[sdiy] Resonance control taper ?

[Tony Allgood]

Further to Michael's post, the Moog ladder filter only uses a reverse 
log pot when the pot is in series with the audio signal. The input 
resistance of the feedback port functions as the shunt part of a 
potential divider.

If you are controlling feedback with a pot wired as three terminal 
potential divider (ie. traditional volume control) then a linear taper 
could be used. However, one needs to consider the effect of the input 
resistance of the feedback port if directly connecting the wiper to the 
base of the right hand input transistor of the ladder. A voltage 
follower connected from the pot's wiper could be used if you want a true 
linear taper. Moog are currently using linear pots with an op-amp 
follower in the Matriarch. Beware of Q enhancement when using such 
followers as even a small amount of phase distortion in the feedback 
pathway will effectively increase resonance at high filter cut-off 
frequencies.

Most VCAs in voltage controlled resonance pathways for Moog ladders are 
linear devices.

Given that the Rossum Evolution probably uses a SSI2164 I would guess 
that it's linearised in some way.

Tony

www.oakleysound.com