EM Theremin Help - Pitch side works, volume side does not.

Posted: 11/23/2011 11:53:46 PM

Joined: 11/23/2011

I'm having difficulties with the EM theremin that I have breadboarded.

The pitch portion of the circuit works (connecting pin 12 of the lm13700 IC to ground produces sound and the pitch changes when I move my hand near the antenna). Unfortunately, the volume side of the circuit doesn't work properly. (No sound at all)

What works:

The volume oscillator is producing a waveform (~450kHz out)
The volume tuning pot changes that frequency slightly.

Problems (the values given here are approximations from memory as I'm not in the lab right now):

- The AC voltage across the variable inductor is lower than expected (<4VAC, should be ~10VAC)
- The DC voltages at the 2n3904 emitters in volume oscillator are incorrect (0.1V, should be 0.6V)
- The signal from oscillator to detector shows ~450kHz @ ~35Vpp; between detector and VCA processor shows ~450-500kHz @ ~250mVpp
- The voltage across the resistor in the volume detector circuit ranges between ~-1 and -1.8V (depending on where my hand is relative to the antenna)
- Connecting a voltmeter between pin 12 of the lm13700 and ground and playing with the variable inductor does nothing significant to the voltage


I've gone over the connections multiple times, swapped parts (transistors, diodes, caps) to see if they were the problem and poked at random components in the board while engaged in intense wishful thinking to no avail.

If any one has any ideas as to what may be the problem, I'd appreciate the help.
Posted: 11/24/2011 3:19:49 PM

From: Colmar, France

Joined: 12/31/2007

450kHz is ways too low, and thus it is clear that there is not enough resonance current through the volume circuit. Try to adjust it to ca. 505kHz and you should find a bigger amplitude.

And take the theremin far away from the metal workbench when trying out the volume loop! :-)
Posted: 5/28/2012 3:09:45 PM

Joined: 1/27/2012

HI, i have the same problem with volume circuilt could anyone help and suggest what to do to fix it?

dmwjorge what did you do to fix it?


Posted: 5/29/2012 12:11:42 AM

From: Colmar, France

Joined: 12/31/2007

1.) Make sure that your theremin is mounted in a way that no metallic parts or furniture around may mute the volume antenna.

2.) Look if the volume oscillator works: Check with an oscilloscope and frequency counter if you have a sine wave, 22Vpp, ~500kHz at the collector of transistor Q6 when the theremin is powered on.

3.) Make sure that the antenna is well connected to the volume oscillator through the linearization circuit: Switch the power off. Check with a multimeter:
- DC resistance at the pins of L9: 15Ohm
- DC resistance at the pins of L7 and L8: 8Ohm each
- overall DC resistance from the volume loop to the base of transistor Q7: 31Ohm

These coils are sometimes broken due to mechanical shock, but they can easily be replaced.

4.) Check the detector circuit: Power the theremin on and connect the multimeter to the pin of diode D1 without the black ring and ground. Keep the voltage in mind (should be around 0V). Now turn slowly the slug of variable inductor L11 (with the red plastic tool) until you find a point where the voltage drops by around 4V to about -4V (still making sure that probes and cables are kept away as far as possible from the volume loop and L7-L9). If you can't find this point although all checks above were ok, replace diode D1 (1N4148). 

5.) Check the VCA processor. Connect the multimeter to Pin12 of IC U3 (LM13700) and check if the voltage goes from -12V to 12V around the same tuning point when the voltage at D1 goes down by 4V. If not, replace U3.

Let us know how far it goes with these 5 tests and we'll try to refine the diagnostics.


Posted: 6/2/2012 10:58:20 PM

Joined: 1/27/2012

the problem is that input signal from volume oscilator to U3 is about 345kHZ, its to high. how could i lower it dow?

Volume oscilator produces 550 kHz sin wave 16Vpp only.

I had been doing everithing according to http://www.cs.nmsu.edu/~rth/EMTheremin.pdf

I still cant get my LM13700 ON to start working. Could it be problem of detector of volume side? I am not geting any signal in U3 pin 13.

