The Active Filter Unit
was an add-on for the Project
60 modular hi-fi system. It was designed
to eliminate unwanted frequencies above or below ("scratch"
or "rumble") a specific cut-off frequency,
thus improving the overall quality of the sound output.
It incorporated two Sallen & Key type active filter
stages, providing a high pass (scratch) and low pass
(rumble) filter for output from the Project 60's twin