# Help finding parts for EM/etherwave theremin.

Posted: 8/18/2011 12:41:32 PM

From: US

Joined: 8/17/2011

I was hoping to try to build the etherwave shown in the article, "UNDERSTANDING, CUSTOMIZING, AND HOT-RODDING YOUR ETHERWAVE ® THEREMIN" by Robert Moog. But I'm finding it nearly impossible to find any variable inductors in the 47uH range. I also can't find a suitable grounded 12-15VAC wall wart. Does anyone know where I might be able to find these things in the U.S.?
Posted: 8/18/2011 3:13:32 PM

From: Colmar, France

Joined: 12/31/2007

The problem is that these variable 47uH inductors from TOKO are not longer produced. Moog music inc. replaced (since board revision 211-D of the Etherwave) them by another brand but this required also modifications in the biasing of the oscillator's transistors.

So there are only a few sources for these inductors remaining, I know of one in the UK: http://www.jabdog.com/toko-ez.htm

The 47uH variant you are looking for has the part number EZ1017 there.
Posted: 8/18/2011 6:11:16 PM

From: US

Joined: 8/17/2011

Ok. Do you think it would work to use a variable capacitor instead of an inductor. For example, On the pitch oscillator use a 94uH inductor with a 1.7nF capacitor and a variable 1.7nF capacitor all in parallel?
Posted: 8/18/2011 6:21:59 PM

From: Colmar, France

Joined: 12/31/2007

No, it's a question of stability vs. thermal drift. And there are no 1.7nF variable capacitors... And the tuning range has to be only small: The total inductance is 94uH, the variable inductor may be tuned for +/-10% with makes +/-4.7uH. So the total inductance varies only by +/-5% which makes varying the oscillator's frequency by only +/-2.47% (Thompson formula).

You could theoretically get the same tuning range using 2 47uH fixed inductors in series, and a 3nF capacitor in parallel with a 0 to 600pF variable capacitor. But the oscillator's frequency will not be stable with such a construction... That's the difference between theory and practice...

Why don't you simply order the variable inductors at jabdog?
Posted: 8/19/2011 2:46:57 AM

From: US

Joined: 8/17/2011

I might do that. If possible I'd like to avoid paying 3 times more for shipping than for the actual merchandise. Are the Slot Ten-5-10's from coilcraft acceptable?
Posted: 8/19/2011 3:10:43 AM

From: Colmar, France

Joined: 12/31/2007

No they aren't - at least not without further modifications on the oscillator's circuits. The oscillators will need a somewhat higher higher bias current and the reactance stages (the additional tuning circuits besides the oscillators) are also to be modified in order to prevent self-oscillation.

I once tried to do such a coil replacement (since I had no others at hands) but I found that the tuning circuits around Q5 and Q8 started oscillating themselves at around 600Hz, creating intermodulation which made the theremin unusable. So I had to be patient and order the "correct" inductors.

Or you may wait until I have some free time and can edit the schematics in order to publish an updated version of the Etherwave with the Coilcraft inductors. But this may happen only towards the end of next week.
Posted: 8/19/2011 4:27:37 PM

From: US

Joined: 8/17/2011

Ok. If you find any time to edit the schematics that'd be great. thanks.
Posted: 9/21/2011 3:03:39 AM

From: US

Joined: 8/17/2011

bump?
Posted: 9/21/2011 3:26:07 AM

From: Colmar, France

Joined: 12/31/2007

When using the coilcraft SLOT TEN-5-10, then you have to use 2 x 2.5mH and 1 x 5mH in the volume antenna circuit (as the newer Etherwave theremins also do, starting on board revision 211C).

Same modification on the pitch side: Use only 3 x 10mH coils in series with the pitch antenna when you built the oscillators with the coilcraft inductors.

The SLOT TEN variable inductors have a slightly different inductance, self-resonant frequency and DC resistance, affecting the oscillator's amplitude and stability.

That's one reason why Moog changed the following resistor's values in conjunction with the coilcraft inductors (the other reason is preventing parasitic oscillation around Q8):

R2, R6 and R16: 1.5k instead of 2.2k
R15, R19: 330 instead of 470
Posted: 9/25/2011 8:48:44 AM

From: US

Joined: 8/17/2011

Awesome. Thankyou. I can't wait to get started. Now I just need to find an acceptable wall transformer. Is it possible to ground the theremin in some other way?, it's nearly impossible to find a suitable grounded wall transformer.