How to make an HPF from an LPF

No title link. Mike Kent sent this to AH. He gave me the ok to post this. I figured others not on AH might like to try it.

"You can probably get a High Pass Filter or Band Pass Filter type of effect
from a modular system that only has a Low Pass Filter. You may need to make
one special patch cable, with its wires crossed to switch the polarity, for
it if your modular doesn't have a way to get reverse polarity outputs.

If you send a signal into a low pass filter and then mix it back with the
original signal with reverse polarity, you can emulate a High Pass Filter.

______ ________
| |--Polarity Switch--->| |
|VCO | _______ | MIX |---------->
|____|---->| LPF |-------->|______|
|_____|

Or:

______ ______ ________
|VCO |--->| |--Polarity Switch--->| |
|____| |MIX | _______ | MIX |---------->
|VCO |--->|____|---->| LPF |-------->|______|
|____| |_____|

The mix levels after the filter are critical. This depends on phase
alignment to cancel frequencies below the LPF's cutoff point. Filters
generally mess with phase so this may not work well on some modules.
Resonance can mess things up, too. But it does work on many systems.

Add another LPF on the end of that chain to get a Band Pass Filter with
variable width (the difference between the cutoff of the 2 filters). Drive
both filters from a common EG to get typical BP sweep sounds.

______ ______ ________
|VCO |--->| |--Polarity Switch-->| | _______
|____| |MIX | _______ | MIX |--->| LPF |--->
|VCO |--->|____|---->| LPF |------->|______| |_____|
|____| |_____|


Regards,
Mike."