Different MS-20's

[Tony K]

  Haible Juergen wrote,

> It is commonly well-known that Korg produced at least two different
> versions of the MS-20.
> 
> So far, they are adressed as old, or "Korg-35-filter version" and later,
> or "LM13600 filter version".
> But when I looked at the schematics a little closer over the weekend, I
> found that there are
> other differences as well.
> 
> old one:
> 
> * Custom Korg-35 SIL ICs directly on main board for vc-filters
> 
> * Single-transistor VCA
> 
> * Fixed 8.8kHz LP filter after VCA (100pF, 180kOhm)
> 
> 
> later one:
> 
> * LM13600-based vc-filters on separate board
> 
> * Dual transistor VCA
> 
> * 338 Hz / 6dB  deemphase filter after VCA
> 
> 
> Quite different !
> 
> Someone said he heard both and there wasn't a difference. I heard
> someone else saying
> some MS-20s are more noisy than others, but he didn't know why. 
> Well, the schematics give some clues at least ...
> ... and also a lot of questions.
> 
> 
> A lot of questions:
> 
> Where is the coresponding preemphase circuit in the later MS-20 ??
> How much is noise improoved by the 2-transistor VCA ??
> For which parameter did Korg select the two transistors ?
> 
> Are there more "older" units out, or more "later" ones?
> Which ones do list members own ?
> Which one do you prefer ?
> 
> Any feedback welcome ...
> 
> JH. 

I've got the LM13600 butterworth filter version which I reverse 
engineered many years ago (changed HPF to a LPF for two separate 12 
db filters,very Tim Orr'ish) and I can't find the pre-emphasis 
network either.

My friend had both old and new machines and he says the only 
difference was the controls for the Q and EG2 had to be set up 
differently on the Korg -35 one,otherwise he didn't "hear" any 
difference.Now this might just be trimmed differently on a 35 
version.

What is the function of Q4 in the dual 2SC945 VCA?Is it used as a 
diode across BE junction of Q3 main VCA transistor ?That's what I 
see.

BTW,my LM13600N's are marked with pink paint,and the 2SC945's in the 
vca are blue.

Tony K
IC1 Electronics