Silicon Chip Theremin

A fairly simple but nice theremin plan from the Australian Magazine Silicon Chip. Normal 455 kc IF transformers are used as osc. coils. A Kit is available from Jaycar Electronics kit #KC5295, for about AUD 50,- / EUR 35,-. Which seems a fair price to me.

Silicon Chip August 2000 theremin kit modifications

The theremin kit as sold by Jaycar has good value for money. You receive a complete theremin with both pitch and volume control.

However I found some aspects of this theremin less appealing and tried to modify the original circuit to deal with them. I've drawn all mods into the original circuit diagram for you to compare.

[see gallery]

I will run trough the mods explaining what and how:

Power supply - Bad design. A 78xx regulator should not be loaded with such large capacitors as oscillation may occur. I've put one 470 uF cap at the regulator's input and only a 100 nF at the output. Some of the original caps are not located near the 7805, it's a bit of a search to find the right ones..
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Volume antenna - A bit small... I've replaced it with a traditional loop as seen on most theremins (like the etherwave). This has a positive effect on volume control as well. And it looks much better..

Volume control amplifier - This one is difficult to adjust. Therefore I made some mods here as well. Before playing the theremin you need to do some fine-tuning to compensate for drift in the volume oscillator and to adjust the response to your needs. It seems a common solution to use a panelmount pot instead of the trimpot VR2. This is not wise though. First, this pot is tricky to adjust and second, it has a entirely different function!
The way the volume control works is as follows: T3 is set to a certain frequency. That frequency will get lower if you approach the antenna. T4 is adjusted to a frequency slightly higher than T3. Therefore the output of T4, which is a band filter, will decrease when the frequency of the oscillator gets lower.
This voltage is then amplified and level-shifted by the LM358. VR2 is part of this amplifier and takes care of the level-shifting.
Now, fine-tuning the volume oscillator has to do with making shure that the frequency of T4 is slightly higher than the frequency of the volume oscillator. Thus obviously fine-tuning has to be done either around T4 or in the volume oscillator. That last option is the easiest.
The oscillator can be fine-tuned by slightly varying the drain current of the FET. This can simply been done by replacing the fixed 100 ohm resistor at Q3 with a series connection of a 100 ohm resistor and a 500 ohm pot. This pot can be panelmount to allow adjustment from the outside.

In addition I've changed the 1 Meg resistor between pins 2 and 3 of the LM358 into 220 k. With the original 1 Meg resistor there is way to much gain with as result that little handmovement has already a big effect on the volume. With the new resistor I have about 15" playroom to go from silence to max. volume.

Adjustment procedure (keep distance from volume antenna):

1 - Set the core (slug) of T3 and T4 in their top position, set the new volume fine-tuning pot in the mid position.
2 - Measure the voltage on pin 3 of the LM358 and adjust T4 for max. voltage.
3 - Measure the voltage on pin 1 of the LM358. Adjust VR2 so that the voltage reads about 4.0 volt*.
4 - Turn T4 a bit back, so that the voltage just starts to drop to about 3.5 volt.
5 - Check the volume responce by moving your hand. If the control area is to small tweak T4 slightly.

*) If you adjust in such way that 4.3 volt is measured when you move away from the antenna, the volume response is not very smooth, therefore I recomment to adjust the instrument in such way that you end up with 3.5 volt at step 4.

Fine-tune possibility for pitch - It's hard to exactly tune the instrument with adjusting T1 and T2, and beyond that, one needs to be able to tune the instrument without opening the box. fine-tuning can be done by varying the length of the telescoping antenna. However I replaced that antenna with another rod and thus have lost that option. The pitch oscillators can be fine-tuned by slightly varying the drain current of the FET. This can simply been done by replacing the fixed 1 k resistor at Q1 with a series connection of a 470 ohm resistor and a 1 k pot. This pot can be panelmount to allow adjustment from the outside. It may be wise to use screened wire to connect the pot and somehow fix the wires to the cabinet because vibrations in the wires may modulate the oscillator and thus the audio signal.

