Tom Henry Tunable Noise Source

WeAreAs1 at aol.com WeAreAs1 at aol.com
Fri Nov 12 20:00:30 CET 1999


Hello all,

Since we're on the subject of Thomas Henry, I thought I should mention that 
Mr. Henry has an article that has just been published in the November 1999 
issue of Nuts and Volts magazine.  It's a DIY tunable noise source, and it 
looks pretty interesting.  It's kind of similar to the "Clatter" section of 
his ADV-Cymbal project (http://mall.lakes.com/~map/cymbal.htm), but with some 
key differences -  

The new noise source derives its noise from the standard reverse-biased 
transistor, but then uses this noise to modulate (FM) an LM566 VCO.  The 
modulation and tuning CV are routed through a simple 
non-temperature-compensated two-transistor expo converter, which is scaled at 
roughly 1 volt per octave.  His circuit, as drawn, doesn't have an external 
CV input, but it would be easy to add one.  The VCO has square and triangle 
outputs (he recommends using the triangle for most noise apps).  When you use 
the tuning pot to "tune" the noise, what you are really doing is tuning the 
VCO, but if the modulation is set deep enough, it sounds like the noise pitch 
is changing.  His circuit has a modulation amount pot which lets you set the 
amplitude of noise modulation (thereby also setting the apparent noise 
volume).

This circuit differs from other tunable noise sorces that we have seen in the 
past in that most of them use some kind of pseudo-random digital shift 
register which is clocked by a tunable high-frequency oscillator.  While 
these do have their interesting sonic qualities, they are still not truly 
random noise sources, as is Tom's new design.

Anyway, check it out if you happen to see the magazine anywhere.  Hopefully, 
Tom will also put something up about it at his web page, one of these days 
(but so far, he isn't offering to sell a kit or a PC board).  If anyone tries 
building it, please post something to the list about the results.

Michael Bacich




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