New Vacuum Tube Theremin Design

Direct coupling of FPO an VPO is probably best done electrically in a controlled manner via physical (i.e. a component) capacitance (and/or resistance), rather than magnetically.  Unless there are non-linearities involved, it should boil down to the same thing.

You are probably correct.  My knowledge of inductor coupling is sketchy. I wonder though, if the Hall effect on the coupled coils would have non-linear effects on the harmonics of the oscillators, due to the change in inductive reactance with frequency and the effects on the harmonics due to parasistic capacitance.  Just thinking out loud.

I don't know much about the Hall effect, but my feeling is it likely has nothing to do with anything going on in a Theremin.  I believe magnetic coupling in coils is just that and nothing more.  Skin effect is pretty surprising though, it kicks in at fairly low frequencies and can really lower Q by raising the AC resistance.

It's kind of perverse really, Theremins tend to have few active devices / simple circuitry, but this often leads to a lot of complex (i.e. hard to predict) interaction in the circuitry.  Kind of like how many old guitar effects pedals only have one or two transistors but do a lot with them and their transfer functions.  I prefer to have more isolation and buffering, makes things more predictable and less dependent on the vagaries of construction.  Digital is the ultimate in isolation.

Check my website for the latest on my design: https://markaudiomusic.com/

OT - While I was over at your website I read about your piano recording technique and listened to the samples.  I very much like the dry recordings, they sound quite nice to me.  I didn't like the added reverb and compression so much.  Reverb is a very subtle thing, I find my ears are unfortunately constantly learning how to spot new dimensions of what sounds unreal / synthethic.  Pianos are tough to mic.  I'm very impressed that you're tinkering around in the tube mics themselves.

If you haven't listened to the "Secret World" CD by PW denizen and Thereminist Amethyste, and pianist Doug Hammer I encourage you to do so (bandcamp link). The piano is very full and resonant yet intimate sounding (and Amethyste's playing is fantastic as always).

Only the Rachmaninoff has reverb.  No compression was used.  It's fun to hear all the natural interactions of partials in the piano when close mic'd.  It sounds like reverberant without the reverb.

My son also prefers the dry samples.

Thanks for your comments.  It helps me to decide how far to go with any processing.

Listened to the Secret World.  Gorgeous stuff, and yes, the piano is well and intimately recorded.  Nice mix.  Interesting theremin tone.  Thanks.

Mark, you are a master at your craft so there is little I can add, as a hacker. To enrich the even harmonic overtones the audio signal should pass through an audio transformer. You  want to skew/flatten the narrow side peak of the audio wave.

When it comes to using antenna coils I understand the concept but never figured out why my own original designs without them at 922 kHz have perfect pitch field linearity. This is using vacuum tubes or NPN transistor oscillators. I did have a special antenna which I dismissed after the engineers at TW poo poo'd it.

Christopher

I've been thinking about your suggestion of adding an output transformer for a while.  It makes sense from an impedance standpoint as well.

I'll play around with some design stuff at 900+kHz.  Sounds interesting.  Theremins are a winter project for me, so it will be a while.