Possible EW Modification board - *continuation part 2*

Drat, I tried to get into the last thread before it hit the magic number but I missed it.

To get us back on topic a bit, not that I disliked the ideas being expressed at all...

FredM, with regard to your Etherwave Add-on, I've discussed this with my dear friend who is far more electronically inclined than I and he has agreed to take me under the wing and help point me to all the things I need to learn to do this.

So, if dumb help is better than no help, I'm in.

In the interim while I await a better theremin (better from my POV) this seems like a good idea.

However, while I'm at it I think it's a good idea to undertake another mod to the Etherwave which I'll start a new thread about.

Ok, Carricre,

[i]"FredM, with regard to your Etherwave Add-on, I've discussed this with my dear friend who is far more electronically inclined than I and he has agreed to take me under the wing and help point me to all the things I need to learn to do this.
So, if dumb help is better than no help, I'm in." [/i]

I think that managing this is going to be quite an undertaking! ... I am starting to think in terms of an on-line project, possibly (hopefully) with more than just you and me participating.. If experiments are duplicated on more than one EW, I think we have better chance of blowing up more than one EW ! {LOL - Only joking! ;-)

I will do a set of schematics over the next few days, and post these on my "scrapbook" - These will range from some simple ideas which can be implemented on the EW board, and should provide further insights on which to base the next steps, to a basic schematic of what I think (at this time) the mod board will consist of.

If you could contact Moog, and get a layout diagram for your model EW, that could prove useful - Also, ask if there is any difference between the schematic of your model and that on the EW mamual.. I dont think there will be, but its best to be sure.

Once we have a functioning modification / board, we will need a PCB designed.. This is not something you can do easily - layout could be critical, and a good PCB editor is expensive (for a task like this almost any freeware editor will do, but a good one gives electromagnetic/electrostatic interaction simulation, which really helps to get a good layout) - So unless there is a PCB designer interested in getting involved, I will do this.

I am going to restrict the time I spend on this to a maximum of 5 hours a week, and my total time I spend on TW to 7 hours a week.. I am finding myself becoming an on-line addict again.. I have been here before.. It is an illness! I am at great risk of having to borrow someones EW when I go to "Hands off" if I dont focus on getting my Theremin Transportable..

So, no more from me tonight - Tomorrow I will run simulations to check my hypothesis about FM/PM distortion, and take it from there..

Update:

It is really looking like a non-starter for now .. Sorry everyone.

I have just run several full simulations - this is the entire EW pitch and VCA / tone sections .. The full schematics were entered, including 'real' oscillators (including equalization coils and a simulated player to test effects on linearity), diode mixer, LM13700 and all that precedes and follows these.. Did not include volume circuit - simply biased the OTA to achieve variable gain.. Likewise with the tone controls, which I biased and tested at extremes and mid positions.

This is a simulation - so one cannot fully rely on the results, but..

1.) The FM/PM distortion due to change in capacitance on the diode mixer, has almost certainly been ruled out as the cause of the 'problem' .. It is quite difficult to be absolutely sure because of its nature and the way the simulator operates in 'timesteps' - but I ran an extremely high precision simulation to verify this (My PC was tied up on this for more than an hour) but found nothing significant.

2.) Oscillator synchronisation is the primary source of deviation from sine on the input signal (signal across C23) - and it is quite severe at low frequencies (I was able to adjust my players capacitance down to get 110Hz - but each simulation takes so long I didnt try to go lower) - there was severe distortion down at this frequency.. This distortion extended up to 1kHz - the distortion decreasing proportional to frequency until, at about 2kHz, a pure sine wave appears across C23. The filter (C23) is set quite high (>10kHz) so the reduction in distortion is not due to the filtering effect of this.

3.) The distortion on the input waveform may NOT be the primary problem - but while it is there, there is no possibility of getting a pure sine wave below about 1kHz .. One can reduce the distortion by increasing the capacitance of C23 and C26.

4.) Thierry is right about the VCA / Distortion stage being the most likely "trouble spot" .. Distortion produced by this can be really horrible - I only ran a few FFT's, but these showed that, although the input waveform is distorted, this distortion mainly consists of musically pleasing harmonics.. Feed this waveform into the VCA/Distortion stage, and what one gets out is a horrible mix of really nasty harmonics under most settings of the waveform and brightness controls.

The effect of the VCA Distortion is entirely amplitude dependent, and amplitude is greatest at the low frequencies, so the distortion is really most noticable at low frequencies, but is present right up to 1kHz.

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PROVISIONAL CONCLUSION
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The experiments undertaken at the start of all this may well be as good as any simple solution can get.

Reducing oscillator coupling has an effect on linearity at the bass end, so any change to the coupling could have big consequences.

