My New Year gift to TW: A new theremin circuit

The BF256B has a too high tolerance for Ugs(off) which goes from -0.5 to -8V. That means that most of them will cut off below -4V. But the cut-off voltage should not exceed the half of the supply voltage (6V), thus only a cut off voltage of max. -3V is acceptable because otherwise the oscillators would not start and the demodulator stage would not output enough AF signal. That's why I clearly stated that you should use the BF256A.

Edit: The J210 should do the job, too, but not the J211 for the same reasons.

You may naturally buy about 100 units of the BF256B and build a test circuit to identify those whose Ugs(off) is smaller than -3V, but that's not very economic...

Hmm apparently I have the wrong datasheet because it shows the same Ugs for all of them (A B C). Okay, I will order the BF256A. Thank you for your help :)

Hi Thierry! Thanks for your help. We soldered it on the through hole board and it works great! The only issue we're having now is in trying to attach it to a speaker. We tried to have a switch between a 386 amp and an 8 ohm speaker, or an active speaker, but the signal seems to go into the 8 ohm speaker whatever way it's hooked up. Plus the little speaker is affecting the parasitic capacitance effect I think. Anyway. We'll figure it out. Thanks for the circuit!

Hello all!

I'm getting ready to build this... quite excited!   I've seen mention in other threads that Thierry has PCBs for this design... should I get one of those... or should I try my first one out on a prototype board... will that do?

I have some prototype boards in my studio (not breadboards).... the type with the little copper circles at each hole... not the strip.

Will that be good enough?

last words from thierry was like: sure, give me your adress and i like to send you one.... never happend. but give it a try and send him a request about those boards he offered.

the dotted perfboard is pretty good enough.

happy building.

xtheremin, you can not say "never", it can theoretically still happen as long as I'm alive...

But I'm actually overloaded at my day job and with the organization of the upcoming Theremin Summer Academy. I know that I have about 5 of these circuit boards here, but I couldn't find them yet. Dotted perfboard will work, too. I built all of my Thierrymins that way, the boards were just made for a few "build your own theremin" workshops which were organized by Wilco Botermans.

thierry, nevermind. just saw the academy anouncement for the next month. see you soon...

aha!  yes. good. 

dotted perf board it is!

wishing you the best with the summer academy!  Wish I could make it to France for that!  Maybe one day.

The oscillators work in a grounded gate configuration which provides best independence of the internal junction capacitances.

Yes, great schematic idea... But, in the matter, the C42 & C43 have more bigger capacitance then, for example, input capacitance of 2N5484 (5 pF) ...

The grounded gate reduces greatly the Source-Drain capacitance which lets just the 150pF (in series with the 1n5 capacitor) determine the resonant frequency, almost without thermally variable side effects from the source-drain capacitance. Second, the gate-source capacitance is not reduced, that is right, but it (and its thermal variations) become negligible with the 1n5 capacitor in parallel.