The EM Theremin - Pitch and Volume Antenna: not working.

That looks reasonable. Finding about 1.6V instead of the calculated 2.1V may be ok, seen that the resistor values (1Meg and 4.7Meg) are similar to the input resistance of the multimeter.

Now we have still to debug the volume oscillator and antenna circuit, so that the voltage at the anode of D1 will not longer exceed -4V. For that, we let U3 still out to protect it.

First thing is to check the oscillator amplitudes. For that you will need an oscilloscope.

a) Check the signal form at the collector of Q6. You should see a ca. 480kHz sine wave centered around +12V with 24Vpp, thus going from 0V to 24V.

b) Check the signal form at the base of Q7. Since it's the preceding waveform but divided down by C14, C15 and R17, it's now a sine wave centered around 0V with 4Vpp, thus going from -2V to +2V.

Please report back your results.

N.B. If we will (one day) get your theremin working, you'll owe me a belated birthday gift!

Thierry, if we will one day get my theremin working, I will have your bust (the sculpture kind of course :p ) built outside my house!

On friday I will have access to an oscilloscope. Until then I am doing a little studying on the circuit. Any suggestions for any helpful and interesting read, would be much appreciated!

Ok, I have the results from the measurements:

a) A sine wave is produced at ~480kHz, centered around +12V with ~24Vpp

b) The signal at the base of Q7 is a sine wave centered around 0V, going from ca. -3V to ca. +3 (a little less than -3 and 3).

Hi Thierry. So, the volume antenna seems to work ok now that I replaced the wrong resistance. I put U3 and it works fine. The big variations at the voltage at D1 must be fault reading of the cheap voltmeter, right?

The pitch antenna problem remains though. When I keep my hand at a steady distance from the pitch antenna, the pitch doesn't stay the same but it changes smoothly and unperiodically. I observed that when I blow towards the linearization coils, the pitch goes up relatively quickly. Does anybody have any idea what might be causing that?

" I observed that when I blow towards the linearization coils, the pitch goes up relatively quickly. Does anybody have any idea what might be causing that?"- Blala

Yeah -

What happens when you blow on something? If the something is warmer than your breath, it will cool - if its cold, it could get warmer.

I think this is the most likely cause of that problem ... So the question.. What coils did you use?

Extreme drift is more likely if there is a lot of current through a component ( coil ) causing it to dissipate a lot of heat - under these conditions blowing on it can quickly cool it substantially - are the coils warm to touch? (they shouldn't be - they should be ~ room temperature) - If they are, you could have another fault.

But it may not be anything related to the above - there may be a diode or something that's getting cooled by a side-draft, or it may just be the position of your face when blowing that's adding capacitance and shifting the frequency.

"When I keep my hand at a steady distance from the pitch antenna, the pitch doesn't stay the same but it changes smoothly and unperiodically."

Perhaps try closing the box or wrapping the board in thermal insulator.. (NOT "Space Blanket" !!! ;-) [Polyurethane foam or similar sheets of white closed-cell foam or even a thick piece of cloth wrapped ' round the board should prove something.] In an enclosed container temperatures tend to be more stable - I wrapped my inductors (or encapsulated them) in polyurethane foam, because they (both the antenna inductors and the ones on the board) are IMO the biggest source of thermal drift...    The most significant are the antenna coils - IF you havent used specified Bourns 63xx or Miller coils (or other good coils with minimum inductance change as a function of temperature) these are likely to drift like hell unless you insulate them and / or compensate in some other way.. And even the specified parts need to be 'isolated' from extreme (a few degrees!) temperature changes

Note - wrapping the inductors doesn't really provide a comprehensive solution, but it does help - it slows the rate of temperature change at least, so in an environment where temperature us varying, rather than having to re-tune every few minutes you may only need to re-tune every hour (if at all).. it all depends on a load of factors - how much heat the component is producing, how good the thermal insulator is and what other thermal conduction paths exist, what other local components are dissipating, air flows / convection paths, the outside temperature etc.

Fred.

Please help me to understand something - Why did you "blow towards the linearization coils" ?  ... There must have been some thought process behind this act... ?  I ask this because I am finding a lot of this type of thing recently, and I dont understand.. I am trying to understand how other people think / relate to the world, because its becoming obvious to me that I am "odd" !    If I blew on a coil  my thought process would be to test the effect of air current (even if I didnt think before I did it) - the only reasonable conclusion I could draw if it had an effect would be that temperature was a possible factor - and I would then act to verify or dispel this hypothesis.

But a lot of people seem to do things that make some sense, but fail to act as if they had any idea about why they did it!  Is there a huge gulf between "engineers" and "scientific thinkers" and "normal" people? Do "normal" people do things like whacking a boiler when it wont ignite, or shake their leg when they get pins+needles, or blow on a coil "instinctively" without knowing the (possibly) sensible reason behind what they are doing, and then get stuck when they get results they dont understand?

Please understand that this is a genuine, non-judgmental question - I am just really interested in thought processes of other humans and myself.

Hi Fred!

