Volume control

I'm thinking of building a Theremin, and I've started looking into info on how they work. I understand (roughly) how the pitch and volume controls individually work, but how are they combined into a single output signal? I've been googling up analog multipliers (http://en.wikipedia.org/wiki/Analog_multiplier) - is running each output into one of those and taking the output as the audio out anything like a standard way to do it?

Hi Mobcake, Welcome to TW!

[i]" but how are they combined into a single output signal? I've been googling up analog multipliers - is running each output into one of those and taking the output as the audio out anything like a standard way to do it? "[/i]

The easiest way to think about pitch and volume combining is probably to think of them as completely seperate functions.. One generates the audio signal by heterodyning the pitch reference and pitch variable oscillators - using an analogue multiplier of some kind as a 4 quadrant modulator (ring modulator or heterodyning mixer) and the output of this is the difference frequency of the 2 input frequencies modulated by the sum frequency of the 2 input frequencies.. one then puts this signal into an audio low pass filter (simple resistor into capacitor is usually enough - R*C set to give roll-off at about 16kHz (time constant of R*C equal to between 50us and 100us)and this "throws away" all the sum frequency components (if one oscillator is running at 250kHz and the other at 254kHz these will be >= 504kHz so easily filtered out) and leave you with the audio difference frequency (in above case, 4kHz).

So you now have the audio.. This audio must be fed to some form of voltage controlled amplifier.. You can use an analogue multiplier IC to do this (in fact a VCA is a form of analogue multiplier), but it is better (and often cheaper) to use a good quality VCA designed for audio gain control, or use a transconductance amplifier such as the LM13700 (http://www.national.com/mpf/LM/LM13700.html) - The data sheet for this gives schematics for a good VCA.

The volume antenna circuit needs to provide a control voltage which is used to control the gain of the above VCA.. This is a DC voltage whos value varies proportionally to capacitance seen by the volume antenna..

This voltage is fed to the control voltage input of the VCA (or in the case of the LM13700 via a resistor to convert this control voltage into a control current) and the output of this VCA is audio whos level is controlled by the volume antenna.

Analogue Multipliers:

AD633 (http://www.analog.com/en/other/analog-multipliersdividers/AD633/products/product.html) - An extremely good and simple IC .. but expensive (£6) .. Only worth using for the mixer stage on a quality Theremin, not ideal (and a waste of money) for use as VCA.

MC1496 (http://www.onsemi.com/PowerSolutions/product.do?id=MC1496D) - a good, slightly more difficult to set-up, cheap analogue multiplier.. This multiplier has adjustable bias inputs and can be configured to distort the output waveform, and also can be used as combined mixer and VCA.. This is the method used on some Silicon Chip Theremins (cannot give link to these - someone else may be able to - I subscribe to Silicon chip, so my links carry an embedded authorization code I do not want to give away! ;-)

I advise using a seperate LM13700 VCA rather than the combined mixer/VCA approach - One cannot implement Preview with the combined method, and the LM13700 is cheap enough, and avoids problems one may have getting the combined mixer/VCA scheme to work well.



[b] The bible: [/b]

EW Technical reference (http://www.moogmusic.com/manuals/HotRodEtherwav.pdf) - This is the article from which most new Theremin designs are spawned.. It shows the full schematic for the EW Standard, and has technical notes which are useful as a starting point.

Note the simple diode mixing - this is the simplest way to get CRUDE analogue multiplication.. I dont like it (in fact, I hate it!) but the majority of Theremins (on this planet anyway) adopt this method.

UNLESS YOU ARE EXTREMELY KNOWLEGABLE ON ELECTRONICS, AND CAPABLE OF MEDIUM / ADVANCED LEVEL CONSTRUCTION, YOU ARE STRONGLY ADVISED TO GO FOR A TRIED AND TESTED KIT

Thanks so much, that's really helpful! I'm definitely not extremely knowledgeable on electronics (GCSE electronics and sections of A-level physics form most of what I know), but I've got a lot of time on my hands and a willingness to spend big chunks of it learning - are there any books you can recommend that'd give me a basis for the theory I need? I [i]am[/i] pretty handy with my maths, so I'm cool with diving into something massively technical up to and including breaking out Maxwell's equations for some good old-fashioned vector calc.

[i]" are there any books you can recommend that'd give me a basis for the theory I need? "[/i]

If you can find a copy of "Musical Applications of Microprocessors" by Hal Chamberlin ISBN 0-8014-5773-3, this book covers basic synthesis and related electronics and although not Theremins, is a great reference (despite its title, it has probably more analogue than digital bias - most circuits are analogue (VCO's VCF's etc) and methods of interfacing these to digital.. the actual microprocessor stuff is quite lightweight.. perhaps a little too lightweight) - but long out of print.

Most of the books I use are quite ancient, and none really deal with the Theremin.. Good mathematics skills should help you a lot - Books like Analogue Electronic Circuits and Systems by Amatava Basak (ISBN 0-521 36913 4) are (in my opinion) too expensive new, but worth having if you can pick them up cheap.. Likewise Integrated Electronics (Millman - Halkias) and "The Art of Electronics" by Horwitz and Hill..

But - You have a big advantage over us old school engineers - You have access to on-line resources and circuit simulators.. I advise you to get LTSPICE (http://www.linear.com/designtools/software/) and join the support forum, download its manuals, and use this as your primary means for learning electronics - It is all free, models for components are available for download free, you can exchange schematics with the many people on the forum willing to help .. and you could probably find people who will collaborate on Theremin projects - particularly as you have something big to offer - you can help them with maths!
allaboutcircuits.com (http://www.allaboutcircuits.com/) has a set of introductory on-line books, and a (usually) helpful forum (just so long as you dont disagree with the high and mighty moderators political views sometimes imposed - I left AAC in protest over what I saw as outrageous bias and their refusal to delete a thread which in my opinion was advocating the genocide of Muslims - I used to spend as much time at AAC as I now do at TW).

If you are really good at maths, please think about contacting me (email is in my picture <---).. I would be interested in having some help, and could give you help on electronics in exchange.

An example of a simple adjustment of the volume tested by me.