Need help: Variable High pass filter with resonance - switch noise

[elskardio]

Hi guys,

I'm presently working on a Variable high pass filter based on ESP project 155 (http://sound.whsites.net/project155.htm)

I added a switch to create a resonance of +/- 5dB around the cutoff frequency (HPF_Bump).

I also added a switch to change the slope to 6 dB/octave. (HPF_6DB)

I built a prototype and everything works but... if the HPF_BUMP switch is pushed and you push the HPF_6DB switch, it makes a loud noise for a brief moment.

It sounds like the opamp is oscillating for a brief moment. I guess it doesn't like not having any signal at the input while the switch breaks the signal before reconnecting.

Any help on the design would be appreciated.
Thanks

 

[L´Andratté]

You can only have resonance in a 2pole or higher order filter, if you switch the first pole out in a Sallen-Key-filter you also have to switch out the positive feedback...

 

[elskardio]

I just shorted Comm2 to NC2 of the HPF_6DB switch and no more noise 

Thanks