It is not the linearization coils that are affected when i blow after all. It seems that L5 (the variable inductor from the variable pitch oscillator - I used coilcraft's SLOT TEN-5-10) and the two regulators at the power supply circuit are affected. L5 is not warm (it doesn't seem warm when I touch it, or the parts around it) but the two regulators are warm. Not that much so you cannot touch them though. Actually the regulators are the only parts that are warm, but I guess that is normal, right?

These two parts have an opposite effect when I blow: if the pitch goes up when I blow towards L5, the pitch will go down when I blow towards the power supply circuit.

I covered the circuit with a piece of clothe, and after some time (more than ten minutes) the pitch seem to be more stable. Still not completely stable. So it seems that I should insulate L5 and the regulators?

As for your last question, while I was writing my previous post, I was thinking "why the hell did i blow in the first place?" but I really couldn't remember. It probably happened by luck. I thought that the cause of the pitch change might be due to high temperatures, but as I said above nothing seems warm by touch, except from the regulators. Also, I thought that if that was the case, then SOMEONE from the forum must have encountered the same problem, but I didn't find any similar topic here. The truth is I didn't put myself in the process of testing my hypothesis somehow.

Temperature drift in theremin oscillators is a normal phenomenon. The circuit design of both oscillators is similar, so that they normally should drift together thus compensating the potential problems. For that, both have to be at the same temperature which is more or less the case if you close the lid of the enclosure. An open circuit is always more temperature sensitive because of possible local temperature variations as you create them by blowing. Drift during the first 15 min. warm-up phase of the Etherwave/EM theremins is normal. That's why I switch mine never off, it's always pre-heated.

Hi Blala, thanks for your answers!

One warning - DONT insulate the regulators! - the regulators need to be able to dissipate heat! - (also, the regulators are common to both oscillators, so if these are correctly built changes to voltage from them due to temperature should have no effect) ... IF the heat from the regulators is coupling to one oscillator more than another, this could cause a problem - but (as Thierry says) in a closed enclosure things should balance eventually.

One thing I have done on other analogue circuits that are temperature sensitive is to place thermally conducting material in critical areas to keep components at the same temperature - As Thierry says - its the thermal difference between like circuits that causes most drift... So I suppose the ideal would be to thermally couple all relevant components to each other, and then insulate them thermally from outside fluctuations.

A standard trick is to heat the whole enclosure to a temperature above maximum ambient and keep this regulated - this is what Moog did with their Source synthesizer.. But this takes a lot of power.. I have used Peltier devices to regulate the temperature to 30C, and driven this either heat or cool as required, but this was on other analogue circuits that could afford the price tag!

Its not the temperature that matters so much as the difference in temperature between oscillator components, and changes to this.

"The circuit design of both oscillators is similar, so that they normally should drift together thus compensating the potential problems. " - Thierry

Makes me think that perhaps you should check - Have you used the same type of components on the oscillators? Particularly capacitors - If in doubt (type is not explicitly specified) , these should all be COG / NP0 - its not good enough to just drop any old capacitor in the circuit even if the values are the same! If you fitted X7R on one oscillator and a Z5U on another, they may look the same, but they wont behave the same - you need class 1 capacitors with a 0 tc (NP0 or COG) or a class 1 capacitor with a deliberately selected tc if you really know what you are doing and had reliable data on the tc of the inductors (which I dont :-) !

Fred.

I think that Peltier-elements are a bit an overkill for a simple and cheap theremin circuit as the EM or Etherwave Standard/Plus design is. These circuits need from 5 to 15 minutes to get on temperature and to stabilize (in the closed wooden enclosure) and that's IMHO ok for an instrument in the <500$ class. There are reasons for other instruments like the Etherwave Pro or the tVox tour being much more expensive. The Etherwave Pro which uses a sophisticated current mirror technology to make sure that both oscillators are always running on the same working point needs still 2 minutes to fully stabilize (although the warm-up drift is already very weak). The tVox tour couples both oscillators thermally by glueing the transistors from two sides on a copper shield (which at the same time gives an excellent electrical decoupling), but it also needs a few minutes to get a stable pitch field.

"I think that Peltier-elements are a bit an overkill for a simple and cheap theremin circuit as the EM or Etherwave Standard/Plus design is. " - Thierry

Oh, I completely agree - it would probably be overkill for even the most elaborate theremin one could design - as I said, "I have used Peltier devices to regulate the temperature to 30C, and driven this either heat or cool as required, but this was on other analogue circuits that could afford the price tag!" - Oh, I have thought about using them and messed about with them for theremins a bit ;-) (They aren't expensive - its the containment that can become expensive!) .. But have actually only used them on one medical product which really needed temperature to be within a narrow band, and played with them a bit on analogue synths.

The only reason I mentioned it is the same reason I mention a lot of things - IMO, the more ideas are in the "matrix" the greater the chance of inspiring others to think "out of the box" - I have seen engineers agonizing over the trade-offs between running a component too hot, and risking ambient being higher than its ideal operating temperature.. Its this sort of situation where Peltier heater/cooler us ideal.

Fred.