THEREMIN CONTROLLED SYNTHESIZER

I want to use my theremin audio signal to replace the keyboard on my Moog Concertmate synthesizer. It has a voltage input that disables the keyboard. I'm looking at frequency-controlled-voltage circuits, at the LM331 specifically. Am aware that I may need to enhance the circuit to match signal level requirements, etc. Anyone done this before? I don't want to convert to MIDI, just use my Moog to enhance my "Theremaniac" simple theremin sound. Thanks  Jeff

Hi Jeff,

IMO, your best bet is to see the circuit in the Moog EW manual at least to get an idea about what needs to be done. You do not say what theremin you have, but even if its not an EW, this circuit could be adapted.

The issue of converting pitch to CV is a bit more complex than just having a frequency-voltage converter .. See my posting in http://www.thereminworld.com/Forums/T/28940/question-about-the-frequency-to-voltage-converter?last=True

Fred.

Sorry - Just seen you specified a Theremaic "theremin". I presume therefore that actual melodic playing and linearity are not of paramount importance to you.

The Moog EW Pitch-Voltage circuit is much more complex than your theremin! - You may well find that using a simple Frq-Voltage IC is adequate for what you want.. But if you dont have an exponential converter linearity will be even worse, so I would advise putting an exponential (Log) converter in your circuit.. However you could probably get away with not having the expensive thermal correction resistor - drift due to temperature will probably be the least of your worries.

If you want your theremins pitch to track the pitch from your synthesiser, then you will need a good pitch-voltage converter with a good exponential converter (as the EW) otherwise things become a bit less critical (if you only want to control the synth, you dont need to accurately track - and you can fiddle with the converter and might actually manage to improve the linearity)

Agreeing with FredM. A pitch-to-voltage converter plus some analogue curve correction and you're easily done.

Is the Moog CONCERTMATE synthesizer capable of continuous, smooth, pitch bend control?

Since it is a keyboard synthesizer, built to be played in quarter tone steps and not designed with continuous control in mind, it may produce a "zipper effect" when triggered by a VC device. 

I don't know if this is the case, but the results from these kinds of experiments (and I have conducted MANY of them) are often not what we imagine they will be when we start out.

"Is the Moog CONCERTMATE synthesizer capable of continuous, smooth, pitch bend control?" - Coalport

Thanks for the above question, Peter - I had mistakenly assumed the Concertmate was a monophonic instrument - but looking at some on-line notes about it, it seems it was a combined instrument, having both a polyphonic "synthesiser" (based on standard organ topology with top-octave generator and deviders) and a monophonic synth (using CV derived from the keyboard or from a CV jack scocket to bypass the keyboard).

The monophonic section is fully continuous - AFAICS an independent 1V/Octave mono synth - One should therefore be able to apply a CV to this scocket and control the mono synth from a theremin.. There also seems to be an external trigger socket - this would need to be driven to enable output (or triggered to drive the envelope generators).

The keyboard, AFAICS, should still be operational for the polyphonic voicing.

Fred.

I have just found a full service manual for the instrument:

http://www.theappreciationpost.com/mg1-servicemanual.htm

I dont have time to study it right now though.. It MAY just be that you could actually play the keyboard with one hand, that this would trigger the mono-synth envelope generators, and play the theremin with the other hand to control the mono-synth pitch - My feeling is that this could be really tasty! -

You would really want to get inside the concertmate and take an output from the VCO pre-VCA*, so that you could add a "preview" (to listen to what pitch the theremin is producing, and feed this to an earpiece) You could then mix the poly and mono voices to taste - The keyboard would only control the poly voice pitch and trigger the mono-voice envelopes, and the theremin would only control the mono-voice pitch (and VCF).

Its a really nice little synth, from what I can see! - I have a couple of synths with similar topology (A Siel Cruise and a Farfisa Synth-Orhestra) but by the looks of it, this little Moog does things far better than either of these.

 In my last I said: "If you want your theremins pitch to track the pitch from your synthesiser, then you will need a good pitch-voltage converter with a good exponential converter " - I think I need to clarify this.. If you want to play your synth AND simultaniously hear you theramin, THEN you need a good accurate pitc-Voltage with exponential converter.. You can then tune / detune the theremin and synth to whatever inteval you want, and they will track each other (Like adding another VCO to your synth).. If you dont have a good converter, the pitch from the theremin will not retain the same relationship to the pitch from your synth (IMO, there are not many musical horrors to compare with tone sources which deviate from each other in this way).

*If you have accurate pitch-voltage 1V/octave converter, then you could just monitor the audio output from your theremin with an earphone to give "pitch preview".

IMO, Pitch preview (particularly for an application like this) is absolutely essential.. You have no volume antenna - so you cannot "bleed" any of the tone - the moment you trigger the envelope, the pitch is heard.. YOU cannot hear it until everyone hears it! You have absolutely no guidance unless you have an audio preview.