New Silicon Chip Theremin 2009 edition

Posted: 3/7/2009 7:29:07 AM

Joined: 3/18/2007

The March 09 issue of Silicon Chip has a new version of their popular theremin design.

This version has added a voicing control, incorporated a larger loudspeaker and increased the power output. They've also changed the power supply to avoid problems with switchmode DC plugpacks.

Link to article (

The article refers to it as a Mk 2 design, but it's really a Mk3 since we already saw a Mk2 released in 2006 (Theremin World forum thread here (

I've scanned the article and put it in a zip file along with the PCB artwork from the SC website in case you want to etch your own board. (4.6MB)

Mediafire (
Rapidshare (

Jaycar has an ad in the same issue for the new design.
Clicky for bigger (

If you are just interested in the schematic, you can have a look at it here.
Clicky for bigger (

Link to Image (
[i]changed too large image to a link TW-Staff[/i]
Posted: 3/7/2009 7:20:16 PM

From: Croxley Green, Hertfordshire, UK

Joined: 10/5/2005


But I couldn't help notice the lack of discussion about linearity, and only a rather grudging admission that more separation between the rod and plate [i]might[/i] be a good idea.

What really caught my attention was the passing reference to a "sensor chair."

I'm just off to start a new thread about it in the General forum...

The Penn And Teller Spirit Chair (
Posted: 3/7/2009 7:51:30 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

First look at the schematic does not give much to inspire me!

1.) I see absolutely no means of linearity correction.. There is no equalising coil on the antenna (unless this is detailed in the article), and if this circuit is complete, linearity will be awful!

2.) There is no seperate VCA.. Volume control is implemented at the mixer by altering the bias.. I would expect this might lead to volume dependant distortion (I am not sure that this is the case).. BUT, more importantly perhaps.. There is no point at which one could tap audio before it is being volume controlled.. so no possibility of simple preview.

3.) The 'voice' control puzzles me a bit - It looks like it works on a oscillator 'sync' mechanism... This could be quite nice, as waveshape would be a function of difference frequency, so would change dynamically... What I have not yet worked out is why this does not cause the osillators to go prematurely into lock.

I would not be surprised if this Theremin sounds great.. Dynamically changing harmonics (as a function of pitch and/or volume change) is, to me, what makes tones interesting -
I would be most surprised if this Theremin was easy to play - It will have better linearity than a RC based design, but linearity will be as bad as any LC Theremin can be! - I also suspect that the 'voice' control will change the linearity at the bass end... I predict rapid interval compression close to the antenna, a 'playable' zone of perhaps 2-3 octaves, and slight interval compression at the bass end - depending on the voice setting this might even extend the linear playing zone a bit...
Posted: 3/7/2009 7:52:06 PM

From: Colmar, France

Joined: 12/31/2007


Technically it seems to be a step backwards. The same basic design has already shown up with an equalization coil and more sophisticated waveform/tone controls...
Posted: 3/7/2009 8:41:06 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

[i]"Technically it seems to be a step backwards"[/i] I agree, Thierry.

A re-design was needed - the VCA in the original and EPE versions is in limited supply / unavailable, and due to RoHS will be completely unavailable soon.. I suspect that this new design was primarily done for that reason. The EPE version tone controls were good but not good enough to justify 3 potentiometers. I am most interested in this new voice control - I think this might be a step in the right direction! .. needlessly Sacrificing linearity was definately a step in the wrong direction!
Posted: 3/7/2009 9:50:23 PM

From: Colmar, France

Joined: 12/31/2007

In the May and June 2008 issue of EPE they used an opto-coupler and not a VCA, so I do not see a ROHs problem.

And I can absolutely not understand why the 10mH coil was taken away, seen that they wrote about the "magical" effect of it.
Posted: 3/7/2009 10:01:04 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

Hi Thierry -
Alas, the optocoupler is a LED with Cadmium Sulphide Photoresistor.. Cadmium is banned.. I have been advised that ALL Cadmium Sulphide photocells are likely to fail RoHS compliance - I understand that there is a request for exemption (on the grounds that there is no other technology which can replace this function) - BUT that this is unlikely to be granted. I found it difficult to get these opto-isolators - most have now been discontinued, as have FET opto-isolators (H11F1 etc)which are the only viable alternative, and have no RoHS reason to be discontinued.
Posted: 3/9/2009 5:52:21 PM

From: Escondido, CA

Joined: 2/6/2008

That "new voice control circuit" ... would that be the pot to alter the bias on the pitch oscillator?

