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posted by martyb on Friday September 14 2018, @08:16AM   Printer-friendly
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.

Source: https://hackaday.com/2018/09/06/you-cant-build-a-roland-tr-808-because-you-dont-have-faulty-transistors/


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  • (Score: 3, Informative) by RS3 on Saturday September 15 2018, @02:46PM

    by RS3 (6367) on Saturday September 15 2018, @02:46PM (#735299)

    Okay, I need to practice what I occasionally preach about doing less speculating (but it's so easy to write stuff online!)

    After some searching: transistor gain, frequency response, etc. have nothing to do with this thing. It's not being used as a transistor (3-wire amplifying device). Q35's BE (Base-Emitter) junction is used as a diode in reverse-bias at the Zener / avalanche breakdown voltage, and amplifying the resulting avalanche noise. One simple reference: https://www.electronics-tutorials.ws/diode/diode_7.html [electronics-tutorials.ws]

    The chatter is that the defective 2SC828s create more "sizzle" - so more high-frequency component. It should be fairly easy to measure the spectral density and replicate that, even in software as JoeMerchant wrote https://soylentnews.org/comments.pl?sid=27590&cid=734766 [soylentnews.org].

    But sticking with the simplicity of the original analog circuit, http://secretlifeofsynthesizers.com/wp-content/uploads/2018/05/TR-808-transistor.jpg [secretlifeofsynthesizers.com] one could vary the bias resistor, 1M R128, which is certainly adding noise, and the load resistor, 300K R131, and of course op-amp gain pot (TM4), and change the sound characteristics. Lower resistor values should give better high frequency response, but a lower R131 will shunt away more of the noise signal, so more op-amp gain would be needed, but op-amp circuit high frequency response might suffer (more study needed).

    It would be very interesting to compare the avalanche / Zener voltage of a defective part to a good one. My hunch is the defective one is lower, so noisier.

    As a kid I learned the hard way that transistors and resistors get much noisier if overheated by a soldering iron. One could build a noise generator just using carbon resistors and an op-amp- the higher the Ohms the better. One could hurt some transistors with external heat, or carefully overdrive some reverse-bias on the BE junction and cause noisier behavior. Maybe I should do that and sell them on ebay / reverb...

    Maybe someday if I'm bored of writing here I'll mock up the circuit, change some values, cook some transistors, see / hear what happens ...

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