coil basics

Now to find that 500pF variable capacitor..

It really doesn't need to be that big. You could find one in an old AM-FM radio and just use that.

The biggest ones I can find at electronic shops in town and online are between 40pF and 100pF... seems to be quite a difference...

I'm about to start looking for old radios to canibalize.

however... this DIY variable capacitor looks much cooler and more legit than the last one I found... probably very time consuming... but looks reliable.. : http://www.instructables.com/id/air-variable-capacitor-from-scrap-aluminum-sheets/?ALLSTEPS

In the world of theremin and discovering the 10,000 reasons it will not cooperate, variable capacitors are high up on the list. They add a small part of the overall thermal drift issue so I am against using them in theremin oscillators. Transistor junctions are #1 so need special consideration. Thermal drift will be the main demon that torments your artistic display.

Using 100 uh or 1000 uh coils is basically the same, one is much easier to wind.

I never found magic in making my own coils large or small so I use these coils today for my solid state and tube theremins alike. It was this angelic sound that I was after and would eventually find but it occurs not because of the coil used. My 10 year journey concluded with this sound, it was then RS Theremin revealed himself as oldtemecula. (-'

My coil below uses a brass screw for tuning after the wire wraps are adjusted for ideal tuning. Brass is the opposite of ferrite, it raises the frequency of the oscillator when inserted.

As time passes I shall call the above my 10K Tips. I use to post my list of 13 Theremin myths but that got people upset. 0-:

Thierry has a pcb, until we know more I favor his design in your application.

Hey Doug, enjoy when locals show up!

Christopher

" do you think I will be able to use it on a mac?"

time to say "oops".

But you can use the online version.

Christopher, as you might know, the oscillators of the Moog Etherwave Pro Theremin are rock stable. But they work with fixed inductors in parallel with a fixed and a variable capacitor. How does that match your experience? ;-)

BTW: Thermal drift in a heterodyning system is not forcibly an issue. If both oscillators drift synchronously, the beat frequency will remain the same and you'll not feel the impact of the drift. That's why both pitch oscillators of the EPro are fed by a common current mirror which makes sure that both oscillators have exactly the same working point (and thus the same temperature and junction capacitances). In the tVox tour, the FETs of both pitch oscillators are thermally coupled through the shield between the fixed and the variable oscillator with the same effect: the oscillators do drift, but by the same amount, so that the beat frequency remains constant again.

Thierry said: "BTW: Thermal drift in a heterodyning system is not forcibly an issue. If both oscillators drift synchronously, the beat frequency will remain the same and you'll not feel the impact of the drift."

Capacitance in the discussion is about making your own, this will have major issues, I was trying to be gentle. (-'

I have no experience with the E-Pro so can not give any opinion.

As far as drift goes my experiments have been in a room all day that can start off at 55º F and at the end of the day at 90° F. Following good design rules my theremin I call the Phoenix held the same musical note over a 10° room variation. My hybrid for obvious reasons is not so forgiving and more like the same musical note over 5° F change. How good other designs are I don't know, it is never mentioned.

Most rooms hold the same temperature for quite awhile, the above stability requires good design and practical tricks.

The EtherWave Standard is good but I never ran any tests on it outside of my 5° F hybrid design.

A solid-state theremin that needs more than a 5 minute warm-up demonstrates a need for design improvement, especially if you leave it and return later and have to re-tune it. Luna in the other thread seems to have run into unexpected environment change issues.

I agree balance is everything, if you have a sizable variable cap on one oscillator it is better to have it on both.

Christopher

"A solid-state theremin that needs more than a 5 minute warm-up demonstrates a need for design improvement..." - oldtemecula

I'll be bold here and state that any warm-up time for a solid state Theremin demonstrates a need for design improvement.

Christopher - thanks for the links and info! 
Ilya - thanks for the online link to coil 32!
Thierry - thanks for the clarification and info!

This drift related to temperature sounds really fascinating... I'm going to look into that... it might be nice to intentionally harness that in another project!

I will admit, some of this is still a tiny bit over my head, but I'm re-reading and following all the links you send, and checking out other forum threads.... slowly wrapping my head around it!  I'll post photos of the circuits as they progress.

Spent a bit of time on the coil spreadsheet, now it uses VB to calculate length, turns, etc. given the desired value, and it automatically enters the wire dimensions based on AWG and coating:

http://www.mediafire.com/download/s1zk9ya761xlvmc/solenoid_design_2015-04-29.xls

"This drift related to temperature sounds really fascinating... I'm going to look into that... it might be nice to intentionally harness that in another project!"  - xoadc

Not to dampen your enthusiasm, but thermal drift is almost always the enemy.  This is a good reason to avoid most coils that employ ferrite - there are some that don't have too much temperature dependence (IF coils for instance) but most chokes and such drift so bad they aren't good for precision LC oscillators.  Printed wiring board real estate isn't cheap, so miniaturization trumps everything, but that usually means highly drifty ferrite.  Self capacitance also tends to go to hell when you miniaturize coils.  There's a reason you see huge honkering coils in Thermin's original stuff (though you can make them a lot smaller by going up in frequency somewhat).

ooooh... that is interesting... i had been reading a lot about ferrite coils, and they generated so much discussion that I was wondering if i should be going that way instead... but i hadn't yet made the connection that the ferrite is what caused the drift.

aesthetically, i like the bigger coils, I don't have my wire yet, but look forward to making some.

i made a coil for a radio a few years back, wrapping wire around a toilet paper tube, then sanding an edge for the tuning wire. 

am I right in assuming I can wrap my wire around any material as long as it is round and not a conductor?  cardboard tube, beer bottle, pvc pipe?

and thanks for the updated spreadsheet!