moog melodia

"I am not a dang scientist!"  - RS Theremin

Oh yes you are!  Engineering = applied science.

dewster said: "Oh yes you are!  Engineering = applied science."

That's a rather nice complement dew, as I have never worked in electronics. I am just a kitchen table hobbyist entertaining the illusions!

Fred,  Don lives 30 minutes south of me and the only one to ever visit my lab. As an engineer in defense he was laid off and I figured he would show up at TW when the time was right. He is our age (around 60) and employment in Calif is not the best these days.

That Vactrol thingy, I have a lifetime supply of new ones (surplus) specific to my purpose.

Christopher

"IMO, the best sounding tube amps are the ones which are built into the same enclosure as the speaker." Fred.

That's exactly what I'm using to get the best sound out of my old beast, which sounds a lot like a Melodia, even though it has silicon transistors.

I have not tried it at high volume. I can't right now because if I wake my wife, there will be blood and pain. Maybe tomorrow.

About the Vactrol. I made some with CDS cell's, high output LED's, and a bit of heatshrink to handle channel switching in an amp I built. On first test, there was a lot of hum. This turned out to be ripple on the LED's power supply. A regulator cured it, but I was surprised that the CDS cell's were fast enough to pick it up.

"That's exactly what I'm using to get the best sound out of my old beast, which sounds a lot like a Melodia, even though it has silicon transistors." - w0ttm

The above sort of goes against my hypothesis regarding the mechanism of that Melodia sound.. Not because the transistors are silicon instead of germanium (I wouldnt expect this to have nuch effect on the sound, except perhaps if the lower frequency response of germanium reduced harmonics in the oscillator signal - but this should reduce rather than enhance the harmonics heard) but because, AFAIK, there is little similarity between the cabinets of the Kustom and the Melodia.

I would (based on my hypothesis) expect the two instruments to sound quite different - I would actually expect the Melodia to sound "better" because of its more solid construction (wood and large wobbling EQ inductors)

But then it might just be something to do with the acoustic resonances of the ferrite slugs being similar - who knows.... I may just be mad ;-)

One thing I can say is that I dont see anything particularly remarkable on the schematic which explains what I hear.

Fred.

 ADDED >>

Couple of possibly noteworthy things I have seen about the Melodia design.. Some which might be important..

 Tuning is performed using variable capacitor connected directly between antenna and ground - this may have several ramefications:

1.) Unlike most later theremins, where tuning is done on the (usually reference) oscillator frequency by changing either the tank capacitance or (through biasing) its inductance, tuning is done at the most sensitive point on the theremin... Any vibration of the plates of the tuning capacitor will be "amplified" giving far greater "microphonic FM " than other designs.

It could (if my hypothesis has any validity) be as simple as that.

2.) Interesting spin-off of the above is that, once the reference oscillator has been tuned, provided it does not change, if the linearity was optimally set when the instrument was first calibrated, any changes due to background capacitance etc can be directly compensated with the tuning control, therebye maintaining linearity - Unlike later designs where the reference oscillator frequency is user adjustable with the result that linearity can change as a function of background capacitance changes. The E-Vox is the only recent theremin I have seen (apart from mine) which tunes the antenna EQ circuit rather than directly tuning the REF or variable oscillators.

FredM wrote: The E-Vox is the only recent theremin I have seen (apart from mine) which tunes the antenna EQ circuit rather than directly tuning the REF or variable oscillators.

There are still the Big Briar Series 91 theremins and the Henk theremins...

"There are still the Big Briar Series 91 theremins and the Henk theremins..." - Thierry

Thanks Thierry - I had forgotten about the 91's, but I never knew that the Henk theremins tuned at the antenna.

Fred.

Fred, I was referring to microphonics in the amp. It sounds good with any instrument I feed to it. In fact, microphonics were a problem in the first version I built. I would oscillate at high gains.

My theremin is also made of wood, but the layout is a lot different, and it's padded with tuck and roll, so shaking it with sound might not change things much.

When I said it sounds a lot like a Melodia, I'm just judging by what I hear. I have not used a scope to compare the recording with my instrument, and I agree that silicon should not make any real difference, except as you have noted.

"Fred, I was referring to microphonics in the amp. It sounds good with any instrument I feed to it. In fact, microphonics were a problem in the first version I built. I would oscillate at high gains." - w0ttm

I bet that was fun!! ;-)

Joking aside - if the microphonics of your amp are sufficient to cause it to go into oscillation, then I can well believe that the sound will be greatly affected by microphonics even at low volume levels..

"My theremin is also made of wood, but the layout is a lot different, and it's padded with tuck and roll, so shaking it with sound might not change things much. When I said it sounds a lot like a Melodia, I'm just judging by what I hear. I have not used a scope to compare the recording with my instrument, and I agree that silicon should not make any real difference, except as you have noted." - w0ttm

Please dont get me wrong - I believe you! ... All I was doing was breaking down my hypothesis in the light of the information you gave..

I have two favorite games - One is coming up with hypotheses, the other is destroying these hypotheses.. In fact, I almost enjoy destroying hypotheses as much as "solving" a riddle and coming up with an answer - theres something sad about answering a question completely!

As things stand right now is almost perfect.. I know that FM feedback makes a big difference to the sound from my experiments - I suspect that this is the mechanism for the sound of the Melodia, but am not sure.. And I suspect that it may be microphony on the plates of the tuning capacitor in both the Melodia and the Kustom which is the primary mechanism for feedback, but I am not sure..

Enough doubt and uncertainty to keep me happy ! ;-)

Fred

ps - I had planned to try having an FM input on my theremins, so one could take the audio out, feed this into a spring or digital reverb, and feed this back to the FM input - I had started playing with prototyping this before health stopped me - all sorts of interesting stuff was looking promising - from feeding the FM with vocals to get vocoder / ring mod effects, and even feeding noise to the FM which gave a breathy tone... I think there could be big potential using CV out to drive a synth, and feeding the synth output back to the FM (for example, driving a VCF and feeding tuned noise back to the FM) - But for acoustic sounds, feeding the theremin output into my Lexicon MPX1 and its output being fed back to the FM input gave some beautiful sounds. Simple tapped delay line being best.. I think that a simple tapped digital delay line, even with quite low resolution, is probably all that is needed - I was (and am) looking at implementing this in the mixed signal IC I was (am) developing.

Note that this does not produce a 'reverb' effect when driving the FM - it simply alters the theremins tone dynamically.

Yes, the "T" on my keyboard is a bit persnickety.  :-)

Why not put a piezo pickup on the theremin box and feed that to another channel on the amp.I put my hand on my EW box when it was playing with the speaker close by..there was certainly vibrations there for such a pickup to process.

Oh and thanks Thierry for referencing the AC127 and Fred and RS Theremin and all you other guys I always enjoy the interesting banter here.