[sdiy] Korg X911 guitar synth

[Motohiko Takeda]

Hi Alwyn and all,

> also, i can't seem to find a schematic of the MS20 filter based on the
> LM13600 chip. i have found some that are based on it that use the CA3080.
> Am i blind?

It's almost same as CA6080. I try to arrange Rene Schmitz method to
LM13600. His original page is here
http://www.uni-bonn.de/~uzs159/ms20.html
and this is my trial.
http://www.aleph.co.jp/~takeda/radio/MS20clone.html
I have no English page about this yet. But you can check some schematic
or pictures.
http://www.aleph.co.jp/~takeda/radio/img/VCF-scheme.gif
http://www.aleph.co.jp/~takeda/radio/pdf/ms20clone.pdf
http://www.aleph.co.jp/~takeda/radio/img/g-vcf2.jpg

About PCB pattern, You should better also check my One Board synth's 
VCF part. It's same schematics.
http://www.aleph.co.jp/~takeda/radio/OneBoardFarm.html

> and which is better in peoples opinion? the korg35 based one , or the
> Lm13600 one?

I also build Korg35 based VCF.

http://www.aleph.co.jp/~takeda/radio/MS20clone2E.html


> and finaly, i was reading in keyboard magazine's 40th anniversary special
> on korg, one of the engineers who worked o the MS20 commented something to
> the effect of
> 
> 'the filter on the MS20 is basicly a resister'  ...
> anyone care to comment?

I made frequency response graphs for VCF that I had made.

http://www.aleph.co.jp/~takeda/radio/farmVCF2.html
It's also Japanese only page, but you can check some graphs there.

"MS-20 VCF Clone" is LM13700 based on MS20 and "MS20 VCF Clone2" is
based on Korg35.
Korg35 based VCF has 2 peaks. I think that it should be caused by
selection of parts or something. but their curve is almost same. And 
their sound is so alike. I can't point out which is which, if I try to
check in blind for myself. 

In a glance, their schematics looks so different. But if you check
them in the point of view of registors, then you can find that
they are almost same as their sounds.
I think that both of them are based on this.
http://www.ecircuitcenter.com/Circuits/opsalkey1/opsalkey1.htm
 
the biggest difference is the feelings of operation. LM13600 based one 
is more smooth, I think.

Cheers,
Motohiko "almost melting" Takeda
Tokyo Japan