My first E.P.E. Theremin build.

Hello everyone. just become a new member of this forum. I have built a digital Theremin which obviously sounded rather harsh, so I am waiting for parts to build a prototype E.P.E. analogue type. My question or concern is that according to the literature, the range of the pitch is only a few inches :-(, so I was wondering if anyone has experience with the EPE design, and has achieved a more useful range of say a couple of feet ?
In the meantime, I'll construct a prototype and report my findings. Icouldnt source BF195 transistors either, so going to try BC182 instead.
Any feedback / comment to this would be appreciated. Thanks

If this is the EPE May / June 2008 design, have no worries (except that there is a minor fault on some of the boards supplied by EPE - a short across the optical VCA).

The EPE 2008 Theremin is really nice, even in its 'raw' form (I have used it as a test-bed for some of my developments).. It has a good linear range if tuned correctly - easily 50cm on the pitch field..

Of the available kits, The 2008 EPE is, in my opinion, one of the best....

The later theremins from Jaycar / EPE are not so good.. they are a BIG step down - Equalising coil is ommitted, linearity is (therefore) crap, and 'tuning' is achieved by altering the antenna length -

Really 2 extremes - bloody good, and bloody awful!

(the above is my personal opinion - I have landed in trouble for making comments "against" the SC/EPE/Jaycar Theremins.. I am NOT trying to 'put down' the 'competition'.. But I passionately believe that taking a good Theremin, and turning it into a useless piece of junk, just to cut a few dollars off the price, is a crime! ... To me, there are two classes of theremins - there are squeeky effects boxes which have little musical use, and can be made extremely cheaply - And there are Theremins which are real playable musical instruments.. The EPE 2008 Theremin is absolutely in the latter class - it IS GOOD... They have taken this real musical instrument and turned it into an expensive squeek box, for less than $5 saving in parts cost - its a damn [b]Crime![/b])

Just been checking for an EPE Theremin which used BF195 Transistors, and found it!

Ok - Forget everything I said above.. This is the 1995 EPE Theremin, Yes?

A.) Linearity will be awful.

B.) I cannot say why one would only get a few inches of playing field.. Without doing maths and running simulations, it would be guesswork - I have built similar single transistor oscillators and not had a range problem - but it all comes down to component selection and the finer design details.

Why do you want a Theremin ? Do you want to play music on it, or just to use it as an effect?

If you want to play music, this is not the design to go for.. it cannot even be tuned without adjusting the VC1 trimmer, and has no volume antenna..

Welcome to TW! I am the nasty engineer here, who posts long incomprehensible theses, and makes everything seem more difficult and 'learned' than it really is! - But I am really just a bag of hot air - dont get frightened by me! ;-)

Fred,
Yes its the 1995 epe diagram.
The parts came, and I built the theremin on the weekend.
However, no audio ...??.. Both oscillators oscillate with a clean sine wave. The variable oscillator has a higher output than the fixed one, but i reckon thats due to the power supply for the fixed. They both " mix / beat " and before the AM detector, I get the typical envelope waveform, and this drifts as your hand approaches the variable oscillator, however, after detection, theres no audio ?/ I 'm using the BAT diodes too, Help please anyone .

Regards
Adrian... oh and thanks again

[i]" Both oscillators oscillate with a clean sine wave. The variable oscillator has a higher output than the fixed one, but i reckon thats due to the power supply for the fixed. They both " mix / beat " and before the AM detector, I get the typical envelope waveform, and this drifts as your hand approaches the variable oscillator, however, after detection, theres no audio ?/ I 'm using the BAT diodes too, Help please anyone ."[/i]

I know you are saying you see the modulation signals - What follows is a general diagnostic routine which I have edited to include your specific component references..

Ok - First - You are getting waveforms from both oscillators.. BUT.. Are these waveforms at different frequencies? This is NOT easy to determine, as they could be extremely close or even 'locked'.. But (with a circuit like this which has unbuffered oscillators) observing / measuring the independent waveforms is difficult (this is one reason I now suggest that beginners start with the SC/Jaycar/EPE kits - They have well buffered signals you can put a 'scope or frequency counter on).

If you are not sure - You need to disconnect the common side (the side connected to TR3's base) of C11 and C12, and connect a dual channel 'scope to each of these 'floating' leads first.. Be sure that the oscillators are independent.

At the base of TR3 there should be extremely little signal - and a DC level of about 0.6V.. TR3 is quite strangely configured.. The voltage on its collector should be the signals from the oscillators mixed in a non-linear manner, to produce a waveform on the collector which is a high frequency (sum of the two oscillator frequencies) modulated by the audio frequency (difference of the two oscillator signals).

If you get this modulated waveform on TR1 collector (you should be able to view this with any reasonable 'scope - even if you cannot see the waveshape inside the modulation envelope) determine what the frequency of this modulation signal is.. it may be too high.. in which case the filters (c15,C16,R15 etc) may be rejecting it.

There is no reason I can see for any problem with the rectification circuit (D1,D2) .. If you are getting an audio modulation waveform whose AC amplitude is >1V on TR3 collector, You should get audio across R17, across VR1, and then on the wiper of VR1.... If you get audio here (and often the potentiometer / wiring to it, turns out to be the problem) then it is the output amplifier stage giving the problem.

If you have a high impedence piezo speaker or earpiece, this is a superb test kit for stuff like this.. probe about and trace the audio (if you dont have one, get one.. Piezo with series 1k resistor.. The most essential piece of kit after a multimeter!