it was invented in 1912 by russian Lev Termen, so that the real pronunciation should respect the real name of the inventor. But English language has modified in Theremin.
Lev Termen has realized this magic instrument in a easy way.
It' basically constituted by three oscillators.
The first two are dedicated to pitch generation and the third one to the expression: volume control.
But in what does it ?
Well, basically is a wood box with two antennas. One horizontal and the other vertical ( as the one of your radio ). If you stand up your hand and you get closer to the vertical antenna, there is a sound emitted by a loudspeaker. The pitch of that sound, will be higher and higher directly related with the distance between your hand and the antenna. The closer you are to the antenna, the higher is the pitch and vice versa.
Sound intensity is given by the distance between the other hand and the horizontal antenna.
Lev Termen was thinking to an Orchestral Director. When his hands are going down, this indicates to the musician to fade out or anyway to decrease the sound level. Vice versa, when the hands are going up, it means: get louder.
So that the closer you get to the horizontal antenna, the less loudy sound will be emitted from loudspeaker and vice versa.
Technically, as I told, is very simple. There are for the pitch, two oscillators. The first one fixed n a determined frequency ( typically 160 kHz, well out from audio range ) and a second one that's free and connected to the antenna, and oscillates at the same frequency of the first.
When you move an obstacle ( as your hand for instance ) from/to the antenna, it variates the capacitance. Dielectric is the air between your hand and the antenna. What happens then ? It happens that in the reality, changing the value of the capacitance, you change also the frequency at which the oscillator is working. A very simple circuitry, very similar to the one that is in our AM radio, detects the difference between the fixed oscillator and the variable one, and it give this frequency to a loudspeaker, by an amplifier. Normally that difference falls down in the audio band and we are able to listen to it.
An example. Suppose that both oscillators are working to 160 kHz. so that 160-160=0 and no sound is emitted.
Well, now we get close to the antenna. Capacitance of one of these, will be increased and the frequency of this oscillator diminishes. Suppose that it reach 158 kHz. Then 160-158 = 2 kHz. and a sound closer to higher C ( DO ), that's 1093 Hz = 2.093 kHz is emitted and we ear it very well.
Suppose now that we get a little far away from the antenna, and the variable oscillator produces 159.560 kHz ... calculating the difference we have: 160-159.560 = 0.440 Khz that's means 440 Hz: the main A ( LA ) and so on. Moving continuously the hand toward and backward the antenna, it's possible to change the pitch as a "Glissando". Exercise permit to reach a really good result.
Note that the circuitry calculating the difference, it calculate positive differences between fixed oscillators and variable one. Not vice versa. So that if variable is higher than the variable, no sound is emitted because there is not negative frequency. It cannot be 160-170 = -10 kHz. It doesn't make any sense.
This is the technical explanation, but you can imagine to watch a musician ( Thereminist ) that is producing sound just moving her hands on the air.It looks alike spooky and magic :-)
Intensity ( loud ) of the sound, works almost in the same way. there is only one oscillator connected to a filter.
This oscillator operates almost to the same frequency than the other two but it's not important. When the frequency get closed to the resonance value of the filter, the tension out of the filter is higher than when the oscillator is far away form the resonating filter's frequency.
So that get closer to the horizontal antenna with your hand, frequency will be lower, and more closed to the resonating frequency of the filter. Voltage out from this filter is higher, and polarize a VCA ( Voltage Controlled Amplifier ) that will change the value of the amplification , in this case: it will diminish it. Getting far away from the antenna, it will be obtained the opposite result and the volume will be louder.
A lot of words spent for this magic instrument. Now, the main producer is Moog, the inventor of Synthesizers, but you can easily build it up.
Here you can find it and buy it if you like: CLICK HERE
and here you can watch something magic:
CLICK HERE it's on Youtube
Do you'd like to have the electric diagram of Moog ? Here below there is it
It's not determinant what type of transistor you are using, due the frequency range that is very low. General purposes transistors can be used. Critical points are the inductances. Keep inmind that if the inductance is not the correct oen, you can get some difficulty in the functioning.
Moog is sold also in Kit and it costs a little less, about 66€ less ( today September the 21st 2008 ), it's a good experience
Moog is sold also in Kit and it costs a little less, about 66€ less ( today September the 21st 2008 ), it's a good experience
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