To clean up the sound, the VCA circuit needs to be changed and the distortion components and controls removed - this can be done without needing a piggyback board.

As I see it now, one could reasonably easily make the EW tone a lot more sine-like just by modifying your board.. But you would lose all tone control and could never get a more harmonically rich sound if you wanted it.. I suspect that most people would be unhappy with this trade-off.

There is a way to overcome all the problems, by taking the oscillator waveforms and re-shaping these, then heterodyning them in a good mixer, and feeding them to a good VCA.. This gives great tone possibilities and everything one could want..

Unfortunately, one ends up with quite a large and complex PCB - I know, because I have made one.. And it would be a waste to put this into an EW.*

I will keep thinking about simpler ways to solve the problems (often, after a few days of stewing in the subconcious, and getting a good sleep, the answer just pops

Hello FredM, Carricre, and all

On the "dumb help being better then no help at all" front - I would like to offer what little I can (too many children, too little time and space :)

I have an Etherwave ready to be blown up, some electronics knowledge (very much beginner), can wield a soldering iron, live down the street from Moog (don't know if that helps or not), and am constantly thinking of Rube Goldberg ways of creating/modifying devices.

FredM's talk (I can't find the quote) of the Etherwave being owned by a lot of people who might be interested in upgrading as opposed to shelling out thousands of dollars for a "pro" theremin, is right on the money, in my opinion. I am in the same situation. I was going to go all out on the Wavefront Classic but kept the Etherwave and dove into modular synthesis, I got to the theremin through synthesizers, so wound up leaning that way. Plus, I could upgrade to the E-Plus and use it in interesting ways.

I would imagine the modboard would need to be compatable with the E-plus and an older Ewave with the Plus upgrade.

FredM

Looks like you posted that last one while I was writing mine, before it looked more bleak :(

If the amplifier is the source of "badness", could one just use the plus upgrade, the pitch preview output (which is not affected by the E-waves amplifier, I think) and an outboard VCA, keeping the heterodyning oscillators for purists?

(I'm still building the modular and haven't upgraded my Etherwave so I can't try it myself, yet)

Dae32..

Dont give up hope..

IF the tone before the VCA is ok, then your suggestion will solve the problem - You would not be able to modify the tone though.. If you are happy with a fixed, lower harmonic tone, then I think this could be facilitated by quite simple modification to the EW board.. You wont get pure sine waves in the lower frequencies, but the waveform would be more sine-like.

Your involvement is most welcome.. I am sure I will have some suggestions in the next few days - But I am no longer so hopefull of having a "wonderful" board which "fixes everything" being the end result of this process.

YOU have a much better option - Upgrade your EW and use the CV to control your modular.. dont bother with the EW tone at all.. tune it so that it acts as a good controller, and use your modular to generate the tones.

Ok - A new idea inspired by Dae..

The expansion connector has raw audio, volume CV and power supplies.. Everything needed..

So, step 1.. take these to a 5 pin socket to the outside world..

Now - Everything can be done in a little add-on box, and nothing is critical or subject to the strange voodoo inside the Theremin.

In this box we have a tunable 2nd order low pass filter - This makes the waveform as sine-like as we want .. But also causes reduction in amplitude at higher frequencies..

Now the bit I am not sure of - Could we follow this filter with a compander or similar auto gain stage.. This could be used to keep signal levels constant over the changing frequency range and, I think, might give an effect similar to a tracking VCF..

The output of the above is fed into a WELL DESIGNED NON DISTORTING VCA, gain controlled by the volume CV.

Now, the interesting bit.. The normal unmodified EW output can still be used.. The original EW with its sounds is unchanged..

Take this audio output and feed it into a simple mixer in the expansion box.. One can then vary 'sine' and 'original' content, providing a whole new sound pallette..

One could also have a couple of switches to change the routing.. For example, one could route the original (distorted) waveform via the filter, to allow other harmonic variation to bwe achieved.

The above scheme has some big advantages... It would be universal (all EWs have these outputs) and PCB layout and all wiring etc outside the Theremin is simple and non critical.

Also, once the connector is fitted, all experimentation is 'external' - so one could continue to use the Theremin as normal.

Thoughts?

That does sound like a fine experimenting idea.

I guess the question would be, 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?

If it comes out to be a simpler, cheaper method to expand the sound, that would be pretty cool. Maybe a box with knobs and in/out jacks that mounts on the mic stand just below the E-wave.

FredM

If I can bounce a couple of side-tracked modular questions off you or someone out there.

- I seem to recall you metioning that if your controlling a VCA with your theremin that you should match (not match actually) response curves between theremin and amp. 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?

- 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.


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.

Thanks