4 Pole Mission Cutoff Range - LP vs. BP/HP

My 4 Pole Mission Shruthi sounds wonderful! Unfortunately, the Cutoff range calibration for LP modes causes the HP/BP modes to be out of whack, and vise-versa.

When calibrating the cutoff range per the instructions, the e-piano patch sounded very muffled. I had to turn the pot nearly all the way clockwise to get something that resembled an e-piano. This worked great, and the cutoff range in any low-pass mode seemed to be dialed in correctly. However, when I change filter modes to HP or BP modes, the cutoff band bandpass ranage seems far too aggressive, with any cutoff value over 40 being far too quiet. Just to see what would happen, I turned the range all the way counter-clockwise, andthe HP/BP modes came to life! Unfortunately, this wrecked the calibration of the LP4 mode when using the e-piano sound, and it’s now so muffled the patch is useless.

I checked the values of R23, R25, R26 and R27: the color codes are correct, with solder joints that are correctly formed. The trimmers are the exact BOM part # specified from Mouser, and their solder joints are correctly formed as well.

A simple way of checking your build is to see if the filter can be made to track the keyboard in LP4 mode with full resonance. If you can tune the V/Oct trimmer so that playing octaves on the keyboard make it play octaves, then I don’t think there’s a problem with your build.

Typical waveforms like saw, square (and even more triangle or wavetable) have little energy in the highest frequency. So above 6 or 7 kHz there’s not much left in the sound and the BP/HP modes are going to be very quiet. But the 6 … 20kHz range is still useful for the LP mode. I doubt there’s a solution except scaling the filter range in software when the mode is changed.

I took the time to re-calibrate the V/Oct - I’m a bit flat on C3, and my C6 is a C6#, probably ~110 cents drift across 4 octaves. That’s acceptable, right? Either way, it sounds like it works.

I was tempted to hack in a switch, but a firmware modification sounds much better. Thanks pichenettes!