My New Year gift to TW: A new theremin circuit

Maybe I do not understand something... let's see to C42&C43... it have high capacity parallel to the antenna and sensitivity may be reduced...

You have 150pF in series with 1n5 which makes 135pF in parallel with the antenna. That is the capacitance which gives enough, but not too much sensitivity for a decent pitch range. The highest playable note (which can be reached before tones come too close together near the antenna) is a little bit above 1.5 kHz which corresponds to 2 1/2 octaves above middle C. More sensitivity would allow to play still higher tones, that's right, but it would make low tones to be away very far from the antenna with an extremely stretched tone spacing. Thus, it's (in musical terms, for playing most common melodies) the best compromise.

BTW: Thinner antennas (as for example in the Theremax design) require less parallel capacitance to obtain the same sensitivity.

It's interesting to see again how a theremin design is seen only with the eyes of a technician or engineer, totally or partly neglecting the fact that the subject is a musical instrument to be played by a musician in the first place... That's for sure one of the reasons why the world has seen so many bad theremin circuits.

I'm still having trouble getting sound out of mine... and I'm a bit stumped as to where the issue might lie.

I built it on pad per hole perf board, and I've used all the identical parts listed in Thierry's design... except my inductors look bigger than the ones on his pcb layout... however they are the same value... This is the part number for my inductors: RLB0912-102KL-ND

I've tested the continuity along the whole circuit path and everything seems good... good connections and no solder bridges.

My transistors and voltage regulator are installed in sockets (I used sockets so that I wouldn't risk overheating the transistors when soldering, and also in case I needed to change them)....

When tuning my variable capacitor, it seems to just turn and turn forever in both directions... is it possible to turn it too far?... or do i just keep looking for a sound?  This is the part number for my variable capacitor: SG3003-ND

I currently have the antenna attached with an alligator clip while testing.

I'm finding it strange that I was able to get sound out of the more primitive and less stable Caglayan circuit... but nothing out of this one.... I'm hoping it's something simple like tuning... Any suggestions on what I should try or test next in order to get a sound would be very much appreciated!

Thanks!

I'm still having trouble getting sound out of mine... and I'm a bit stumped as to where the issue might lie.

I built it on pad per hole perf board, and I've used all the identical parts listed in Thierry's design... except my inductors look bigger than the ones on his pcb layout... however they are the same value... This is the part number for my inductors: RLB0912-102KL-ND

I've tested the continuity along the whole circuit path and everything seems good... good connections and no solder bridges.

My transistors and voltage regulator are installed in sockets (I used sockets so that I wouldn't risk overheating the transistors when soldering, and also in case I needed to change them)....

When tuning my variable capacitor, it seems to just turn and turn forever in both directions... is it possible to turn it too far?... or do i just keep looking for a sound?  This is the part number for my variable capacitor: SG3003-ND

I currently have the antenna attached with an alligator clip while testing.

I'm finding it strange that I was able to get sound out of the more primitive and less stable Caglayan circuit... but nothing out of this one.... I'm hoping it's something simple like tuning... Any suggestions on what I should try or test next in order to get a sound would be very much appreciated!

Thanks!

It is normal that the variable capacitor turns forever. There is one point where both plate sets overlap totally = max capacitance and there is one point where they don't almost overlap = min capacitance. The tuning point is somewhere between these extremes which means that on a 360° turn, you'll find two points where the theremin is correctly tuned.

If you don't find that tuning point, that does forcibly mean that something else is not working. 

This can not a big thing. Wilco built 5 of these Thierrymins in one afternoon and all were working from the very first moment.

The quickest and easiest way to find soldering or connection problems, and/or defective components is using an oscilloscope. A look on the schematics gives you not only an approximative idea of which DC voltage is to find at every node of the circuit, but also which signal, waveform, frequency and amplitude is to expect there. If you aren't experienced enough to estimate that, you might redraw the circuit in LTSpice, a free circuit simulation software and run a simulation which shows you what should happen in the circuit and where.

Thanks Thierry!

alas, I'm on the road and don't have an oscilloscope with me... so that will have to wait until I get back to my studio in December, unless someone in NY state or Massachussets has one and doesn't mind me stopping by sometime.

In the meantime, I'll give LTspice a try.

I have no doubt that the circuit design is good, and that Wilco was able to do 5 in an afternoon... sadly, I have no formal electronics training and while I'm pretty sure I followed your design perfectly... obviously I made a mistake somewhere, and I would just like to find it so I can fix it.

If I lay it out in LTspice and it works... then, I'll still be at a loss for what happened in the real world circuit.  If that's the case, is there anyone in Brooklyn who could maybe lend me an oscilloscope or look it over with me in November?  I'm on the road and will be passing through Acra, Saratoga Springs, and Amherst Massachussets on my way to Brooklyn this week... if any of you are located in the surrounding areas and want to offer a hand and take a look... let me know, and I'll swing by... have electronics and tools (just not an oscilloscope), will travel. :-)

Lacking a scope, I'd look at the DC voltages with a DMM, particularly the voltage out of the regulator, and the voltages across R12 and R42, as well as across R22 and R23.

