[sdiy] Prophet 5 noise sources

[Tom Wiltshire]

Hi All,

Today I've been exchanging emails with a chap who's put one of my noise generator PICs in a Rev 2 prophet. This has thrown up an interesting difference between Rev 2 and Rev 3 Prophets - beyond the obvious fact one's full of SSMs whilst the other's full of CEMs.

The Rev 3 Prophet uses two separate MN5837 noise generators. There's one for the white noise which is fed to the voices, and a entirely separate MN5837 chip for the wheel modulation noise source, which is filtered to make pink noise. Well, pinkish; 6dB rolloff from 100K and 10nF.

The Rev 2 Prophet uses the same source for both purposes, feeding the unfiltered noise to a SSM2020 VCA, and also filtering it for pink noise before feeding it to the wheel mod section.

So there you go! Between Rev 2 and Rev 3, Sequential decided it'd be a good idea to have independent noise sources for modulation and audio. God only knows why!

T.