I’m working on a project and found a filter topology as drawn in this schematic in the red box and I’d like to know more about it (especially how to choose the component values). Rotating the pot in one direction it is supposed to behave like a low pass filter and in the other as a high pass filter.
Unfortunately I lost the source documentation for the filter. Is there anybody who has any references to this type of filter?
I don’t know anything about it, but C6 in combination with R13 is a high pass filter for AC coupling to the next stage. I put the rest into LTSPICE:
and ran an AC analysis:
The solid lines are amplitudes (left axis) and the dotted ones are phases (right axis). Green, blue, and red are pot respectively counterclockwise, center, and clockwise (or perhaps the reverse).
If I’ve done this right: Red is the largest amplitude at low frequency and the smallest at high; green is the smallest amplitude at low frequency and the largest at high. But all three roll off at high frequency. Green also rolls off a little below 1 kHz. None of these rolloffs is much larger than about 1 dB per octave. I wouldn’t describe this as high pass on one side of the pot and low pass on the other.
If I replace the 22 nF cap with 2.2 nF, though, I get this:
That does look more like it. Except that the maximum amplitude change over the entire audio range is less than 3 dB. Maybe some other component value change would improve that but I’m skeptical.
Added: I just remembered the term for this sort of thing (I think) is “tilt filter”. Google that, and check out the Music Thing Simple EQ module for an example.
Thx for the simulation!
I vaguely remember that C1 and C2 originally were of equal value which made it look a bit symmetrical in some way.
After looking for ‘tilt filter’ I stumbled upon a Baxandall filter which shows ‘some’ similarities.
There’s one of those alongside the tilt filter in the Simple EQ. But it has separate controls for treble and bass.
