volume affecting pitch on EM- tech advice needed

Posted: 6/23/2010 11:30:54 AM

From: Brooklyn,NY

Joined: 12/1/2009

Hi everyone,
Ive recently been sending some time trying to actually "finish" some old projects. One is an EM theremin that I built a few years ago. Its quite playable, but at the lowest (snappiest) volume response, the lowest pitches change up to 2 semitones as I approach the volume loop. It pretty minor and doesnt affect my playing, but Id like to solve it nonetheless.

As I stated, I have a scratch built EM theremin, actually I built 2. Theremin #1 (with the problem) has large hand wound coils rather than the iron core chokes listed in the article (32.5 mH large coil + 10mH variable for pitch; 12.5mH large coil + 2.5 mH Miller choke for volume.)
Theremin #2 is more orthodox, using the Miller inductors listed in the EM schematic.
Both theremins have the capacitance values listed for the EM (not the hot rodding the EW values)and both have hand wound variable inductors on the same surplus coil forms. Both theremins perform well with and have really good range and linearity, however (I think) due to the added capacitance of the large coils (and slightly larger dia. antenna)Theremin #1 needs the variable inductors set to max. Around 110uH for pitch, 89uH for volume. To get more than 4-5 playable octaves the pitch antenna is reduced to about 38.5mH via the 10mH variable. With this setting, linearity is quite good (for my playing at least, I perfer it to the standard EM, theremin #2)

Anyway, I think the cross talk problem is a result of the volume oscillators working at too low of a freq.
L11 @ 89uH and c14 @ 2200pF: unless Im wrong I get about 350khz. Way too low.
(pitch I belive is ok: L5&6 @110uh and c1&5 @ 3900pf = 243khz-ish)
Obviously. I dont have acess to a frequency counter or scope.

I understand the pitch circut(relatively), but the volume is throwing me for a loop.(sorry)
Im pretty sure I can just change the capacitance value in the antenna circuit, but I just cant seem to figure it out.. and I feel like Im getting dumber the more I look at it. Can someone please help clear my mental fog and point me in the right direction.
Should I raise the value of C12(1000pf)? or am I completely off?

I hope that this is all coherent.
Any suggestions are greatly appreciated.
thanks in advance.
Posted: 6/23/2010 1:12:55 PM

From: Brooklyn,NY

Joined: 12/1/2009

In this thread (http://www.thereminworld.com/forum.asp?cmd=p&T=4434&F=1) Fred, presents some solutions for pitch changes due to proximity to the ground field. I dont think that this is the issue, unless it is via the loop.
If I place my hand between the cabinet and the outward curve of the volume loop, and move my hand out, away from the pitch rod/ theremin circuit/ ground plane and toward the loop, it still raise the pitch the same amount as from any other direction.
Posted: 6/23/2010 2:55:38 PM

From: Colmar, France

Joined: 12/31/2007

In the E-Standard the pitch circuits operate at about 290 kHz (285kHz free-run, raised somewhat at the moment when the negative impedance of the linearization coils and the pitch antenna is added and successively lowered towards the initial 285kHz when the hand approaches the pitch rod.

The volume oscillator works on a more or less stable frequency around 500kHz. In parallel to the oscillator's tank circuit there is a series resonant circuit formed by the volume loop and its linearization coils which has a somewhat lower resonant frequency which is just far enough away from the oscillators frequency to not to drop too much energy from the tank circuit but to allow the flow of a resonant current whose intensity is detected by the diode in parallel to one of the coils. If you approach your hand to the volume loop, its resonant frequency goes farer away from the oscillator and the resonant current will be less and lesser. Those slight current changes are amplified in the second half of the LM13600 or LM13700 in order to drive its first half as a VCA.

[sing] It's so easy... [/sing]

If you have floating pitch when approaching the volume loop that means that either you or the theremin is not enough grounded because the grounding potential seems to float in function of the capacitance between you and the loop.

So make sure that your theremin's internal ground is (as it was in the older Etherwave power supply) connected to the third (ground) prong of the wall plug. This should solve the problem.
Posted: 6/23/2010 9:32:19 PM

From: Brooklyn,NY

Joined: 12/1/2009

thanks Thierry,
Grounding was my first thought, but everything seemed to check out when I did continuity tests between the third prong and various ground points in the circuit...though Ill check again. There's always something I could have missed.

Im fairly certain that the house wiring is ok. (Theremin #2 works fine) ..Its way better than my last place. Most of the electrical work was contempory to Theremin himself, fabric insulated wiring with ceramic junctions and wire nuts... and of course, no ground. Here, I can at least plug in a grounded cord.

as for me, Ive been told many times ( and in more or less words) that I wasnt propery grounded...

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