy to build theremin plans and kits on the web.

I think ours should be special. Worthy of it's name.

As we discussed privately, uncle Bob's oscillators are fine by me.

We also discussed adding harmonics, and I remember you commented here (I don't remember where) on Bob's use of IF transformers to get harmonics.

Additive synthesis at RF sounds good to me. I have a partial prototype on my bench, but I've set it aside while I work on my Kep.

The volume section could be made adjustable for range and "snappiness" with op amps. Linear range is assumed in a quality instrument.

If the theremin is constructed very modular-ly, say on 6 separate pcbs, Each function of the design on a separate board, it would allow the design to evolve better, and be more customizable. Things like adding a effects loop would be more trivial later. And would allow pitch only, or different quality sections versus cost. And would be easier, from a learning perspective, to see how it works. And allow for division of labor. A volume control for instance, is a volume control, regardless of the input source.  I think it would be a worth the wiring mess.

As far as range, 4 - 7 octaves? Min 3 foot range? As adjustable in tone as an etherwave (or at least more than a b3)?

As far as difficulty of construction, as long as it doesn't require more than a cheap scope, the boards can be etched at home, no surface mount, and no lethal voltages.