Moog 24dB Highpass

[R.Fahl]

Has anyone seen schematics for the highpass vcf used in the EMS Synthi 100?
I have this nagging hunch that it's a highpass version of their infamous
diode ladder lowpass.


>   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