Variable Inductors for EM Theremin

Posted: 10/10/2017 7:58:00 AM
rob reido

Joined: 6/22/2017

I am currently in the process of populating the circuitboard of the EM theremin I am constructing as per these instructions, and have reached a *slight* hurdle. The leftmost variable inductor has instructions as to its wiring, such as cutting off the middle leg. Do these instructions apply to the other two, as they are unmarked?

Thanks in advance

Posted: 10/11/2017 7:27:36 PM

From: züriCH

Joined: 3/15/2014

hi rob,

found these two similar questions/answers for you: link1 and this one link2 .

hope that helps. 

happy building and let us know how it comes out!

Posted: 12/16/2017 11:38:07 PM

Joined: 12/16/2017

I am testing my Etherwave Theremin kit and found the Volume Oscillator circuit not Oscillating, zero output. The Pitch Oscillator circuit is functioning at 260KHz. I verified this with an Oscilloscope probe near the Tank circuit. The only discrepancy I found is in the schematic. The schematic shows Q6 and Q7 have R15 & R17 470 Ohm Base to ground resistors. My circuit board has Q6 Base resistors R15 330 Ohm and Q7 base resistor R17 470 Ohm.

Can someone please verify if the 330 Ohm is the correct value for Q6 base resistor?

Posted: 12/17/2017 12:57:09 AM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

To add to the mystery...

I get off my lazy ass holding a glass of scotch. My 10 year old EWS has R15 @ 330 ohms & R16 @ 1500 ohms. These limit the current through the transistors and would not think the values are critical. But this big difference makes me think otherwise, maybe to keep the osc from stalling.


Posted: 12/17/2017 9:32:49 AM

From: Colmar, France

Joined: 12/31/2007

Moog modified the resistor values around the oscillators from time to time. The first time was to handle the different DCR of the SlotTen variable inductors after the TOKOs had become obsolete.

Independent of the exact resistor values, the oscillators will always start at room temperature when the “extra load” from the linearization coils and the antenna is removed. This can be checked by unsoldering one leg of the first linearization coil.

Where the different resistors make a difference is mainly at extreme temperatures and when the additional tuning stages which allow to tune the fixed pitch and the volume oscillators with a potentiometer, simulating a variable capacitance, come into play. If the respective oscillator’s drive is not adapted, these reactance stages tend to start oscillating themselves at around 300-400Hz which results in a dirty, intermodulated sound.

Thus, if one of the Etherwave’s oscillators won’t start, it’s at 99.9% not the resistor’s fault. 

Posted: 12/17/2017 1:33:46 PM

From: France

Joined: 11/13/2017

Before my last etherwave build, i tried some different coils.

I have build just the oscillators as a prototype with the values 1k5/330R for the resistors.

Then i tested with coilcraft slot 7(104 and 683), "Filterspule 60uH" from Box73 aka Funkamateur and others i had in stock.

Allways without a second series inductor, i changed the tank capacitor to get the frequency i wanted.

On the workbench all worked fine, without problems.

In my final build i have choosen the coilcraft 7M3 ones.

I read a post here, i think it was Thierry, that using those 7mm inductors can cause problems due to "wiring capacity"(sorry, i dont find the correct term, my english is not the best).

Actually, all works fine, perhaps also depending on the PCB layout in the case of different routed PCB´s

Posted: 12/17/2017 1:48:09 PM

From: Northern NJ, USA

Joined: 2/17/2012

Simulation of the EW oscillator (without EQ coil attached) in LTSpice shows reducing C3 / C7 to 0.001uF (from 0.01uF) makes the thing start up quite a bit faster.  Reducing R3 / R7 to 33K (from 47k) helps too.  

Oscillator sims don't always track harsh reality, and I haven't investigated this thoroughly, so YMMV.

Posted: 12/17/2017 2:02:53 PM

From: France

Joined: 11/13/2017

"Simulation of the EW oscillator (without EQ coil attached) in LTSpice shows reducing C3 / C7 to 0.001uF (from 0.01uF) makes the thing start up quite a bit faster.  Reducing R3 / R7 to 33K (from 47k) helps too.   Oscillator sims don't always track harsh reality, and I haven't investigated this thoroughly, so YMMV."


I´ll try this out, i am on a ondes martenot "thing", with am mix of the EW oscillators with 1mH coils and variable air caps.

I dont have any simulation software installed, i am going the "try and error" way, calculating the necessary and testing a real prototype after.

What means YMMV?


Posted: 12/17/2017 2:11:37 PM

From: Northern NJ, USA

Joined: 2/17/2012

YMMV - your mileage may vary, caveat emptor, take with a grain of salt, etc.

LTSpice is free and works well for this kind of thing.

One thing you might try is to wind a long air core inductor, get it oscillating in series mode, and slide your finger along it.  I've done this in my lab (with a 100mm long, 48mm diameter, 6mH coil) and it seems to work well as a linear pitch input.  Kind of a blend of Theremin and Ondes.

Posted: 12/17/2017 2:36:14 PM

From: France

Joined: 11/13/2017

Really interesting approach...

i´ll try this out... at the moment, i am at the point, that something has to be "moved."

A multiturn pot, a variable air cap or something via a nylon string for example.

i dont want to buy a "ribbon controller".

Later, i am planing a Therervox rebuild with some filters, oscillators and so on..inspired from Mfos....


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