My New Year gift to TW: A new theremin circuit

That's not oscillation of Q41 which you see, it's signal stray back from Q11. I can't tell you if it's a faulty transistor or if the J311 is perhaps not really suitable for that circuit and Q11 works only by chance/luck. Why don't you use the recommended 2N5484 to prevent any doubt? Although they are out of production, they can be easily found on eBay. The Dutch Thereminist Wilco Botermans buys around 350 of these every year for similar purposes.

Yes, I´ve ordered them when I started building it, but takes a month, wanted to finish it before xmas since it was a gift. Well, one last try will be to test if another J113 does the job, otherwise I´ll just have to wait. Thanks for your help, appreciated.

FET parameters can be all over the place, I'd try swapping out the one that isn't oscillating with another. 

(FET parameters VT and IDSS are actually pretty easy to measure on the bench, you might want to do a quick characterization of the bad one and the good one to gain some insight into what's going on.)

Well, of all possible components it was a faulty inductor, which had a resistans of 1.2 K, the working one hade 10....
So now both oscillators work, and I get sound.
BUT....it´s very high frequent, the lowest I´ve managed to get is about 17 KhZ
How do I troubleshoot that? Have tried to vary the length of the antenna, but not so much luck. Can´t really hear the sound being altered by my nearness to the antenna. I´ve twisted the adjustable capacitor a considerable amount of times too.

Sounds  like things are working good. This is Thierry's beast so will let him bring you home. This is similar to what affected my build, it was my choices.

I have always shy'ed away from FET's as the NPN and myself get along great.

Christopher

I measured the frequencies at both oscillators, the antenna oscillator is running about 460 kHz and the oscillator with the trimcap is running 470-478 kHz.
If I touch the antenna the frequencie goes up and matches the other oscillator and sound is heard. So I need to match the oscillators.
Even with the antenna at minimal lenght the frequency is too low.
I´ve ordered some larger trimcaps, could that do the job? Or am I ruining something else that way? A small cap in parallel with the trimmer?
Guess I´ll just try the last one.
The correct transistors are on their way, maybe they´ll solve everything, but if I get it running with the J113 I´ve got parts for even more Thierrymins 

EDIT: Ok, a 22pf cap in parallel with the trimmer did the job. Now it works.
Just curious, how big a range (pitch) should I be able to get? And how long a "working distance"
Now I´ve got a decima in pitch range, and working distance is from 1-50 cm.

If the antenna oscillator is running so much lower, there could simply be a problem with the capacitive environment around the antenna, so that it does not only "see" your hand but also other (electrically conductive) stuff. To work well, a theremin should be operated with the circuit board placed slightly below the antenna, and power supply and/or audio cabling running straight downwards from there) but out of that with a free space (clear of any metal) of about 1m around the antenna. With that, you'll get the optimal sensitivity.

Hi all,

I've been trying to give this schematic a go, yet I'm having issues I don't know how to resolve given my rudimentary knowledge on electronics.

Issue 1: I'm getting audio output only if ground is also earthed, otherwise nothing.

Issue 2: Audio output is extremely low, almost can't hear it. I've tried all sorts of speakers so issue is not there. Somewhere in the background I can also hear what sounds like a radio station

I'm using J113 fets as these were the only ones available here. Before those I had BF256B, audio output was good but still had the earthing issue. I then read in one of the posts that these are won't cut it, couldn't find BF256A and switched to J113.

I would love to know how to start measuring it and figure out what I did wrong, so any help is welcome. I triple-checked my connections, soldering etc, couldn't find anything wrong there. Most if not all parts are as in post #48.

Thanks!

Petros,

Yes you need a true earth ground connected.

This theremin's audio output must drive an amplified speaker, try one of your computer speakers.

Show us a picture of your build, that might reveal something interesting.

Then share a sound byte if you can.

Good Luck

Christopher

The audio output has line level, which means about 150mV and is designed to be connected to a suitable music instrument amplifier which provides at the same time the grounding/earthing from its three prong wall plug via the shielded audio cable. Computer speakers are not grounded, so that's not a good solution.