My New Year gift to TW: A new theremin circuit

I think that one hasn’t to be a brilliant engineer to look at a data sheet and to see the the MPF102 has a gate cutoff voltage of -8V which makes it absolutely unsuitable for an oscillator circuit running at 6V power supply and with an intended gate bias of roughly -0.45V.

Hello Thierry,

You created a very interesting circuit, it was the first that I found that included an operation guide with it, it really helped with the troubleshooting. However, I ran into a problem that I can't figure out how to solve, the audio output of my circuit has a constant humming sound, no matter the position of variable capacitor.

I don't have access to an oscilloscope, but since the problem is audio related I tried using a cellphone app that can tell the frequencies of ambient sound. I prepared everything in a silent room and verified the ambient noise with the circuit turned off, which showed nothing. After tuning the circuit to the clearest sound output the app shows a bump of frequencies from 200 to 3700Hz and two bigger spikes at 236Hz and 1313Hz (yes, 1313Hz).

It sounds like a ground loop, but I've used different power supplies for each circuit (5V for the amplifier and 12V for the theremin circuit, which passes through an LM317 to step the voltage down to 6V), the only connection between the two parts is the ground that goes to the amplifier's input (I tried not using this ground, but it sounds even worse).

I tried different antenna lengths too, but no significant difference. The only thing that makes it completely silent is touching the exposed metal tab of the LM317 (that is connected to the 6V output), only then it is playable without any humming sound (even tested with the app, no noise detected).

I would appreciate help with this problem, the circuit uses all the components listed, including the 2N5484 transistors, only the voltage regulator and the inductors (I used two 470uH inductors in series) are different form your schematic. The amplifier I used is a PAM8403.

GuiFS.

Hello Thierry,You created a very interesting circuit, it was the first that I found that included an operation guide with it, it really helped with the troubleshooting. However, I ran into a problem that I can't figure out how to solve, the audio output of my circuit has a constant humming sound, no matter the position of variable capacitor.I don't have access to an oscilloscope, but since the problem is audio related I tried using a cellphone app that can tell the frequencies of ambient sound. I prepared everything in a silent room and verified the ambient noise with the circuit turned off, which showed nothing. After tuning the circuit to the clearest sound output the app shows a bump of frequencies from 200 to 3700Hz and two bigger spikes at 236Hz and 1313Hz (yes, 1313Hz).It sounds like a ground loop, but I've used different power supplies for each circuit (5V for the amplifier and 12V for the theremin circuit, which passes through an LM317 to step the voltage down to 6V), the only connection between the two parts is the ground that goes to the amplifier's input (I tried not using this ground, but it sounds even worse).I tried different antenna lengths too, but no significant difference. The only thing that makes it completely silent is touching the exposed metal tab of the LM317 (that is connected to the 6V output), only then it is playable without any humming sound (even tested with the app, no noise detected).I would appreciate help with this problem, the circuit uses all the components listed, including the 2N5484 transistors, only the voltage regulator and the inductors (I used two 470uH inductors in series) are different form your schematic. The amplifier I used is a PAM8403.GuiFS.

It could be the swithching amp causes the problem.
https://www.diodes.com/assets/Datasheets/PAM8403.pdf
On page 4 it states the switching frequency is 260 kHz,it might interfere though the current from the power supply line, or the ground line.
Solutions could be using an analog amplifier IC, or decoupling the supplies, using different supplies, or just feeding the audio to a real consumer audio system first to find out the cause.

Kind regards,

Leon

"I don't have access to an oscilloscope..."  - GuiFS

Ah, there's your problem right there.  A scope is pretty much mandatory for this kind of work.

dewster said: "there's your problem right there.  A scope is pretty much mandatory for this kind of work."

GuiFS - Go back and read my earlier post. Leon Theremin never used a scope in the 1930’s and accomplished wonderful things. The scope allows someone to see the audio wave shape after mixing. It is here and before where the beautiful sound or wave shape begins. Listen to the sound samples of anyone giving advice to know their understanding of how the analog theremin works. A simple sound sweep of 30 Hz to 1000 Hz will reveal a lot.

Christopher

dewster do not miss the point again, someone with curiosity in building their first $30 theremin does not need a scope. What a new builder needs is the special knowledge you never found so you went digital. Post your own 30 to 1000 Hz sample here to show us how you have destroyed what is beautiful about the theremin. You have this need to prove something to me, I am sad for you.

"Leon Theremin never used a scope in the 1930’s and accomplished wonderful things."  - oldtemecula

As usual, dispensing the worst possible advice.  Theremin didn't use a scope back then because he couldn't, not because he eschewed them.  Literally no EE (Theremin was an EE) would refuse a scope.  The addition of a scope to one's design, testing, and troubleshooting will never, ever make things worse.  Christ.

If you're building a Theremin you need a damn scope.  Really nice ones don't cost that much anymore, and are entirely within the reach of even a semi-serious hobbyist.  If you're really poor try to borrow one, or buy a used one.

"...someone with curiosity in building their first $30 theremin does not need a scope."

When they don't have a scope they end up here asking us to debug things 3rd hand with a blindfold on.  Which is exactly what's going on this second.  And you're recommending he not try to get his hands on a scope?

"You have this need to prove something to me..."

You flatter yourself.  It's nothing personal, I respond to BS and it just so happens that you constantly spew it.

Nurse: "Doctor, he seems to have broken his leg - should we X-ray?"
Doctor oldtemecula: "Absolutely not!  Hippocrates never used X-rays!  Prepare some leeches..."

dewster —  An oscilloscope is as important for a hobbyist as a multimeter, it is possible to get stuff done without one, but the amount of trouble you will have trying to figure out how to understand the signal you want will certainly be as big if not bigger than what you would get by just going through the troubleshooting of the circuit, it is simply much easier to just use the correct tool. The one I had started to get unstable readings and I decided to sell it to get a more decent bench oscilloscope, so at the moment I don't have the access to one.
I just though that, considering the circuit has been around for so long, someone would have already got a similar problem and found a solution for it, even though I didn't find anything specifically related to my problem in the posts.

Leon Pijpers — I appreciate the suggestions, I will try those.

Oldtemecula — I tried using the radio and it worked as you described. For me just measuring the frequency with a multimeter at the drain of the Fets worked too.

"The one I had started to get unstable readings and I decided to sell it to get a more decent bench oscilloscope, so at the moment I don't have the access to one."  - GuiFS

Sorry, you got caught in the crossfire.

I have an old Tektronix from back when they started making digital desktop LCD DSOs, and it is still very reliable.  My main scope now (a visit from the scope fairy) is Rigol DS1054 that I can strongly vouch for.

I would also highly recommend using an oscilloscope with a resolution in the megahertz range. You need to watch both the RF and the AF. For example, with a simple resistor as a probe in the air, you can see what antenna signals and their mixed products are occurring, hum modulations, etc.

I do everything with my Hantek 2D72 3in1 Digital Oscilloscope Waveform Generator Multimeter USB Portable 2 Channels 70MHz 250MSa/s Multifunction Tester.  It wasn't expensive and works very well for what I need it. I bought it directly from China for around $130. I opted for a portable solution since I'm also asked to do theremin service "on the fly" during Theremin gatherings everywhere in Europe.