[sdiy] Exponential Keyboard (KORG MS-20)

Tony K kalotony at videotron.ca
Sat Sep 1 11:42:06 CEST 2007


Hi list,

  I'm building a keyboard controller for my MS-20 clone project using 
a PAIA 37-note mechanism. It already has the diodes wired
into a 8 X 5 matrix and I'm tempted to use that with a uC and do a 
digital scanning version with DAC (8051 based - I know I should
really graduate already, but those MMT8's make great development 
systems - oops the secret is out ! ).

Analyzing the MS-20 keyboard circuit I'm a little confused about how 
the scaling factor (427/403= 12th root of 2) is actually
'multiplying' the subsequent network. I tried this on paper by 
starting off with 1.059 V, low C, and doing the math, but I don't get
the expected next value of 1.122 and so on down the string. I've 
re-drawn the network and used the voltage divider rule it what seems seems
to be a simple network. Is the network 'on the left' affecting the 
voltage as seen from the right (output) - it shouldn't as each key closure
is the 15 volt regulated supply. I don't remember but was the KORG 
high note priority ? The legacy is last note priority but I'm not 
sure it emulates
the keyboard faithfully (in single trigger mode).

Also the resistor values of 403 and 427 ohms are not available from 
the usual sources -strange. The JH-720 use 430/402= 1.070 to four significant
figures. Any other ratios that work as well as the original ?

I got excited about using a DAC-88 exponential DAC but those aren't 
true log/exp DAC's only u-255 law and for audio use. I suppose the simplest
approach is to use a multiplying DAC and a look up table for those 37 
keys (already calculated each keys voltage output).

All this leads me to scrap that part of the circuit altogether and do 
a straight forward 1 V/OCT design by replacing the exponential input 
scaling resistor R38 with a 24 turn MT trimmer - not ideal but I know 
this works. Only things that concern me is the lower impedance seen 
by the exponential converter and now FM wouldn't be Hz/V changing the 
character of the machine (?).

I guess I'm thinking out loud at this point, but any opinions or 
advice is welcome.

Tony K

P.S If anybody has a source for those precision resistors (403 and 
427 ohms) I would love to know.




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