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from the a-feature-is-something-that-is-broken-in-a-useful-way dept.
That headline sounds suspect, but it is the most succinct way to explain why the Roland TR-808 drum machine has a very distinct, and difficult to replicate noise circuit. The drum machine was borne of a hack. As the Secret Life of Synthesizers explains, it was a rejected part picked up and characterized by Roland which delivers this unique auditory thumbprint.
Pictured above is the 2SC828-R, and you can still get this part. But it won't function the same as the parts found in the original 808. The little dab of paint on the top of the transistor indicates that it was a very special subset of those rejected parts (the 2SC828-RNZ). A big batch of rejects were sold to Roland back in the 1970's — which they then thinned out in a mysterious testing process. What was left went into the noise circuit that gave the 808 its magical sizzle. When the parts ran out, production ended as newer processes didn't produce the same superbly flawed parts.
(Score: 2) by RS3 on Friday September 14 2018, @01:48PM (1 child)
It will be interesting to find out the noise spectra of the original transistors. There are certain frequencies and harmonics that are more pleasant to most people's ears- more chordal, less dissonant. It may be that some kind of EQ could replicate the sound, but maybe not...
Was it simply clipping?
(Score: 4, Informative) by Rich on Friday September 14 2018, @03:25PM
Yes, iirc with some unpleasant TL082 phase inversion thrown in. The original "module controller" (distribution board for four voices) uses two primary noise amplifier stages (IC3 and IC4), of which the second has a 470k feedback resistor against a variable resistance from 2.2k to 49.2k. I had to lower the 470k R33 to 100k to get into a range where I could properly adjust the clipping range. Speaks for the quality of the original 2SC945 ("selected") vs what we now get as BC547. :) But I guess they've had it at some point, because a revised edition module controller uses an OTA with autogain, with the trimmer gone.
You can easily find all the old Roland "Service Notes" on the web. Seems the noise source was a continuing source of trouble for them, because the TR-909 uses two CD4006 shift registers in a digital feedback scheme (but with some rather odd unbuffered half-analog CMOS logic around it).