A musical Instrument for the kid in all of us who likes electronic adventure

RS Theremin

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Perfect Theremin Pitch Linearity is Real

A Mystery Explored at Theremin World! 
gnsmith116 does Lev Antenna research


When I originally wound my own oscillator coils with 30 gauge enameled wire as seen in this Ultimate 2b theremin constructed by Dominik, I could not get much more than three linear octaves.

Visit Dominik's webpage and listen to his sound samples in the upper left corner of his page. He is a master theremin builder today and uses built in mechanical spring reverbs for a nice affect.


I upgraded my Ultimate 2b pitch oscillators from 30 gauge wire on an air core coil to 22 gauge wire on a ferrite core in my later RS Illusion theremin design. Using two heavy gauge 100 uh choke/coils per oscillator to give me a center point and to my surprise, it expanded the pitch field from three linear octaves out to just over five linear octaves. This was using the Lev Antenna on both, with no other circuit change. A five octave linear spread over a 18" to 20" pitch field distance is an ideal number for arm reach and playing distance.

Going from an air core coil on the left to a split ferrite coil for a center tap on the right, the number of linear octaves increased from three to just over five. The oscillator circuit configuration and values remained the same.

Note: in a linear pitch field there are no almost linear octaves, five linear octaves is five from Null to the Antenna.

I tried a no oscillator tank capacitor approach but PN junctions introduce their own capacitance which caused a distorted wave shape, it is important for me to use the pure heterodyne generated signal for sound and character.

My guess is the heaver 22 gauge enamel coated wire on a ferrite core improved the circuit Q factor considerably which in turn delivered more current into the antenna tuned as part of the tank circuit. In other words reduced coil wire resistance.

Ferrite core coils track or counter balance one another very nicely, it is the transistors used that cause issues.

My tube/valve theremin runs a hotter signal at the antenna and interestingly the linearity effect is even better with more octaves.

The oscillator split coils I use in my RS Illusion theremin were not shielded and so I must place the pitch and fixed oscillators farther apart. This is why my two oscillators are on separate boards. The two oscillators interacting within close proximity to one another can cause unwanted wave shapes or distortion in the detected audio wave, sometimes good but most of the time very bad.

THE KEY: One improvement seldom used in very early electronics that lead directly to the improvement of modern RF electronics is the use of ferrite cores for higher Q circuits. The ~ 900 kHz theremin operating frequency keeps the design practical.

The amount of energy available to the pitch antenna determines the number of linear octaves. In a Lev Antenna pitch field there are no non-linear octaves just more or less octaves. Normally this is 3 to 7 octaves.

Here is one of several interesting letters I received from a dear friend of Lev's. It hints at how he had trouble finding parts in his later years of experimenting in Moscow. Lev Sergeyevich is not often thought of as experimenting with transistor circuits and ferrite coils but without doubt he did. He experimented until his death in 1993.

The true marvel . . . 

"Octave & key spacing is an early "man made control" of the piano and the theremin embraces it." The Termen Effect is a good phrase to capture this very unique relationship.

Below are some important behaviors worth mentioning.

Note 1: Fine tuning for the Null point on a transistorized heterodyne theremin can be accomplished by slightly varying the circuit current using a variable resistor on the emitter or base of the oscillators of  a bi-polar transistor. This is manipulating the junction capacitance of the transistor similar to a varactor diode which controls the oscillator frequency. 

Note 2: Variable caps should be avoided in a theremins pitch or fixed oscillator to avoid extra temperature drift issues caused from room temperature fluctuations.

Note 3: In a transistorized theremin it is important to use two oscillator transistors that come from the same manufactures batch # so they have very similar characteristics. Most thermal drift develops from the mismatch here. To minimize thermal drift you want the two oscillator transistors to behave like identical twins.

Note 4:
There "are not" extra non linear octaves in the perfectly linear pitch field, the linear octaves exist between the Null point and the pitch antenna. The pitch field spreads out wider adjusting the pitch tuning knob as usual but it still remains linear. In the phrase "three linear octaves", linear means they line up in a relationship to a musical scale or parallel to piano key spacing. The notes or intervals are all the same width right up next to the antenna.

Linearity gives you precision note control of the theremin. I adjust my Theremin to play one octave lower with my hand closed which gives me a predictable one octave higher jump snapping my fingers open. One octave of notes is held in the fingers of my hand. Here is an old sound sample from my tube theremin Linearity Test.mp3 (200k)

Master those seven and play with the same accuracy anywhere in the theremin playing field. Similar to the Carolina Eyck method. Your pitch adjuster allows you to narrow or widen the octave width to match the size of your hand for accurate aerial fingering.


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