Improved PT2399 Filter

Quick post to share this variable, buffered, 2 pole low pass filter design that can be used with one of the PT2399's internal op-amps.

The op-amps' positive inputs are tied to Vb (2.5v) internally which really limits the range of configurations they can be set up in. It's basically inverting op-amp or multiple feedback filter. I was searching for a variable MFB low pass filter design for a while before just experimenting with values, using this extremely useful online calculator.

This is the design, the two pots represent two halves of a 100k dual gang potentiometer. On each half of the pot the connected lugs are 1 & 2, so turning it CCW lowers the cutoff frequency.:

If you input these values into the calculator (with R1 and R3 varying from 10k to 110k depending on the pot position) you'll see that the cut off frequency varies from 5032hz to 1517hz, which is decent range for PT2399 filtering.

You can also modify it so that R1 and R2 (in the calculator) are the variable values, while R3 is consistent at 10k, with very similar results.

The downside is the Q factor and damping are also changed with the pot turn (you can see this in the calculator) but this isn't so bad IMO.

You could always try different values with the same basic blueprint, it can even be used with the minimalist datasheet design, where the other op-amp acts as a mixer for the delayed/ non-delayed signals. The ability to generate a hgher Q factor through value selection (at least, within a range) makes this filter effective at pinpointing and filtering out unwanted noise from the delays.