DIY: Micromoog/Multimoog Oscillator "fault" correction

Caloroso, Michael E CalorosoME at
Fri Nov 19 22:56:41 CET 1999

A week ago I wrote:

> Well, last night out of curiosity I hooked the Micro up to the oscilloscope.
> Very interesting trait (or "fault" if you prefer) about the oscillator; the ramp shape starts losing its form starting at 8'.  The rising slope changes from a straight line to a curve as the frequency gets lower, the "elbow" of the curve rises toward the top.  When you get into the 32' range, the ramp looks more like a distorted 90% pulse waveform.  This trait is not dependent on the octave setting, it's a function of frequency.
> This is *far* removed from a Minimoog ramp in the bass range.
> Anything above 8' and the Micromoog ramp is a laboratory pristine example of a ramp waveform.
> That may explain why Micromoogs and Multimoogs are better for leads than they are for bass, and why they cannot duplicate the Minimoog Bass.
Rene and Juergen were right.  This is due to high pass filtering from the AC coupling cap at the audio input of the filter.  If I had checked the square wave it would've given it away, it wasn't immediately obvious with the ramp.  I probed the charging cap directly and found a nice ramp, and then I traced that ramp through the buffer, the waveshaper, the input to the filter - BINGO.  That's where the bass is diminished.

It's a simple fix.  Just parallel C506 (located smack in the middle of the large board) with a 100uf 50v electrolytic cap.  Make sure you observe the polarity of the cap and that you have sufficient clearance.  I tack-soldered it on the underside on the PC board and it fit.

This lowers the cutoff frequency of the composite high pass filter formed by the input cap and associated impedances and results in better bass response.  NOW it's a decent bass synth.

This should work for Multimoogs since they use the same board as the Micro.

Sounds similar to what Juergen found in his OB-8...

Opinions (and mistakes) expressed herein are my own and not those of my employer.

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