Oscillators Tunning

Posted: 5/7/2014 4:20:37 AM

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 WaveCrafter.com . ...................................

Joined: 12/7/2007

"We really could use a simple TW Theremin, something made with readily available components, something you would have to really screw up in order for it not to work.  These parallel tank / series EQ things are just too picky and weird for the average novice builder." - Dewster


But it needs to be said that a simple theremin which meets the above requirements does exist IMO.. The Silicon Chip / Jaycar kits are easily built and low cost, use readily available components, and dont require any coil winding etc, and enough people have built them an published modifications to the basic kit to allow them to be improved.. The only essential modification being to fit the board in a bigger box and add a tuning control or threaded adjustable antenna. full details of  modifications are in the article by Max Baars here on TW. http://thereminworld.com/article/14272/silicon-chip-theremin-modifications

In fact, the S/C theremin was the basis for the Enkelaar theremin which was well liked by many thereminists, and I have documented the required modifications to change a S/C into an Enkelaar, with schematic and other documents (photos of board, modification details etc).

Yes, it would be great to have a TW theremin - But I dont think TW is harmonious enough to manage such a venture - I think the S/C is a good-enough simple design - I dont think its a good theremin, but I think its good enough (it only becomes bad if one tries to add an antenna EQ coil, as was done with the EPE 2008) ... The Moog / EM is a level up for more advanced constructors with the required knowledge and equipment IMO, and is a MUCH better theremin.


Posted: 5/7/2014 11:14:31 AM

Joined: 5/4/2014

"Yes, it seems you didn't have enough adjustment range in the local (fixed) oscillator." - dewster

According to Moog's article, this is the range I would be able to get. I am in fact able to get a higher pitch when near but I might get a little off from the 0Hz and there is sound that might start to be audible.

About the simpler Theremin, I also agree. I didn't looked at many theremin schematics but I'm sure the oscillators can be replaced by other type of oscillators. There are plenty of different oscillators circuits and I'm sure a good portion of them are reliable and do not requires coils. I am not an expert in electronic so about the linearization coils tough I don't know what could be done. I think there was a post here about someone that wanted to get rid of the coils in a theremin.

When I'll have time in the future, and when my theremin will be working, I might start looking at modifications to do. I'll come back now and then if I find something.

About the electromagnetic pulse and the oscilloscope to find the resonant frequency, has someone checked this? By the way, I haven't said how I generated this pulse. It was simply me, my cotton vest and a plastic chair with metal legs. By that I mean that it was simply an electrostatic discharge.

See you soon,

Posted: 5/7/2014 4:15:59 PM

From: Northern NJ, USA

Joined: 2/17/2012

"(it only becomes bad if one tries to add an antenna EQ coil, as was done with the EPE 2008)"  - FredM

It seems Max Baars adds an air core EQ coil?  But for some reason he doesn't reveal the actual value nor construction details in the description of his mods.

"There are plenty of different oscillators circuits and I'm sure a good portion of them are reliable and do not requires coils."  - billJohn

For Theremins you need to hit the sweet spot of pullability vs. stability, which unfortunately leads you almost inexorably to coil based oscillators.

Posted: 5/9/2014 11:36:26 AM

Joined: 5/4/2014

Hi, I tuned the pitch a little more and it has a little more range in the frequency domain and in the distance domain too but the linearity is not incredible. I learned that this is directly linked to the selection of the coils in series with the antenna. Is there something else to do else than changing these coils? It's not my main concern tough.

I tried to tuned the volume though and it kind of works decently (at around 380kHz). However it only works when the oscilloscope is connected to the net L7-C12 and to the ground. I tried lowering R14 from 1M to 910k to increase the current in it thus having an easier to trig amplification. It didn't worked.

Does someone has an idea to how I can modify the circuit to make it work? If one would have a good explanation, or a link to one, to how the oscillation is transferred to a DC Voltage for the volume I could try to figure it out myself. This part is still a little too vague for me.

Thanks in advance,

Posted: 5/9/2014 1:40:48 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 WaveCrafter.com . ...................................

Joined: 12/7/2007

OT - Re EPE / SC

"It seems Max Baars adds an air core EQ coil?  But for some reason he doesn't reveal the actual value nor construction details in the description of his mods" - Dewster

Yeah - I wonder about that... Before I fully understood the interactions required for an antenna EQ to work, and before I realized the implications of operating the series LC in its inductive 'zone' and that, in effect, it was the tanks L which was changing as a result of changes on the antenna C, I never understood the strange results I was getting from the EPE 2008.

Sometimes it worked - but only when set up so critically that it tended to 'flip' into an unstable or 'latch up' state - and tuning far enough away from this point did little (if anything) to improve performance.

I only got the damn circuit to work correctly when, at last, I understood the operation - Getting it to work involves reducing the tank L and increasing the tank C - with the EPE / SC theremins using IF transformers, changing the tank is not a simple matter.

It wouldnt surprise me if Max Baars had similar problems (it would surprise me if he didnt have) and kind of skimped on the details.. I dont know what his linearity plots are based on - whether this was with or without antenna EQ - But if he got these with EQ, he did well if he managed to keep the theremin from 'latching up'..

