[sdiy] Minimoog non-linear keyboard response

Eric Frampton eric at ericframpton.com
Mon May 2 00:00:26 CEST 2016

A follow-up to this:

For starters, I’m on the old oscillator board.

The floating component mod for the key/envelope board has not been done on this unit.

It’s only oscillator 1. #2 and #3 seem to be tracking fine.

Furthermore, if I try tuning Osc 1 so the low and high F’s are in tune, not only are the notes in between them slightly out (especially in the 3rd octave), but the half an octave above the top F is out of tune as well.

I’ve put in a matched resistor set on the incoming CV lines, so that’s probably not it.

I’m seeing similar 20mV changes in octaves when measuring pin 6 of the summing 741’s on all three oscillators. Not identical, of course, but not wildly different on any one oscillator.


> On Jul 23, 2015, at 12:15 AM, Eric Frampton <eric at ericframpton.com> wrote:
> …or “unevenly exponential” might be a better way to put it.
> If I can get the A’s on each end of the keyboard to scale up perfectly for each oscillator, and I can get the octaves on all three oscillators to switch evenly, what would cause the internal intervals between those A’s to not be linear? In other words, if A1 and A4 are correct, A2 might be a little sharp and A3 might be a little flat, but it’s enough of a deviation that when switching octaves, the oscillators don’t stay in tune with each other.
> The same situation happens whatever octave it’s in - the keyboard itself seems to be triggering non-linearly (non-exponentially) internally, no matter where the tuning on a given oscillator is set. If the ends are right, the middle is wrong.
> I’ve got a buffer board installed, so I don’t think it’s that.
> Thoughts? Time for new resistors on the actual pratt-read assembly? Or was this what the field service mod on the keyboard PCB was supposed to solve?
> e

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