DIY Etherwave (weird VCA problem + bleed-through)

Posted: 1/9/2017 1:33:01 PM
Jannik B

From: Netherlands

Joined: 1/9/2017


Learned a lot the past few weeks/month in this forum and decided to build my own Theremin. After a few initial problems (concerning finding the wright parts and a few building faults) I could get everything working thanks to the search-function of this forum.


BUT, I can't seam to figure out the last problem I have: If I boost the volume via an external effect I hear the 3 oscillators as "bleed-though" in the audio path. Waving over the volume antenna makes the volume oscillator (which I hear) go up or down in pitch. So I know that's working properly. If I come to a certain point via the tuning I hear the VCA rapidly go up/open with the theremins audiosignal as it should be and immediately close again (sounds like a "pop", but changing my hand over the pitch antenna or changing the wave or timbre settings I can hear that it's the theremins audio with rapidly opening/closing VCA.) If I leave the tuning there and come close to the volume antenna I can "make the VCA pop" when coming close to the antenna. Works better if I make a quick hand movement. If my hand moves to slow the "pop" doesn't occur at all. (same for tuning the coil, or tuning via the panel.)

I already replaced and tested the LM13700, which was/is working as it should. Also the wave and timbre options work properly.

I also tested and replaced the (already working) diode D1 in the volume antenna circuit.

And I tested if maybe the volume antenna is too long or short. But that isn't it either.

Could it be that maybe C12 is faulty?

Or is it because I'm using 3 and not 4 coils for the antennas. I first read that one should use 3 coils when using "Coilcraft Slot-Ten" Coils, only after I build it I read in another post that this only applies when building on a PCB, not when building on prototype-board. But it still seams that the pitch and volume oscillators are working like they should.

I also wonder if the bleed-through is normal? I boosted the signal quite a bit to hear it, so concerning that the normal output is line-level, this might be normal.


BTW: I am using the "hot-rodding your etherwave" schematic with a few changes suggested by user "Thierry" when using "coilcraft coils Slot Ten" in another post on this forum and a few suggestions from this post like the C10 and C11 = 100uF to ground from the 3 schematic.

Posted: 1/9/2017 4:29:03 PM

From: Colmar, France

Joined: 12/31/2007

First, "bleeding through" comes most times from poor or bad grounding.

Second, the volume oscillator should work at a fixed frequency. Waving your hand above the volume loop should only modify the resonant current through the coils (and thus the voltage detected by D1*) and NOT the volume oscillator's frequency which should remain stable around 500kHz and only variable by +/- 500Hz with the volume knob. Check with a frequency counter.

* -4V when hand far away = full volume, 0V when hand close = no volume, measured between the cathode of D1 and ground. This span (0 to -4V) is then translated into -12 to 12V by the first half of the LM13700 before it feeds the VCA (the other half)

Posted: 1/9/2017 7:18:51 PM
Jannik B

From: Netherlands

Joined: 1/9/2017

Thanks for the quick response.

I grounded the theremin with an extra ground connection, but will check grounding connections within the theremin tomorrow.

Checked the DC voltage and it seams to stay around -13 mV. I checked again the VCA "spikes" and the volumecontrol seams to smooth out the "Attack-Decay" behavior I'm getting.

I also checked again the volume oscillator and my initial thoughts, that the frequency changes with hand movement doesn't seam to be the case. Only if I had the frequency turned really low, but it's more an "interference-oscillator-noise"-thing than the volume oscillator itself.

Any ideas what might be the problem?


Posted: 1/9/2017 11:00:34 PM

From: Colmar, France

Joined: 12/31/2007

You could do the same as some other Dutch Theremin builders did before you: Plan a touristic weekend trip to Alsace/France, make an appointment and let me have a look onto your circuit. It's much more efficient than guessing at distance.

Posted: 1/11/2017 8:39:31 AM
Henk Brand

From: Schiedam The Netherlands

Joined: 12/22/2014

Going to Colmar is always a good idea and always rewarding but maybe I can help you as wel .

As I live in Schiedam it may be more convenient to visit my place. If your interested send me a comment.

Posted: 2/6/2017 12:32:24 PM
Jannik B

From: Netherlands

Joined: 1/9/2017

Hello again,

sorry for the late response. Had a lot to do the past few weeks. But today I'm back at the Theremin.

Thanks for the kind offer. Will keep it in mind as a "last resort-before-I-go-crazy"-option.

Posted: 2/6/2017 2:16:26 PM
Jannik B

From: Netherlands

Joined: 1/9/2017

Got it!

R23 had a bad solder joint. After fixing it the VCA works perfect. The signal is (finally) at line-level and the transition is as excepted.

HOWEVER: Now I have a constant line-level + the level from the VCA. As if the output is a sum of original audio and the VCA output.

Any ideas?

Posted: 2/19/2017 12:38:09 PM
Jannik B

From: Netherlands

Joined: 1/9/2017

After visiting Henk Brand the Theremin is (finally) working as it should! :)

I forgot one cable which connects +12V to R35/R36 of the VCA-Processor.

Weird how one can linger hours over a circuit board without noticing the (obvious) problem.


Posted: 2/19/2017 2:10:22 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Weird how one can linger hours over a circuit board without noticing the (obvious) problem."  - Jannik B

That's the beauty part of our profession.  Applies to code too.

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