As you suggested:

1.) theremin is mounted in a way that no metallic parts or furniture around may mute the volume antenna.

2.)Volume oscillator works: Checked with an oscilloscope and frequency counter: sine wave, 16Vpp, 550kHz at the collector of transistor Q6 when the theremin is powered on.

3.) Antenna is well connected to the volume oscillator through the linearization circuit

5.) U3 works, when conected function generator it provides signal to output.

What should i do now? could u suggest something?

thanks fot yor help, and sorry for my bad english.

Posted: 6/3/2012 12:28:14 AM

From: Colmar, France

Joined: 12/31/2007

VCA Processor:

On pin 13 of U3, there should arrive mainly a DC voltage around 0V (slightly positive with RF artifacts) because the signal from the volume oscillator is rectified by D1. In that case the voltage at pin 12 is near -12V and the signal is muted because that is the control voltage for the VCA.

When the Volume antenna is (together with the linearization coils) in resonance with the volume oscillator (that happens between 465kHz and 505kHz, depending on parasitic capacitances), the voltage at pin 13 becomes negative (can go down until -4V, but so much is not needed) and the voltage at pin 12 goes up to nearly +12V and the audio signal is enabled. You must adjust the frequency of the volume oscillator to find this point.

This control voltage from pin 12 is then injected via pin 1 in the other half of U3 which is the VCA. You can temporarily short pin12 to ground. In that case, the VCA works at -6dB and you should hear the audio signal from the mixer, even if there is a problem with the volume oscillator or the VCA processor. If you hear still nothing, check the pitch oscillators and the mixer (0.6Vpp audio signal at pin 3 of U3). But if you hear something (and the pitch can be varied with the pitch antenna), the problem is in the rectifier around D1 and/or the VCA processor or the volume oscillator is not in resonance (frequency too high or too low).

Posted: 10/9/2012 4:28:13 PM

Joined: 7/5/2012

Hey Thierry, first of I want to say thank you for all the instructions you post on TW. If not for them I wouldn't be as far as I am with my theremin.

I followed the instructions you mentioned above and am able to get the pitch and volume working together with the VCA. However on my setup I noticed that the voltage drops significantly when the pitch increases to about 3kHz (without hand on volume antenna). Is this normal for the EM theremin.

Also whats the max peak voltage seen from the EM theremin.

Posted: 10/9/2012 7:45:10 PM

From: Colmar, France

Joined: 12/31/2007

You have to understand the VCA... The raw audio signal arrives at Pin3 of U3 and has about 600mVpp. The VCA itself is an OTA which modifies the waveform by asymmetrically biasing the inverting input (waveform pot) and by symmetrically biasing both inputs via Pin2 (brightness pot). The output voltage depends mainly on the current (not the voltage!) which flows into pin1. With 12V at Pin12 you get about 0.5mA through R30 (47k) which gives the maximum output amplitude of about 600mVpp on Pin8. With 0V at Pin12 you get about half of the current and half of the output amplitude. With -6V it's a quarter, -9V an eighth and so on.

To get the full dynamic range you must make sure that the voltage at pin12 varies from +12V (hands far away from the loop) to -12V (hand close to the loop).

And yes, it's normal that the output amplitude decreases above 3kHz, there are two low pass filters with 4k7 and 10n, one formed by R31/C24 and the other by R34/C26.

Posted: 10/9/2012 11:37:55 PM

Joined: 7/5/2012

Thank you Thierry for the reply. I'm able to see a max voltage of about 600mV but this is achieved with the voltage at pin12 roughly +11V to -11V.  I'm sure the reason for this is because of all the lab equipment in the vicinity affecting the tuning of the volume antenna. Will tune in a better environment later.

Thanks again

Posted: 4/17/2015 8:44:57 PM

Joined: 4/10/2015

Hey Antman,


I am somewhat in the same situation as you are. I am wondering what type of inductor you used for L11? Please reply as soon as possible? Was it a coilcraft slot ten kind?


Thanks for your time,


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