Linearizing the pitch sensitivity - I found that the upper octave was much compressed and that the highest notes I wanted to play were so close to the antenna that accurate vibrato wasn't possible. A way to linearize the response is to put an inductor in series with the antenna. I've drawn a graph of the original situation and the corrected pitch linearity. The vertical axis shows distance to the antenna in cm, the horizontal axis shows the octaves.

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Here are some examples of antenna dimensions and their resulting coil values. I'm using an air-would coil, mounted vertically. This online Coil Calculator can help you to determine the number of turns. Be aware that the examples only apply to the SC theremin kit since the pitch oscillator frequency is one of the variables in the calculation.

Sound - The original sound of this theremin is a rather clean sinewave with some buzz-like distortion at max. volume (caused in the LM358). I don't want a clean sinewave but more a string-like sound.

I replaced above mod. with the following:
I removed the 10 k trimpot and 220 pf capacitor. Then I took some 1 mm massive wire, two pieces of 10 cm. I bend these into an L shape, with the short end being 1.5 cm and the long side 8,5 cm. And soldered one at the 100 k resistor rear Q1, at the side that connects to the 68 pf cap. The other wire is soldered to the 100 k resistor near Q2, again at the side that connects to the 68 pf cap.
The are soldered in such way that the 1.5 cm part is vertical and the 8.5 cm part is horizontal. The horizontal parts are in parallel and have a space of about 0.5 to 1 cm. These now can be bend closer to each other or further away to modify the sound. If they are to close you will loose the lower notes. This is a trade off just as the old mod. was.
This new mod. works similar to the previous mod. but has a different sound as result which I like better.

To modify the sound further, I replaced the 1 k resistor between pins 2 and 3 of the MC1496 with 2k2. This reduces the conversion gain somewhat, which has a positive effect of the max. volume distortion and at the same time makes low volume signals slightly more 'raw' like slowly bowing a violin.

Next I created a low-pass filter between the MC1496 and the LM358 by putting a 100 k trimpot in series with the 0.1 uF capacitor and add a 10 nF capacitor between pins 4 and 5 of the LM358. This allows to take the sharp edge of the signal and makes it more like the sound of strings. It also slightly reduces the signal level which further cures the distortion at max. volume.

Results - are hard to describe in words. Here are some sound examples. All recorded with the same adjustment. These examples are recorded on tape first and are not the worlds best quality I'm affraid. Also please keep in mind that I'm a novice on the theremin.
These samples are of the theremin with the old sound mod.
Example played with fast attack, slow decay (hand movement) (476k MP3)
Example played with medium attack and decay (hand movement) (614k MP3)
Example played with slow attack and decay (hand movement) (579k MP3)
Example played with portamento (sliding from note to note) (1394k MP3)
Example with a slight amount of reverb added (877k MP3)

Before you start soldering, please be aware that you can do damage if you make mistakes. Please only modify your circuit if you know what you are doing. After all it would be a shame if you would break it (and I don't want to get the blame if you do).

Enjoy! Max

 Thank YOU, sir! Far out, man. Finally something about that theremin. I bought one in July this year. Built a very nice cabinet for it, including a self made volume loop antenne out of red 6 mm copper. The cabinet is made of plywood and is finished in black. I've added a blue power LED and the VR2 is now on the outside next to the volume control. Both antennas are easily detachable (butterfly nuts) and it has a hinged lid. Altogether a lot of work but it was worth it; it looks very nice. In the bottom I made a whole, so that the thing can be mounted on a microphone stand. HOWEVER (and that's one of the reasons why I'm so glad that you put your article in the air) the theremin itself is a big problem. I've replaced about every component and sometimes it does its job for half an hour and then it gets quiet and dead. Even brought it to an electronics shop to see if they could find/solve the problem, but the problem was and is, they don't know what a theremin is. Now... well, I just keep trying to get it going, because I like this Thing very much. I'm sure someday it will. Besides, of course I would like to have an Etherwave or equivalent, but I can't afford that. So, it's not a matter of taste or something like that, that I bought the SC theremin. But, still a nice sound for that little money (100 Dutch Euro's) for a theremin. Thanks and kind regards, charles

Schematics

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