It could be the FET pinouts, some have the gate in the middle (where god intended), some have it on one end.

Ok great!  I have a multimeter... so I will post my parts, their component name and the multimeter readings. 
Please let me know if you see something that seems off.
Thanks!

======================

battery : 9v : reading 9.27 volts

I will list my part as follows:
what's listed on the schematic   :    what I have and part number details    :    volt meter reading of both pins

using the black lead of my DMM on the negative connection of the battery and the red lead on the pins.

comp 4-22 pF :   Cap Trimmer 2.2-22pF 100V TH   SG3003-ND  : 0.00 , 5.76
Q11 2N5484 : 2N5484 JFet-N-Chan UHF/VHF AMP T092 : 0 , 1.34 , 5.78
R12 470: CF12JT470KCT-ND  470K   1/2W  5%  carbon film :  0, 1.32
C13 1n5: 445-4740-ND    1500pF 50V C0G Radial   : 0, 1.32
C12 150p : 1238PH - ND   Cap cer 150pF 1Kv  Y5R  Radial : 1.32, 5.78
L11 1mH : RLB0912-102KL-ND  Fixed Ind 1mH   200mA 4.3 ohm TH : 5.77, 5.77
C14 3p3 : 445-8584-ND  Cap Cer  3.3pF  50v C0G Radial : 5.78, 0
R21 2k2 : CF14JT2k2OCT-ND   Res 2.2k Ohm   1/4W 5% Carbon film : 0,0
R22 8k2 : CF14JT8K2OCT-ND  Res 8.2 ohm 1/4W  5% Carbon film : 0, 1.05
R11 100 : CF14JT100KCT-ND  Res 100k ohm 1/4W  5% Carbon Film : 5.78, 6.10
C11 10u : P15799CT-ND Cap Alum 10uF 20% 25V Radial : 0, 5.78
C21 10u : P15799CT-ND Cap Alum 10uF 20% 25V Radial : 0, 1.05
Q21 2N5484 : 2N5484 JFet-N-Chan UHF/VHF AMP T092 : 0, 1.05, 5.03
R23 8k2 : CF14JT8K2OCT-ND  Res 8.2 ohm 1/4W  5% Carbon film :5.03, 6.10
C22 1n5 : 445-4740-ND   Cap cer 1500pF 50V C0G Radial  : 5.03, 6.10
C23 10u : P15799CT-ND Cap Alum 10uF 20% 25V Radial :5.03, 1.19
C32 10 u : P15799CT-ND Cap Alum 10uF 20% 25V Radial : 0, 6.10
C31 10 u : P15799CT-ND Cap Alum 10uF 20% 25V Radial : 0, 9.10
V31 78L06 : 296-31461-1-ND UA78L06A CLPR   IC Reg LDO 6.2V 0.1A T092-3 : 9.10, 0.0, 6.10
R41 100 : CF14JT100KCT-ND  Res 100k ohm 1/4W  5% Carbon Film : 6.10, 5.79
C41 10u : P15799CT-ND Cap Alum 10uF 20% 25V Radial : 0, 5.79
C44 3p3 : 445-8584-ND  Cap Cer 3.3 pF50V C0G Radial :0.0, 5.78
L41 1mH : RLB0912-102KL-ND Fixed Ind 1mH 200mA 4.3 Ohm TH :5.79, 5.78
C42 150p :1238PH-ND   Cap Cer 150pF 1Kv Y5R Radial : 1.30, 5.79
R42 470: CF12JT470KCT-ND  470K   1/2W  5%  carbon film :0.0, 1.30
Q41 2N5484 : 2N5484 JFet-N-Chan UHF/VHF AMP T092 : 5.78, 1.30, 0.0

pitch antenna : 5.78

===================

okay... so going through that exercise... I might have found a problem, but there may be other problems too,
please let me know if any of those voltages seem off or wrong.

In listing all my parts, I see that for R12 and R42 which are listed as 470 on the schematic, I have 470K.... :-/
also... R11 and R41 are listed as 100 on the schematic, and I seem to have 100K.... damn letter K!!!

I'll go to radioshack tomorrow and see if I can buy some new proper resistors... hopefully that is the only error.
I'll watch out for the k more closely next time!

Aaaaargh... Using 100k instead of 100R for R11 and R41 and 470k instead of 470R would explain why the thing doesn't work. Who'd expect that?

yes indeed... feeling pretty stupid right now.  i bought my parts in a rush, and missed that completely when ordering. :-/
This will be a good time for me to download LT spice and build the circuit with those 470k and 100K resistors, as well as the proper ones, and compare the difference to see what I actually just did.  A learning opportunity.

like they say I guess... a letter is worth a thousand units.