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The Lev antenna coil spring naturally has parasitic capacitance between the close
turns of wire loops. There is the possibility that the closely gathered
turns of the antenna spring coil causes the entire antenna coil to "self
resonant" in the 900 kHz to 1000 kHz frequency
range. The antenna is
physically tunable with minimal stretching between 1/2" to
3/4", recommended.
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If I was to construct another pitch only theremin model for
mobility and linearity demonstrations I would use the small $80 pre built board by
Chuck
Collins.
With Chucks Theremaniac board I would raise the
operating frequency to 950 kHz and would be able to achieve just over three linear
octaves. This is the number Clara Rockmore was most comfortable with. Chucks
pitch only theremin board layout generates a very
clean sine wave.
When I originally wound my own oscillator coils with 30 gauge 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.
Observation:
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 my surprise, it expanded the pitch field out to just over
five linear octaves, without any 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.
My guess is the heaver 22 gauge enamel coated wire on a ferrite core improved the circuit Q factor considerably which in turn drove more current into the tuned antenna. Here is a
close up view of
my RS Illusion theremin board showing my pitch oscillator split coils
which generate 5 linear
octaves. heavy gauge.
The oscillator split coils I use in my RS Illusion
theremin are not shielded and so I must place the pitch
and fixed oscillators further 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 audio, sometimes good but most of the time very bad.
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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. Others will improve the look and
design of the Lev Antenna. |
1. From observation I believe perfect theremin linearity
is in direct response to the use of very high Q
tuned circuits.
2. The theremin oscillators should also be designed with the "highest Q factor" in mind for maximum energy/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 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.
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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
like a varactor which affects oscillator frequency.
Note 2: Variable caps should be avoided in a theremins pitch or fixed oscillator to avoid extra temperature drift problems 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 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 like piano keys
on a musical scale.
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 a sound sample
from my 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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