Possible EW Modification board - *continuation part 2*

[i]"- If one were to attempt to synthesize the classic theremin tone (RCA, Rockmore - Golden tone? Impossible, I'm sure ;) Has anyone thought about the waveform makeup more than I have? Sine, Triangle? How it changes over pitch and amplitude? I know the "Claramin" was a special case, though."[/i]

The original Theremin and RCA instruments all had a sine wave that was distorted by the VCA, causing the tone to become more complex. This is the design that the Etherwave emulates, although the distorted harmonic content is slightly different due to the use of transistors instead of tubes.

If you have a pure sinewave and get rid of the distortion, as Fred keeps talking about, you will end up with an instrument that sounds like a Tvox or a Theremax, both of which sound nothing like the original tube instruments. The Ethervox in it's current state sounds much more like the originals.

The answers I give below may not be correct - they are my best guess at this time:

[i]"would this box come out more or less expensive for someone then just upgrading to the plus and getting some modules? If it is more expensive does it offer more options or do something better?" - dae23 [/i]

The box should be a LOT cheaper than modules (VCO+VCF+VCA) and does not need the plus upgrade.

I believe that the + upgrade and good modules will be better than the box - it will have more options and would allow all the sound synthesiser functions provided by the modules .. but one is probably talking about £200+ to achieve this.. I expect the box to be (probably a lot) less than £100 parts cost.

[i]" Maybe a box with knobs and in/out jacks that mounts on the mic stand just below the E-wave." - dae23 [/i]

This would be a good place for it - Anywhere accessible which does not interfere with the Theremin fields.

[i]"As in, if your theremin has a linear volume response then one should use an exponential VCA. Is this correct and if so, does anyone know the Etherwave Standards response?" - dae23 [/i]

We percieve sound level exponentially - doubling the [b]power[/b] from an audio amplifier does not sound like a doubling of loudness - the percieved increase in loudness is probably about 10%. However.. We are FAR more sensitive to pitch change than we are to volume change, so this is not nearly so critical.

The present EW VCA is nearly linear as far as I can see.. So if one is happy with the EW volume response, then a linear VCA should be fine.

[i]"- If one were to attempt to synthesize the classic theremin tone (RCA, Rockmore - Golden tone? Impossible, I'm sure ;) Has anyone thought about the waveform makeup more than I have? Sine, Triangle? How it changes over pitch and amplitude? I know the "Claramin" was a special case, though." - dae23 [/i]

Yes - People have played with dynamics to hone the Theremin sound on analogue synths - I think Kevin Kissinger is probably the best person to give advice on this.


[i]"And one more for fun.
- Could someone, using a ring modulator, some oscillators, and filters,
make a crude heterodyning oscillator to be played on a keyboard (maybe even with a CV theremin)? For no paticular reason,
just because you can." - dae23 [/i]

No. Not unless the oscillators were tuned to > about 50kHz and the ring modulator and following filters could handle the resulting frequencies.. Even IF the above was ok (which it wont be) one needs some fancy modifications to the variable oscillator.. The 1V/octave frequency change will NOT give a 1V/octave difference frequency out..
in My heterodyning VCO, this was (and to an extent still is) the biggest technical challenge in the design.

Example: Reference oscillator = 200kHz
with 0V CV, tune variable oscillator to (say) 200.032kHz (32Hz difference).. Now, in a normal VCO, adding 1V to the CV would double the VCO frequency .. With this VCO, only the DIFFERENCE frequency must be doubled, to take it up to 200.064kHz.. Difficult enough to achieve - but extremely difficult because one needs to incorperate the exponential converter and only work on a fraction of the actual oscillator frequency - I have not yet got perfect tracking - but it is as good as some VCO's. Perfect tracking is not essential for the Theremin, but it is essential if the CV is also going to drive other VCOs as well.

[i]"If you have a pure sinewave and get rid of the distortion, as Fred keeps talking about, you will end up with an instrument that sounds like a Tvox or a Theremax, both of which sound nothing like the original tube instruments. The Ethervox in it's current state sounds much more like the originals."- Eitherspiel [/i]

Has anyone actually run a spectum analyser on signals from the "original" tube theremins and compared these to the EW?

I suspect that, if one was to examine spect

Yeah, the harmonic distortion of tube stages is rather perceived as "warm" while on transistors and op-amps it's rather qualified as "cold". It seems that the rather square-function response curve of tubes (and some FETs) gives nicer harmonics than the exponential-function response curve of transistors. OTAs have also an exponential response since there is no feedback loop as on "normal" op-amps.

I should perhaps tinker a FET amplifier stage which allows to play around with different working points so that one may find the desired amount of quadratic distortion. I have still a "nude" Silicon Chip theremin with a nearly sine wave output which will help me evaluating the results.

Good idea Thierry.