My H1's (nearly) all had antenna EQ's - A couple worked extremely well and ran close to resonance without latch-up - but the majority needed to be detuned significantly to be stable - One reason I came up with my adjustable antenna was that it allowed me to tune the antenna resonator, making it possible to get the best operating point for each theremin - I also undertook modifications to the theremins while the show was on, reducing the tank L and increasing the tank C ... In general though, I wouldnt bother using an antenna L on a SC based theremin unless one is doing something 'special' and significantly altering the build 'round the oscillator.. Even in 2010 I didnt fully comprehend what I was doing - the science / theory behind my actions was vague .. It was only after playing with an Enkelaar which I attempted to "improve" that, after returning the butchered beast to it owner, the whole thing clicked into place - and I swore loudly!  ;-)


Posted: 5/13/2014 12:25:38 PM

Joined: 5/4/2014

I'm happy to see these constructives comments about improvements, but I would appreciate a little help with my problems.

I have the same issues as stated in my last post tough now I added a resistor from the net L7-C12 to the ground. This emulates the probe of the scope and it make the theremin kind of working. It makes sounds at least. The main problem now is that the sound is way too low and it's hard to hear. If I amplify it at the end then I will also amplify the 60Hz noise and it's really annoying.

I still didn't found good explanations for the way the oscillations are transferred into DC voltage for the volume. If you have a good explanation, feel free to answer.

Thanks in advance,

Posted: 5/24/2014 1:35:30 PM

Joined: 5/4/2014

I've red somewhere that there is a band-pass filter in some theremin to transform the volume oscillation into a DC voltage. That was a guess I did before but I can't find what components might make this filter. It seems like a good way to transform the oscillation into DC. If I was to create the circuit I would use a diode bridge or simply a diode to add a DC component to the oscillation and then I would add a band-pass so when the frequency changes to one side or the other, it would make the signal decrease of amplitude. This decreasing amplitude would also make the dc component decrease. The DC can then be extracted with a capacitance connected to the ground. With the VCA processor I could then amplify the DC voltage to range from 0V to 12V or whatever the range is suppose to be at the end.

The problem is, I am not able to see if this is the case in this circuit or if it is even remotely close to what I think it might be. This volume antenna is the last mystery I have to solve and I would be glad to get rid of that resistor that pollutes my circuit.

Thanks for help,

Posted: 5/24/2014 3:49:20 PM

From: Colmar, France

Joined: 12/31/2007

I can't remember how often the prinicple of operation of the EW's volume circuit has already been described in this forums, be me and by others. People, learn to use the search function!...

The EW's volume oscillator works on a fixed frequency around 480 kHz and feeds the series resonant circuit formed by L7 to L10 and the volume loop as a variable capacitor. As the capacitance of the volume loop varies, the resonant current will also vary. And so will the RF voltage across L7. The latter is then rectified by the combination of diode D1 and C12 as a loading capacitor. When everything is well tuned, that means that the series resonant circuit is in resonance with the volume oscillator's frequency (hand away from the volume loop), you'll find about -4V on C12, while if they are out of resonance (hand close to the volume loop), you will find 0V. R14/C27 acts as an additional low pass filter before this voltage sees an offset added (R35) and is amplified to cover a span from +12V (full resonance/volume) to -12V (no resonance/volume) at pin12 of U3B. While the amplification factor of this OTA is basically fixed by the current into pin15, determined by R36, and theoretically no feedback would be needed, there is a feedback branch formed by D5 and R37 to make the response curve somewhat more logarithmic.

The internal volume CV from -12 to +12V which was obtained as described above is then used to control the amplification factor of U3A by creating a proportional current into pin2 through R27.

There is no miracle and no difficulty in understanding if one looks carefully at the EW schematic and if one makes the effort to study the LM13600/13700 data sheet and application notes...

Posted: 5/24/2014 6:30:09 PM

From: Colmar, France

Joined: 12/31/2007

Addendum: there are basically two construction principles of volume RF circuits in Theremins:

a) fixed frequency volume oscillator and variable band pass filter

b) fixed frequency band pass filter and variable volume oscillator

The current Moog Theremins (EW, EWS, EWplus, EPro), the RCA Theremin and other Lev Termen build instruments, the Henk Theremins, work all following the principle a.

The EPE/Silicon Chip and Enkelaar Theremins, all B3 Theremins (Standard, deLuxe, Pro) work following principle b.

There is a third variant, only used in Paia's Theremax, where there are two heterodyning volume oscillators which generate a second audio signal which is sent through a simple RC low pass before being rectified to obtain the control voltage.

The Moog series 91 and the Ethervox Theremins, and the tVox tour Theremin work with a variant of principle a: there it's not the amplitude after the variable band pass which is directly used, but there is a phase detector which transforms the phase shift of the band pass filter into a control voltage. The latter principle makes the volume circuit much less sensitive to AM distortion and interference captured by the volume loop.

Posted: 2/5/2021 2:36:18 PM

Joined: 2/5/2021

Dear theremin enthusiasts: I need a lot of help in building the theremin based on the old EMTheremin scheme and updated to the new Etherwave Theremin scheme. The pitch part works great but the volume part is dead. There are no oscillations in the oscillating volume part. On the emitter from Q6 and Q7 I have -0.6 V instead of +0.2 V. I checked all the components and they are Ok. On pin 12 of U3 I have -12 V, and it does not change when adjusting coil L11 (68 uH). For this reason, the Theremin is muted and also blocks the pitch that works if pin 12 is connected to the GND. In the pitch part to the control part, the old circuit was modified by replacing some capacitors and connecting the C11 capacitor to 12 V instead of GND. Why not do the same with the volume control part with C18? Why is the volume oscillator not built the same as those on the pitch side, with capacitor C14 in the oscillating circuit parallel to coil L11? Thank you for your attention and I would be indebted for some suggestions from you. Please excuse my faulty English expression because I use Google Translator for translation !!

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