Moog 24dB Highpass

Don Tillman don at
Fri Dec 19 18:48:08 CET 1997

   From: Martin Czech <martin.czech at>
   Date: Fri, 19 Dec 1997 09:04:22 +0100 (MET)

   I found the moog 24dB highpass patent

   But this is only a "priciple of operation" drawing.
   Has anybody a real schematic ?

There's one somewhere on the web, I can't remember where though.

   1st order analysis:
   The circuit is not differential and has not much to do with the famous

Gosh, I think it's a lot like the famous ladder.  It's not
differential for the signal, but it is differential for the control
voltage and thus the main signal distortion mechanism is going to be
symmetrical.  Both circuits rely on varying the effective impedance of
a pair of emitter followers by changing their operating current.  And
both circuits are four cascaded buffered single-pole sections.

I think it's clever how much the high-pass circuit is like the
low-pass, yet they look so different.

   The patent does not show any feedback, I guess this can be a simple
   voltage divider pot at the output that is fed back to the input.

True, there's no feedback.  I was surprised at that.  I'm guessing
various phase shifts in the circuit make feedback a problem.

   I heard people say that the sound is quite "dirty" and oscillation
   is possible, but non sine shaped, much more partials. 

Without feedback oscillation is not possible.  

(Also since there's no major resonance, accurate tuning, temperature
tracking and all that, is not necessary.)

   -- Don

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