My New Year gift to TW: A new theremin circuit

Posted: 4/10/2018 11:55:33 AM

Joined: 4/2/2018

I am not very active but dislike lonely threads. 

I appreciate your intervention, thank you

Home PCB making is a good thing to learn. I would never recommend construction of a high frequency analog project like this on one of those white proto boards due to unwanted capacitance coupling between components.

I understand that. I know that building a theremin is better done on a PCB than on a breadboard. In fact, since I have very little experience in soldering but I have used breadboards a lot in the past, I was thinking about trying to make this circuit on a breadboard first, before go further to solder it. Besides, in this thread and on the internet there is a lot of evidence that on a breadboard it can work as well.

In theremin design an ugly sound vs a beautiful sound are only separated by a thin margin of knowledge. 

Now that is exactly my point. What I got is not even close to ugly sound, nor is the noise I'm getting in any way affected by my hand. The circuit as it is now on my desk is very far from being a theremin. Since Thierry's idea, undoubtedly, does work, I must assume something in my implementation has gone wrong.
But is that really the use of a breadboard? Wouldn't that only affect the quality of the sound, instead of the working of the whole circuit? Also, if it's the breadboard, by disassembling the circuit and putting it together again in different rails I should hear some difference, or, if I'm very lucky it should even get better. This behaviour, though, I'm absolutely not seeing.

Posted: 4/10/2018 12:42:02 PM

From: Colmar, France

Joined: 12/31/2007

Just a few quick remarks (not much free time actually):

5V is not enough supply voltage, stabilized 6V are needed, that's why there is a 7806 voltage regulator in the schematic. With the original 5484 FETs which have been explicitly chosen for their extremely low pinch-off voltage of -1.5 to -3V, the circuit is known to work from 6V on. You decided to use incompatible FETs with a pinch-off voltage which can even go beyond -6V. How can you expect an oscillator to work if the required source-gate cut-off voltage of these inappropriate components exceeds already the available power supply voltage? 

Besides all that, a quick check with your oscilloscope would have shown you that the oscillators do not work this way because of too high quiescent current due to a too low Vgs.

C44 has to be identical to C14 and both have to be 3.3pF. If not sure about their marking, take your capacitance meter and check them.

Thus, build the power supply section as required by the design, buy the needed FETs, and then write back, because then there are other issues to fix like the circuit layout: In Theremin circuits, each 1/100 of a pF matters. Breadboards are a no-no. And the antenna connection has to be placed far away from all other wiring, that's why on my PCB design, it is in a corner. With the antenna wire close to the audio cable, and the antenna not mounted vertically above the PCB, it can simply not work.

Posted: 4/10/2018 12:55:46 PM

From: Northern NJ, USA

Joined: 2/17/2012

Leave every other row clear on the breadboard, and set the breadboard on a plastic box - both of these will reduce parasitic capacitance.  If the capacitor is small and only has a '3' on it, it most likely is 3pF.  What inductors are you using?  I've had poor experience with the types wound on ferrite bobbins.  Also, get your hands on a scope, if only for a long enough to debug things, without it you're flying blind.

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