Corrections, improvements, caveats, and other thoughts from my builds. What can you substitute in
various designs? Let's find out!
I had been putting the project off for too long and when I decided to build it i did not have the
odd value resistors or even 1% ones. No LF444 either. Investigating the schematic I decided that the
constant current source would be the most demanding, but it has a trim pot... However, my keyboard
was modified earlier with 100 Ohm 1% resistors. Probably smart not to skimp there if you have a long
keybed (mine is 4 octaves).
As far as I understand this - the critical part is the resistor ladder in the keyboard and
the constant current source driving it. Since there is only a finite current, voltage drops
when you play lower notes (since resistance and current is fixed, voltage must change). The
rest of the circuit is just a sample and hold for storing the value (no critical values here),
and a rather clever circuit to get gate and trig from the same voltage (KBUS), no critical values.
So I used a TL074s instead of the LF444 (and TL084) and a LM741 instead of the TL071 for U1
The 3 Kohm resistors are just for driving LEDS, so I just choose a more common 4,7 KOhm. 300 Kohm
became 330 Kohm, and 20 became 22 (and so on). My only fancy capacitor is a poly-something 0,47 uF
which is quite a bit larger than Rays. It works fine however. Performance is a bit worse than his
measurements, but I can live with a 0,01 Volt drop after 30-45 seconds.
One of my hangups with MFOS projects are the resistor values. It seems that Ray assumed that most
people keep the entire E24 series in stock. Why would someone use 3K resistors for LED:s? 3,3 Kohm
is far more common, and 4,7K Kohm will probably work just as good. For a beginner this can be a
pain to source all values, but a more seasoned builder might just substitute stuff. My point is
that you can often get away with standard values. Try it, and replace if it doesn't work!
Updated 2020-05-01
