[sdiy] EN 4 pole filter resonance issues

[Dan Snazelle]

so tonight i built the electronotes 4 pole vco/vcf (quadrature) from the Engineers notebook (also in one of the old EN issues)

anyway....it works great UNTIL i turn up the resonance high enough to sound good at lower cutoff levels.

first off i am using 13700's NOT 3080's and in order to get resonance i had to switch out the 10k resistor that goes INTO the input out for a 5.6k


anyway, the resonance at higher settings gets VERY screetchy. and the res pot is so sensitive. in other words to get it sounding nice, i have to turn the res up high enough that when i open the filter all the way up, it oscillates in a not so nice way. 

anyway, this is my first experience with this filter...before this i built only ssm2044's or small 13700 state variables...so i am not sure if i am doing something wrong. also the cap values in EN seem pretty small (150pf)

i did notice that this filter doesnt have any of the 3pf feedback caps i see in the ENS76 vcf option 2 so i may try those.

i am sure some of you have experience with OTA 4 pole filters so ANY advice would be much appreciated.

thanks



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