Tuning
Tuning the Oscillators - Coming Soon
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Right click mouse on picture, open in a new tab for
larger view -
I
am not happy with thermal drift so Pot-3 (blue in my
pic) changed 1k to 2k,
R9 changed from 470 ohm to 1k |
The two green
terminals off the left side of the board allow me to test different
Inductor behavior before they are soldered permanently into place. They
also allow for a place to add extra capacitance when you want the Pitch
board frequency to lower down to be a Volume Board.
I use
inductor L3
to pick up the L1
& L2 magnetic
fields which are mixed in the D1 diode.
My two RF oscillators do not need buffering as they have no component
connection between them, just earth ground. Between
the circuits L1
& L2 magnetic
fields will interact with one another. This can be controlled by the
amount of current driving the coil and the L1
& L2 & L3 coil
orientation to one another.
The
transistor emitter current determines the strength of the oscillator RF
inductive fields. Too much you could get over coupling between the
oscillators
which causes unwanted distortion or too strong of a heterodyned mix.
This L3
inductive
approach is unique and original. It allows me to control the amplitude
and mix of the magnetic fields to get the best heterodyned audio wave
shape out of the D1 diode.
The D2
diode is a future
experiment. This
allows for a second channel using the audio section of another board and
mixing it back into the final audio for possibly a much fuller or
interesting sound.
The board
can still use 360uh IF transformers but variety allows more 330uh
workable inductors.
- The
fixed ferrite coils that look like big resistors in the green terminals
on the left side I like so far -
- This
project is the true meaning of serendipity -
This first
test we
are only powering the two RF oscillators on the left side of the board.
You must have the antenna properly setup and connected. Earth ground
connected is a must.
Do not
substitute earth ground for a stretched out cable on the floor.
With an analog theremin that will prevent you from having the
fullest Classic Theremin sound. In digital theremin's this is
not so important, listen to how they sound in comparison. |
Using a meter that can measure
frequency, take the signal from the ungrounded end of the L3 - 3300
uh choke. The Q1/L1 side frequency should be from 850
kHz to 920 kHz. (900
kHz is ideal) Measuring
the frequency anywhere else you will load down the circuit and stop oscillation.
330uh
sniffer choke setup
I use an "analog"
AM radio tuned to around
900 kHz and listen to the blank spots in the background static while
finger touching the L1 & L2 coils. If the Q2/L2 oscillator is out
of range of matching the frequency of the Q1/L1 oscillator you want to add
or remove capacitance from the Q2/L2 side. One thing that can cause this
mismatch is circuit variables like
the 10% tolerance of the coils, etc.
Nice
LC Frequency Calculator
I use the MPSA42-AP
300 volt transistors for a slightly better sound and thermal drift
control.
I changed the R8 & R20 4.7k resistor to 2.2k. If you must use a
2N3909 NPN you might want to keep the 4.7k for less range with Pot 4-1k. If you need even less swing when Nulling with Pot-4
1k it can be
paralleled at R25 with a 470 ohm. The fact that the MPSA42-AP reduces the Pot-4 range tells me the transistor will
be less sensitive to thermal drift.
Thermal
drift is reduced by the choice of transistor and matching
the hFE values. Then balance the current
through the two transistors using the 2k Pot-3 at the bottom of the
board. At first set it half way so it matches the R9- 1k resistor value. With this method you
can actually reverse the direction of drift were the Pitch can drift in
the other direction.
The
natural theremin sound is a wave shape as seen on the scope with the proper bends and curves
here and there. It is not the mixing of saw tooth waves with sine waves
and what not, that is more like mixing noise.

.5 msec/div
10 mv/div
This clean
audio signal is un-amplified measured after the detector diode, taken at
pin-2 of U1 LM358
My
original heterodyne method allows for the ideal mix, shape and amplitude of the
audio signal after the
detector/mixer D1 diode. I can increase the raw audio signal amplitude up to 500 mv, not desirable.
4.5.20
- I do not have the virus, rather a heart/lung issue caused by diabetes. I can barely walk from room to room, breathing
can not keep up. Trying to finish up my last version of our musical
instrument. It will be sent to the same artist who lives in the town
of the original 1919 inventor and friend of the Theremin
family.
I said 20 years ago that Clara's Voice
was a gift to me,
that is what kept me going. Jokingly I would say, ask Dorit
Chrysler if
she still has it.
I am having fun, if ever you saw that movie "The Book of Eli", that is how I feel, I
must finish the journey in the West and write down what twenty
years revealed to me. Did something metaphysical happen along
the way, I will
leave that up to you to decide.
|
Visit
the Special Pitch Electrode
Electrodeum Pitch
Antenna
This gives
me the ideal Pitch Field Linearity
This was
my first unique theremin discovery in the beginning, I looked upward and
said "are you serious"
- above
these two lines is the new build progress as I go, build
started 3.27.20 -

The 10
light tan capacitors on the left side oscillator section are 47pf,
except C7 & C31 are 100pf, C19 22pf
Mouser
Parts List
List is for a
Single Board Construction
Always
Check Parts List For What is Out of Stock!

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