discoveries of the weekend

[Haible_Juergen#Tel2743]

Hi!

I spent some hours to take a close look at a friend's enormous
service docs' collection, and here are some things I have found:

Korg MS-50:

This ultra-rare beast's Lowpass Filter is based on an integrated
diode ring (CA3019)! It is just the common (Sallen&Key?) LP
filter, with the resistors replaced by diodes. An extra feedback
path is added for variable resonance.
The MS-50's VCA is also built with a CA3019 diode ring.

Korg PS-3100:

Now I have seen how those famous Resonators are built!
Speaking of active components: dead cheap. Again a simple
one-opamp BP filter for each resonant peak. But the resistors
are replaced by photocouplers - a quite common technique in
Korg synths of that area. I haven't seen an extra resonance
control part - but they might individually control the two resistors
to get differt Q's - have to look again at home.

Roland SH-5:

Now this was the great surprise! While the later SH-7 has 4-pole
ota-filters, now guess what's inside the earlier SH-5:
It's a *diode ladder*, very similar to the TB-303 and the EMS
Synthi A.
(There are additional BP and HP filters - it's the LP filter I was
talking about.)

JH.