<= home        Website will be active until March 31, 2021

Always use Ctrl-F5 to update this page

The Oscillator Design
Left Side of Board

theremin2@oldtemecula.com

Under Construction

 Tuning
Tuning the Oscillators - Coming Soon

Excellent

Poor

Use Ctrl-F5 to update this Image

steel strip for shielding

- 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!


.

<= home

.