My understanding is that vacuum tubes "squash" the waveform in the original RCA designs, whereas transistors tend to "clip", hence the difference in sound in an otherwise analogous design.

However, in experimenting with the Etherwave, I suspect part of the "coldness" has to do with the harmonics that Dr. Moog chose to filter out when he designed it. I have been able to get a tone with the standard that is similar to the RCA by pulling harmonics through post processing in my soundboard - the sound is similar, but becomes different in the highest register, which is where the distortion decreases in the Etherwave and it becomes much more sine-wavish.

The E-Pro has horrible filtering applied which sounds like a super-bright high-pass filter, which makes it sound extremely cold - I have never found a way to make it sound warmer other than making the sound extremely dull.

Fred -

Good point about the vintage speakers and amps used in the original equipment. However, I would venture to say that it is one of the components that makes them sound so sweet - if one could replicate the harmonic content in a complete RCA system, that would be a feat.

I do not think RCA's had a tone similar to a Tvox. Peter owns a couple of RCA's - I would love it if he were to post some sound samples through modern equipment without any of the processing he normally applies.

[i]"However, in experimenting with the Etherwave, I suspect part of the "coldness" has to do with the harmonics that Dr. Moog chose to filter out when he designed it." - Eitherspiel [/i]

Please, enlighten me.. What bit of the EW circuit does ANY selective harmonic filtering?

[b] Edit > [/b]

[i]"if one could replicate the harmonic content in a complete RCA system, that would be a feat." - Eitherspiel [/i]

To do this, one would start by tailoring the frequency response - this, in its most primitive, would be a filter to attenuate high frequency components - resulting in a more sinuosoidal waveform being heard from the instrument...

QED?

[i]"Please, enlighten me.. What bit of the EW circuit does ANY selective harmonic filtering?"[/i]

According to Dr. Moog's own design analysis of the Etherwave:

[i]"C24 and C26 roll off the high-frequency harmonics...."[/i]

You might wish to download the Etherwave "hot rod" manual and read especially page six, which contains much of Dr. Moogs explanation of the design.

Another interesting thing he states about the EW circuit:

[i]"The audio waveform is applied to the input of Section A, at a level which is high enough to clip it. This has the effect of reshaping the audio waveform from a skewed sine to a quasi-rectangular wave, which is very similar to the waveform of Theremin's original instruments. P3 varies the input resistance of Section A, which determines the amount that the audio waveform is clipped"[/i]

Although he is correct about the waveform being similar, the vacuum tube design of the RCA has the effect of "squashing" the waveform instead of clipping it, and this is why the RCA sounds a bit different from the Etherwave, even though the waveform is "similar".

Here is an interesting question:

What if you could somehow replace the transistors in the EW with tubes (a voltage nightmare, I am sure). Would the sound be much more like the RCA?

A tube Etherwave! I can see them sticking out of the top now - Awesome!

A whole new meaning to not touching a theremin while you play.

Eitherspiel -

I do not really want to get into any arguments about any of this - I know you love the EW tone, and I know you trust what is written in the manual as gospel - and that is fine..

But please - comments like "You might wish to download the Etherwave "hot rod" manual and read especially page six, which contains much of Dr. Moogs explanation of the design." are a little laughable and insulting.. I have on my desk 3 printed copies of this manual, marked up and highlighted.. On my PC I have a fully functioning simulation of the entire EW pitch and audio sections..

This does NOT mean that I am beyond learning from others.. Often a beginner can say something which gets an expert to see something in a different light..

The description of the circuit operation is simplified and written at a level which can be understood by hobbyists - it is not the kind of description or detail that Bob Moog would have exchanged with other Engineers.

"C24 and C26 roll off the high-frequency harmonics...." is a statement which almost does not need to be said .. but they DO NOT DO ANY [b]SELECTIVE[/b] HARMONIC FILTERING ! - Also, No mention is made of the fact that the roll-off of C24 is determined by the current into the OTA.. Which demonstrates the TRIVIAL and INCOMPLETE nature of the "analysis" presented in this document. THIS IS A DOCUMENT FOR HOBBYISTS, NOT FOR ENGINEERS!

Also, the section you copy "This has the effect of reshaping the audio waveform" could, much more accurately, be written as "This has the effect of [b]Distorting[/b] the audio waveform" - The use of "reshaping" is verging on fraudulent - no other technical description I have ever come across calls distortion "reshaping" - Technically, the use of this word is borderline.. Reshaping usually applies to w waveform which is chopped up and re-assembled differently - like if one chops up a triangle wave and forms a double frequency ramp wave from this.


I am not out to denigrate Moog or the EW - I am hoping to design a simple box for those who do not like the sounds available from it.. You love the EW sound pallette - so this should be of no interest to you.

My goodness Fred - I think you misunderstand.

I wish you nothing but the best with your theremin endeavors and hope to see a great instrument soon from you.

Please take a deep breath and realize I am "on your side" here. :)