Moog Theremin T-Shirts

oscillator-less theremin

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Posted: 7/17/2005 3:11:59 AM
jon
From: melbourne, oz
Joined: 6/10/2005

threads posts

just wondering if anyone has ever had success building a theremin that doesn't operate on the heterodyne principal (ie, two high-frequency oscillators producing a 'beat' frequency when shifted slightly out of tune).

i'm thinking of using a capacitance sensor (like the qprox QT301 chip) to convert the antenna/hand capacitance to a variable voltage, then feeding this into a voltage controlled oscillator - probably a sine wave function generator like the XR2206 waveform generator.

if anyone has tried doing something like this before i'd love to hear from you!

thanks.
jon.

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info on the QT301:
http://www.qprox.com/products/qt300_301.php

info on the XR2206:
http://www.exar.com/product.php?ProdNumber=XR2206&areaID=7
Posted: 7/17/2005 7:44:40 AM
Charlie D
From: England
Joined: 2/28/2005

threads posts

Apparently the Moog 91 series didn't operate on Heterodyne principle:

"The 91 (A, B and C all had the same innards, just different cabinets) was basically an analog synthesizer. It was not a heterodyning theremin at all but used a single oscillator to produce tone. How Bob Moog came up with a space-controlled analog synthesizer is beyond my ability to understand or explain."

(courtesy of Charlie Lester)
Posted: 7/17/2005 9:36:10 AM
Jason
From: Sammamish, Washington
Joined: 2/13/2005

threads posts

I think that meant that the sound itself wasn't produced by using the heterodyning principal, but I'm pretty sure it was still used in creating the space control field. Instead of piping the raw beat frequency out to an amplifier, he probably turned it into a control voltage type signal that determined the pitch/volume of the synthesizer module.
Posted: 7/18/2005 2:57:12 PM
Etherdave
Joined: 2/21/2005

threads posts

I would like to think that dual-heterodyne is considered the 'traditional' circuit topology, given the instrument's history and development. But what makes a theremin a theremin is its space-control system, which is nothing more than a pair of variable capacitors. Logical circuit designs use no heterodyning oscillator circuits, nor, of course, would a PC-driven software program, employing the space-control system as an external control device.

The theremin was born out of early radio technology, before transistors or integrated circuitry had been invented. The heterodyne principle was, at the time, the most efficient way to create audible electronic tones from an economic collection of electronic parts. When it became possible to generate low-frequency tones on a single, low-frequency oscillator, this technology became obsolete.

Using the dual-heterodyne system in a theremin circuit today is probably neither the most efficient, nor the least costly, and it is certainly not the electronic state of the art. But it does demonstrate the original method by which these instruments operated, and serves as a good educational tool for demonstrating early radio technology.
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