That does change the waveshape. Nothing else there seems to do that, once the transformers have been tuned.

I see what you mean about pitch-preview, Fred. There's no way to do that since the bias input of the 1496 is used to adjust the output level.

7809's would be hard to find in the US, but it is better to use a linear regulator for something that draws so little current. Switchers won't work without their minimum load, and the minimum load often way exceeds the circuit you are trying to power!

Posted: 3/9/2009 10:26:37 PM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

In Topic: jaycar/silicon chip troubles! ( (spex299) asks: "any suggestions on how to tune the damn thing!"..

Looking over the circuit, I note that the “pitch range” potentiometer (VR2 on the EPE design) has been removed..

This may be the biggest and most absurd change to this new design.. There is NO WAY (that I can see) to tune this instrument other than adjusting the core of T1.

So – This design has removed the linearising coil, and removed the tuning control..
I think that even the Gakken has a tuning control, and will have equivalent linearity.

[b]In my opinion, this Theremin is not worth buying.. the whole redesign is focussed on absurd cost cutting and lack of attention to detail - it looks like a design thrown together by someone who has absolutely no idea about the practicalities of Theremin playing .. Theremins need to be re-tuned whenever they are moved to a location where background capacitance changes (ie everywhere!).. Ommitting a tuning control is the sort of error even a novice would not make![/b]

Checking out the EPE schematic against this latest incarnation, I believe I may be wrong in my last comments regarding the voice control... This control is present as the 'skew' control, in the EPE version. I expect that the removal of the Equalizing coil is what gives this control greater depth..

I may be completely wrong about what follows, but I suspect: 1.) Redesign was primarily done due to VCA/Opto supply problems. 2.) Cost cutting was the primary objective for the 'improvements' (LOL) 3.) In order to increase the range of the voice/skew control, perhaps the EQ coil was a problem - removing this also cut cost. 4.) The Tuning control (1k potentiometer in series with 220R on D of Q1, in EPE)has been replaced with fixed 1k on D of Q1 in this incarnation - WHY? Cost? - or because it affects the 'new' wider 'voice' control?

Costs "saved" by making this design virtually useless: 1.) Tuning pot (<£1) 2.) EQ Coil (<£5)
The EPE version was more expensive in terms of components - but a re-design using an LM13700 for VCA, and changing the biasing scheme on the tone controls, could still have produced equivalently good Theremin (to the EPE) for less than price as the EPE (say about £10 more that this useless version).

Anyone wanting to modify this new SC Theremin could start by replacing the 1k with a 1k pot and 220 R, to bring back the tuning function... This pot should have minimum wiring length..

When (if) the above works and the 'voice' control still gives a good range (The Voice control is shown as a preset on the schematic here - I would want this to be an accessable potentiometer) I would add a 10mH Equalising coil (220pF to antenna connection point, other side to coil) and see if the voicing control still works.
Posted: 3/9/2009 11:49:43 PM

From: Sydney, Australia

Joined: 10/29/2007

Hi all,

This looks very much like the original 2000 circuit with a few additions-more like a V1.1 than a version 2. The "Voice" control circuitry looks identical to the Jaycar deluxe/EPE circuit, which is a good thing-it lets you change the sound from a near-sinewave to an RCA buzz. Changing the regulator to 9 volts from 6 will get much better performance and power from the LM386 amplifier chip.

As what's missing from the Jaycar deluxe circuit-it's missing the second FET in each oscillator, the linearising coil on the pitch rod, the two Etherwave-style distortion controls (not a great loss) and the vactrol VCA element (a pity but maybe inevitable if the Eurocrats ban CdS). Don't forget that the linearity and lack of tuning controls in the original 2000 design was easily overcome with some well-documented mods on the net-several of which were copied back into the deluxe design, and should be easily applicable to this one.

Don't forget too that the Jaycar deluxe design failed in the marketplace here (on catalogue for less than a year)-it gives my Etherwave a real run for its money in tone and playability but was just a bit too pricey for a mass-market kit, likewise the Jaycar MIDI Theremin from 2005. Also the linearising coils may be cheap but they're a pain to wind and probably not ideal for a kit aimed at electronic novices.

As for tuning controls-that's what the extensible antenna is for! :-) I don't want to pick a fight but there's no comparing this design to the original Gakken-my 2000 Jaycar is non-linear but has a decent control range and sounds much, much better.

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