[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 one.
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 alike as same as their sounds.
check both of them.
http://www.aleph.co.jp/~takeda/radio/mp3/BlueBlue.mp3
http://www.aleph.co.jp/~takeda/radio/mp3/SpringRain2.mp3

Both base tone sounds same, But I use LM13700 base VCF for "BlueBlue"
and I use Korg35 based VCF for "SpringRain". In the Tunes, You can't
point out their difference, I think.

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.