Theremin auto-tune

[T3sl4co1l]

Anyone ever built a Theremin?

For all the circuits I've seen around, I don't think I've ever seen one with a PLL though.  Which is strange, because it's really easy, and awfully handy!

Background: http://seventransistorlabs.com/Theremin/

The present model has independent pitch mixer and level control, so I can pre-distort the tone for timbre that doesn't vary with level.  I can also take the DC level of the pitch, and null that to zero through an error amplifier and varactor -- in other words, phase-locking the pitch oscillator to the reference.  This isn't very useful, but if I cut bias to the error amplifier, it just sits where it was: effectively, an analog track and hold for free.  So I've got one pushbutton that enables bias and sets the varactor voltage.



Besides saving the hardware for an explicit S&H circuit (analog switch -- JFET or CMOS), one thing that's neat about this is how it effectively does nothing with no bias current, then as bias increases, it gradually "computes" the correct operating point.  (Slew rate is limited by bias current, and the 47k resistor in the pole-zero feedback.)

Also, an unusual aspect of this application: it doesn't matter what polarity the error amplifier runs in.  The mixer output alternates in phase, so it settles on the phase it's stable on.

Tim

[T3sl4co1l]

*Bump*

Improvements:

- As built, it was drifting at, ehh, a few Hz per minute or something like that.  Not bad enough to be unplayable, but needs to be reset every song or two.

- Maybe the "gain" transistors (the top right PNP, and the NPN directly below it -- these drive the feedback capacitor and MOSFET gate) were leaky?  I tried a smaller type: MMBTH10/81.  Absolutely worse (slewing at 10-100 Hz/sec!).  Well damn.  Not surprising though, they're only 25V devices.  Ok, maybe a high voltage RF part: I next tried BFQ221/241 (100V 100mA 1GHz complements -- pulls from a CRT monitor!).  Same as original.  Well, it's no improvement, but at least it's not the BJTs.

- Maybe the MOSFET is bad, leaky or something.  Seems really unlikely, but whatever.  Replaced with a 2N7000 that was handy (the TO-92 of the 2N7002).  No effect.

- So... oh, I have a 0.01uF polystyrene cap in my junk box, what the hell, let's give that a try.

Now it's drifting maybe a few Hz over 10 minutes.  Leaky freaking polyester cap!  Could be dielectric absorption at work, too.

Lesson learned: polyesters may not be particularly low leakage.  This S&H application doesn't need low absorption (it's sampling pretty much the same voltage every time), but it does need low leakage, and I didn't realize how sensitive that was!  (A few Hz oscillator drift corresponds to mV of varactor drift, so out of a 30V supply with an average 15V bias, it's in that regime of precision stuff.)

Also shows that you can use BJTs (even pedestrian 2N3904/6s) for surprisingly good S&H, with a carefully thought out circuit.

Tim

[Bud]

Anyone ever built a Theremin?

Yes, built one at the age of 14.

It is good to see Theremins are still of interest to people.

[KJDS]

I've seen John Otway play one often, didn't realize they needed tuning.

[T3sl4co1l]

The oscillators will always drift with temperature, plus setting up the instrument in a different environment necessitates at least a little tuning.  The drift can be finessed with a very good design, but you'll still have to adjust a trimmer or something to deal with setup.

I don't know that I've seen an auto-tune circuit for one; but it seems like a good idea, and to some extent, solves both problems (drift and setup) with the mere touch of a button.

Tim

[Yago]

I don't know that I've seen an auto-tune circuit for one; but it seems like a good idea, and to some extent, solves both problems (drift and setup) with the mere touch of a button.

Tim

Wouldn't it become a different instrument?
Like putting frets on a violin.

[SirNick]

Is this auto-tune as in, "everyone play concert B-flat"... or auto-tune as in "do yoooouu beeLLIIEEEvvee in liiiffe after loooOOOVe..."?

[KJDS]

Evidence that Otway's theremin needs tuning.

[T3sl4co1l]

Autozero... autotune would be trivial, just DAC into the varactor and disconnect the antenna altogether

Not that you really "play concert B-flat", but the nonlinearity in antenna response means it's a lot harder to play slightly off zero from what you're used to.

Tim