Tuesday, 19 August 2014

TTSH (ARP 2600 clone) build notes - Noise (DIY)

Part 14 of my TTSH (Two thousand six hundred) build which is an ARP 2600 clone synthesizer.
You can see my full build thread HERE
There are two awesome TTSH threads in Muffs.
2600 clone - Two Thousand Six Hundred (TTSH)
and
Two Thousand Six Hundred (TTSH) Project General Build Thread
The DSL-Man site has a very good TTSH thread
And of course the official build thread is here

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 The Noise module pre-population


The ARP 2600 used the 4022 submodule to generate noise.
Here is a link to the schematic care of ArpTech
The transistor Q1 (2N5172) acts like a zener diode. The base-emitter junction of any silicon transistor can function as a noise-generating zener diode if its emitter base junction is reversed biased.
The zener produces random noise. The noise is then fed to an amplifier (LM 301)

The official TTSH build states that "a 2N5172 should be selected for maximum noise. But the 2N5172 may be too noisy!... causing clipping and distortion. Any other transistor will probably do. Try a BC547 ??? (Beware the pinout though, the 2N5172 is BCE and not CBE/EBC as most other transistors)".

DSL man suggest using a bc337 instead and/or soldering a 10K resistor to the 1uf cap.


RESISTORS FIRST

Music from Outer Space have a great page devoted to noise generator circuits and they make special mention of the 2N5172 transistor.

Quote:
"These 2N5172 NPN transistors have been tested and selected for their emitter base reverse bias noise. They give greater than 3V peak to peak noise with a non-inverting gain of 100. While testing I observed a wide range of noise levels with > 3V p-p as what I consider the lowest acceptable level. However several of the tested transistors caused clipping at a gain of 100 because they are very noisy. Due to this fact it is often necessary to reduce the gain in the white noise generation section of many MFOS circuits when using one of these selected 2N5172 noise transistors."

Anyway, I'll stick with the 2n5172 for now. I've cut up some female headers to make a socket for the transistor. The original ARP 2600 schematics use a "selected 2N5172". I may have to try out a few 2N5172s before I find one I like.


IC's and transistors installed.


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The ARP 2600 Noise section - part of the sample/hold & clock section.

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