Etherwave Theremin Mods

[Harry Bissell]

Update... project completed. I use two micro mint toggles to switch in the
lower octaves.

VCA: I put a 10K:1K divider before pin 3 of the LM13600. This gets the VCA
out of overdrive mode... I had to parallel the 4.7K (to ground) on the
inverting input to sort of balance the DC again so the "waveform" control
would work again. No changes to the Brightness control.

Because the VCA is no longer limiting, there is more volume loss as the
brightness control is used. Waveform gives different tones, but not like the
old sine wave.  It doesn't sound quite the same as stock (who cares... why
do you think I modded it?).

The outputs of the 4040 (divide by 2 and 4) go through 100K and .012uF RC
filters (low pass) then to the toggles, then to the summing point (the
positive input of LM13600). Remember there's that 1K resistor so this is a
massive divider (good thing those are 12V square waves)  One or both can be
on at once. There is no switch to kill the original wave (which might be
nice BTW)  Divided square waves have a very pure "no even harmonics" sound.
The original wave is a "skewed sind wave" says Dr. Bob. BullSH1T... I say
its a sawtooth wave with some missing high harmonics... The brightness
control can get a very dark sound, but with a lot of volume drop.

Gotta hand it to Bob Moog...there is not ONE component in that unit that
doesn't do at least double duty.  Modifying it was a bitch... but worth it
???  Well at least MY etherwave doesn't sound like anyone elses... (and I
still can't play it...)

At the extreme low frequency end... the sub octaves give rythmic click
sounds. Until the lowest octave is in the human "pitch regognition" range...
the thing sound like a race car or motorcycle revving up.

Great... now I can leave the vibrator/toothbrush at home...

H^) harry

See you at AHMW2000 if you are there...

Harry Bissell wrote:

> I'm modding an etherwave theremin. Wanted to share tips...
>
> 1: I'm installing an octave divider. The etherwave audio is about 500mV
> pk-pk.
> I tried a CMOS "linear" amp using the 4049UB, with three stages cascade,
> 470K input and 10meg feedback.
>
> BIG BOO-BOO !!! This is a BAD thing. First off, it draws a whopping
> 20mA. Second, while it does amplify, it is prone to oscillation at the
> transfer point (1/2 Vcc). And in the nasty RF environment of the
> etherwave, it oscillates big time.
>
> The next stages cannot trigger on this. And you can't filter it because
> it is right at the trigger threshold. CMOS schmitt triggers just "clean
> up the oscillation. Result, a nasty ass distorted noisy ring mod kind of
> sound... Maybe like the vibrators on steroids...
>
> I used the 4040 divider to get the octaves... each sucessive divider has
> a lower noise spectrum.... but nasty!!!
>
> I trashed this and used a LM311 with a 470K input and 10meg for
> hysteresis. No problemo !!! Now the trigger works perfectly.
>
> The VCA in the etherwave is seriously overdriven. so when I inject the
> dropped octaves they nearly disappear. UNDERWHELMING !!!
>
> Tomorrow I will re-build the VCA so that it is cleaner and can pass the
> more elaborate waveforms. I'll bring the result to the AHMW2000.
>
> Just wanted everyone to know that the linear CMOS trick sucked... IMHO.
> I just wanted to try it once in my life to say I did it. I won't do it
> again. Honest !
>
> H^)       (My new sign... replaces :^) Harry)