Building EM Theremin / rebuilding EtherWave circuit

[i]"Is the Moog EtherWave certified?" - Scotty [/i]

I dont know!
I imagine that it would have required FCC certification at least -
If it does not comply with EEC standards, then entities importing these units into Europe are technically breaking the law!

[Rant]I think European manufacturers suffer far stricter policing than importers - this is an entirely unjust bias, as I must incur large expenses to comply, but completely non-compliant products from the East (and some other places)are readily available and on sale.. It is almost as if Europe wants to kill its manufacturers (particularly small start-up enterprises) and just consume crap made elsewhere.. I registered my company with primary objective being design consultancy and electronic product manufacture.. BIG MISTAKE.. I have been contacted by every standards enforcement agency since I filed this.. and had demands for estimates of how many tons of products I will produce anually (LOL!) so they can bill me in advance for waste disposal![/Rant]

Hi FredM and Thierry,

thank you for your answers. Indeed, first I'll have to build a device, and then I can think further.

Anyway, back to topic. Schematic capture started, and I'll post a result when I'm finished for a review.

Regards,

Scotty

[i]"During the years I have seen lots of people who head already difficulties with much easier designs such as the EPE/SC theremin or the Theremax." - Thierry [/i]

Thierry - I am just curious about your assessment of the EPE/SC and Theremax versus the EW.. In my estimation the EPE-2008 is about equal in complexity to the EW, and the Theremax is much more complex than the EW.

I do agree with everything else you say in this posting..

Every EW 'clone' I have built has worked first time without problem - even when built using "Road runner" point-point wiring - and every sub-section I have played with has behaved with far less problem than other circuits.. The oscillators in particular are incredibly tolerant of component variations.. Conversely, I have found the Theremax is a real pain - The EPE/SC designs are less tolerant than the EW, but can still take quite a lot of 'bending' before things stop working.

I actually think that, as a kit, the EW is as easy to get working to make a good Theremin as any of the other common designs.. After all, it was originally designed by Bob as a do-it-yourself without even the luxury of a PCB!

Please excuse me if I failed to express my thoughts correctly. I didn't want to say that the Etherwave circuit was more critical than others. But I wanted to say that the way from a working circuit board to a well playable instrument might be longer. The EW is the only of the above cited circuits with linearization coils. The advantages of this desgign are well known. The disadvantage is that since there is no possibility to tune the series resonant circuit formed by the linearization coil and the static antenna capacitance, even small deviations of the latter may decide about good or bad linearity and thus playability. As every EW owner who tried to optimize his instrument knows, even the small screws which keep the cover in place have an important impact the pitch response. And there are still the big screws which hold the cover together in itself.
So I meant that even a perfectly working circuit board is no warranty for the same good playability as a ready built EW. Perhaps B. Moog didn't yet know about what could be thé optimum when he published this as a diy design.

Thierry, please dont get me wrong - and no apology is needed.. I completely agree with you that building a Theremin, even if designed by Bob Moog, is absolutely loaded with potential hazards... And that producing a kit for others to build from scratch could be an easy road to ruin..

The good kits on the market are supplied with comprehensive documentation - and this is essential.. But even with this, I think we probably see more cries for help here at TW from those building a kit from parts than from those assembling a Moog EW.

The EPE-2008 does have an EQ coil for pitch, and on-PCB components to assist tuning the oscillator for optimum linearity - Despite being about the same complexity as an EW, this is an easy kit to get working, and in my opinion its an extremely good Theremin for its price..

BUT, I am not really well placed to judge how 'easy' it is to get a Theremin kit to work .. There is, for example, a small error on the EPE-2008 board which shorts out the audio signal (pads connecting the opto isolator VCA are merged) and an inexperienced constructor may spend many hours debugging this - or give up.

So I suppose the bottom line is that it all depends on ones knowledge and expierience - and building a Theremin from scratch can require quite a lot of both!

I (foolishly) sold a couple of spare kits I had bought.. It has cost me far more in supporting the purchasers to get these kits running as good Theremins than I can justify - even if I had sold these kits for a profit.. Big net loss.. except that some donations were made to http://www.streetkidsrescue.org/ which made me feel better about it! ;-)

BUT - In terms of design.. I see no reason why a [b]well engineered and documented and supported[/b] kit based on the EW design should be any more complex or difficult to get working than the SC/EPE or Theremax.

Hi,

schematic capture is mainly done :)

By drawing the schematic, my understanding of the circuit has grown. As far as I can see, it's not possible to connect the circuit directly to a PC or something like that, because the audio signal is separated from the volume circuit. is this correct?

So, I think I need a additional circuit to control the volume of the audio output and condition the signal for a input to a PC. Does such a circuit already exist?

Regards,

Scotty

The common experience is that connecting a theremin into a PC does make problems. Theremins are very sensible to grounding issues, the harmonics of the computers SMPS power supply and so on. Since the EW has a line level output, you should connect it to a keyboard amp. If you really MUST go into the PC (why ?) then use a buffer preamplifier or a mixer in between.

Recordings are done with best results by the acoustic way: Play the theremin through 1 amp, add the accompaniment through another amp and record the acoustcally balanced mixing with a good microphone. Peter Pringle has shared much of his professional experience under the nick "coalport" here in the forum. Just use the search function.

[i]"As far as I can see, it's not possible to connect the circuit directly to a PC or something like that, because the audio signal is separated from the volume circuit. is this correct?"- Scotty [/i]

I do not really understand what you want to do..

The AUDIO-OUT from the VCA (via R33 and the filter / attenuator C26 and R34) is nominally at line level, and the amplitude of this signal is controlled by the volume antenna.

Problems can occur if all HF is not removed prior to taking this signal to other equipment - particularly digital processing equipment where HF MIGHT interact with the sampling frequency of the A/D on this equipment.

R34 can be replaced with a 5k Log potentiometer to give a output volume control.. Wiper of this potentiometer taken via a non-polarised 10uF capacitor.

Adding a good active LPF with roll-off at about 16kHz will ensure that no HF gets to the audio equipment.. But for best results this should not be in the same box as the Theremin circuitry - or if it is in the same box / on the same PCB, Extreme care must be taken with the layout and shielding if this circuit.. There is no point in having a LPF if HF manages to leak into the wires connecting this circuit to the output socket!

Grounding is also absolutely critical.. but much has been said about this elsewhere.

I connect my Theremin outputs to a PC soundcard regularly (for quick crude analysis of waveforms / spectrum) and also to PC via digitising 'scope box (PICO ADC-212).. I have not had problems even when I dont go via an external LPF.

(one thing I always do is to have a low ESR 1n capacitor strapped across the audio output socket .. this greatly attenuates any HF picked up between the board and socket - it MUST be LOW ESR.. a 100pF LOW ESR is often enough.. it is the ESR [Effective Series Resistance.. Capacitors are never perfect, they have inductance and resistance in series with the capacitance.. One wants as near to perfect "capacitance only" as you can get, as any series element reduces the effectiveness at high frequency] which is of paramount importance)