More questions from the newbie.. Regarding linearity..

Hey all - 

Okay, back with another question.  I know the linearity of the EW isn't going to be perfect, but I'm wondering how close/far off it should really be?  It seems like when I have it relatively tuned, my low end range/distance is considerably farther than it is as I move up in pitch.  

I just took a break mid-post and moved the theremin a little further from some potential interference and it seemed to help a little bit, actually.  

My question is - is the internal tuning process something that will help with linearity, or does that only affect pitch range and distance?

Thanks again for enduring my questions!

~John

"My question is - is the internal tuning process something that will help with linearity, or does that only affect pitch range and distance?"  - kc5string

Thierry can probably answer this better than anyone, but my $0.02 is this:

There are three LC resonances to deal with on the EW pitch side.  The first is obviously the LC tank of fixed oscillator that beats against the variable oscillator.  The second is the LC tank of the variable oscillator.  The third is the LC formed by the series inductor L and the antenna / hand capacitance C. 

The variable inductor for the fixed oscillator gives you coarse control over the first resonance, and the pitch knob on the front of the EW gives you fine control here.  The variable inductor for the variable oscillator gives you coarse control over the second resonance.  The wire connecting the PC board to the pitch antenna travels over a grounded piece of aluminum tape, and bending the wire up / down gives you some kind of control over the third resonance.

The trick is to get them all aligned in a way that maximizes perceived pitch linearity, particularly I believe near the antenna (but I could be wrong).  Good luck!  The EW is a tricky beast.

FredM's tuning approach was to tune via the third resonance by saturating the series inductor, and I believe Theremin's approach was to also tune via the third resonance, but with an explicit variable capacitor.  Moog's tuning approach is clearly to tune via the first resonance, which is sub-optimal IMO.  (My approach is to avoid this analog kludge altogether.)

Thanks for the reply, dewster!

I think I have a headache now.  Most of what you said flies right over my head - my electronics knowledge is approximately zero.  I suppose I should learn some if I'm going to tackle this instrument.  

You did answer my question, though, in that tuning should at least have some impact on linearity.  I don't know that it's a real issue right now, since I'm brand new to the instrument and working on some basics.

Thanks again! 

~John

John, there should be a certain amount of non-linearity both very close to the antenna and very far away, but in the maybe 2' to 6" zone you should get roughly the same semitone response to an open/closed hand gesture.  I'd try small adjustments of the second resonance, then adjusting first to get the beat back until you achieve that.  Keep notes.  Report back.  You might want to drill holes in the case to facilitate the adjustments as the wood is somewhat capacitive and interferes with things when you remove / replace it.

Linearity has a lot to do with playability, so I urge you to get that at least halfway right before going to all the trouble of learning to play on a mis-calibrated instrument.  Life's too short and all that.

john, maybe you read the legendary hotrodding manual and try to understand what dewster said in the first reply. (man, these two cents are worth dollars) tuning to greater note range helps to get the higher notes a bit more away from the rod, so the higher register is better to play. but as more higher notes you add the smaller the spacing between the notes. the whole tuning is a awkward, time and nerve consuming experience, a bit like fret intonation but fretless with no neck and a invisible string with variable lenght!  only use the proper plastic tool if you adjust the variable inductors. i second what dewster said about small (sometimes really small) adjustments and taking notes of what happends. i twiddled around like there's no tomorrow and tuned through hell and back again...

i replaced my pcb with a brand new one (from 211-D to 211-J: they sold a lot!) yesterday, so it's still factory calibrated. i knew i had to adjust the internals to compensate some additions i made. zero beat was about 8cm away from the rod.lol. i have a drilled case, to make tuning easier. the wire that goes to the pitch rod is bent upwards, following about 3 fingers above and paralell to the tinfoil. (so it's as far away from seeing some ground to early. this maximizes the entire note range.) i only tuned the fixed oscillator (L6) to move zero beat away from the rod. i guess your etherwave is new too, so you'll have about the same range: i have a E1 as lowest playable note( below it's motorboating or geiger-counter) and then 5 octaves up to E6. E6 is about 5-6 cm away from the rod. the higher rest is cramped. it feels like the middle 4 octaves are not so bad linear out of the box. or more likely: my ears control my hand position a bit too. :-) i think it's not so bad to learn the basic theremin technics, scales hand position etc....independent which hand technique you use, to get the higher notes more spaced you should move your body towards the rod, like peter pringle or thomas grillo and most do, for example. 

later on: adding thierrys module and his fine tuning might be a further option, to really increase range and linearity. specially if you like the basses. but the module also cuts that buzzy timbre when the brightness knob is turned far cw.