Moog 24dB Highpass

[Charles A. Mann]

Don Tillman wrote:
> 
>    From: Martin Czech <martin.czech at intermetall.de>
>    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
>    ladder,
> 
> 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

Happy holidays all,
the schematics for the modular Moog are here:

http://aupe.phys.andrews.edu/diy_archive/schematics/synths/moog/index.html


the highpass filter is page 14.  As per the warning, downloading this
file caused my browser to crash.  Saving to a temp directory
then using a viewer solved this problem.  Viel glück.
Sincerely,
Charlie Mann