Fwd: Nice Analysis of Minimoog Filter and Oscillators

[Martin Czech]

SNIP 
> One "technical type" reporting to duty....sorry for the long append.
> 
> I then did a side by side comparision.  The single factor that dominates
> the differences is not the bias resistors in the ladder (1K), not
> whether the CA3046 array is in the design, but it is the differential
> stage, the feedback path between the top of the ladder and the input
> transistor opposite the audio input at the bottom.
> 
> Feedback has more impact on a circuit's soronity than most people care
> to acknowledge.  
>                       
SNIP

A year ago, I experimented with a v.c. state variable filter.  This is
a completely different filter design, but the same applys:  The
observation is that increasing resonance "steals" volume at the bottom
in lowpass-mode.  (The same happens in every filter with feedback, only
frequencys at Fc are boostet, the rest is decreasing).  My idea was to
insert a simple RC highpass filter (Fc ~ 100 Hz) into the feedback loop
(a better idea is to use a cap/pot for tuneable bass).  Of course, this
decreases resonance at the low end, and it also enhances the bass
frequencys.  Before the filter had the typical "thin" or "airy" sound,
after insertion of hp there was a "fat bottom". WOW!

The six-string people claim, that old tube amps become "brighter"
because of drying out electrolytic caps. Now, I don't own any vintage
synth, but could it be that such caps are used for the feedback loop ?
(There are two ways where caps are used : from the ladder and again
back to the ladder). If so, is it possible that they have lost capacity
due to drying out?  This would mean that the mini gets fatter and
fatter as time goes by.

SNIP
>The Mini (or 904A) is the simplest and is all discrete components (no
>op-amps), and it sounds the best to most analogue die-hards.  The
SNIP

Yes , and the oldest, too.

I can't understand how a darlington output stage may alter the feedback
at *low* frequencys, but this is *linear* thinking, and this darlington
differential might introduce considerable distortion when the amplitude
rises (feeback rises).

m.c.