[sdiy] HP from LP ? -> Mother-32

[Michael Zacherl]

Hi Florian,

> the Behringer "Highpass" is appearently simply the 'mix with inverted signal'-thing;
> sounds more like a notch filter with less low frequencies than a highpass.

you mean less low end than a true notch filter?

> Audioexample
> https://youtu.be/3pRiUSf_QFw?t=2214

The other synth in question was the Mother-32.

I looked up the Mother-32 manual (https://www.moogmusic.com/sites/default/files/Mother_32_Manual_Web_1st_Run.pdf)

Quote:
"The High Pass Filter is a classic Moog 24dB/Octave non-resonant Ladder Filter. This is useful for bright sounds that cut through a mix, or dynamically removing bass frequencies to thin the sound.

NOTE: In HIGH PASS mode, if the RESONANCE control is turned up at all, the High Pass Filter will NOT function fully as this will reintroduce bottom end into a sound, which may or may not be desired.

TIP: You can provide resonance in HIGH PASS mode by patching the VCF output into the EXT. AUDIO input. The MIX control then becomes the resonance control for the High Pass Filter, even increasing to self-oscillation. For this to work, keep the RESONANCE panel control turned fully counterclockwise.

TIP: With the CUTOFF control at minimum in HIGH PASS mode, you are basically bypassing the Ladder Filter and passing the output of the mixer straight to the VCA. If you are looking for the pure sound of the VCO, this is a way to get it without patch cords."


Quote from the Mother-32 review in Sound-On-Sound:   (http://www.soundonsound.com/reviews/moog-mother-32)

"In contrast, the high–pass response is anything but standard. If the resonance knob is in any position other than zero, it sounds similar to a peaking filter but, even without resonance, there’s still a great deal of signal passing at mid–frequencies, even at the highest cutoff frequency (at which very little audio should survive). However, you can patch the output from the VCF to the external audio input to create a feedback loop that generates something more akin to a conventional resonant high–pass filter response."

So, it seems Moog have taken a shortcut as well, to get this "other” filter response?

OTOH, the Moog 904B is a true HPF but with no resonance control.
Is there a practical reason for that?

Thanks for the great thread, everybody!
m.