AW: SEM Style knobbies

[gstopp at fibermux.com]

     It would be a single pot.
     
     The selection of one signal or the other when the pot rotation is 
     maxed out to one side or the other is determined by the output 
     impedance of the sources that drive the ends of the pot. In this case, 
     if you connect the lowpass output of the VCF to one end of the pot, 
     and the highpass output to the other end, and each filter output is 
     the output pin of an op-amp, then if you turn the pot all the way to 
     the lowpass side for example you will get 100% lowpass signal and 0% 
     highpass signal at the wiper of the pot.
     
     If the pot is 100K, this would be the same as using the lowpass output 
     with a 100K resistor to the highpass output. The lowpass output op-amp 
     will completely control the situation, because there is zero ohms to 
     the lowpass driver and one hundred thousand ohms to the highpass 
     driver. Okay, op-amps aren't really zero ohms, but darn close due to 
     negative feedback. Anyway with the pot rotation in the middle 
     (assuming a perfectly linear pot) you will get a Notch output at the 
     wiper due to phase cancellation effects, with 50K impedance to each 
     filter output. The reason to buffer the wiper would be to provide 
     unvarying low output impedance from the wiper instead of zero-50K-zero 
     as the pot is turned.
     
     If the filter outputs have 1K output resistors on them, you could 
     still do the notch-pot thing but at min and max rotation you would not 
     get 100%/0% mixes, you'd get 99%/1% mixes or something like that. Not 
     too far from perfect, so you probably couldn't tell the difference 
     sound-wise.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Re: AW: SEM Style knobbies
Author:  Christopher List <Christopher_List at sonymusic.com> at ccrelayout
Date:    6/5/96 1:54 PM
     
Wait a sec, I'm missing something here. Are we talking about a single-ganged 
pot or a dual-ganged? 
If it's dual-ganged, I already knew this answer. 
If not, what's the one wiper connected to? A summing opamp? A follower? I 
either case, won't you still get some of the the other signal feeding through 
when the knob is turned all the way to one side or the other?