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previous: GFR Sorry for taking too long to post t... -- 9/8/1998 7:40 AM View Thread

Re: Wah Phasor! Who Made One?

9/8/1998 4:30 PM
Mark HammerRe: Wah Phasor! Who Made One?
Back in 1979, Craig Anderton offered a "bonehead mod" (his term not mine) for his Super Tone Control in EPFM. The Super-Tone control was simply a state-variable filter similar to the design you have at your site, only with switches to select between LP, HP, BP and Notch. The 2nd edition of EPFM hopped up the design, replacing the switches with pots and a mixer stage, but the essential design remained the same.  
 
The mod consisted of replacing the twin series variable resistors used to tune the centre frequency (dual 220k pots in your schematic) with a dual-ganged pot wired up as voltage dividers rather than variable resistors. The frequency range was limited by a low-value resistor between ground and the ground lug of the pot (I used 33-100 ohms). The wiper went to the input of the op-amp, and the other lug went to where your schematic shows the 10k resistor. The nice thing about the mod was that it increased the sweep range by about 10 (from 100:1 to 1000:1) and was relatively insensitive to the actual pot value used (you could use anything from 10k to 100k and get pretty much the same outcome). The other nice thing about this mod is that it has such a wide sweep range that you can stick a dual pot in place of the low-value ground resistors mentioned earlier, and use it to tune the filter within that range. Stick a second ganged variable resistor in front of the sweep pot, and you can use the combination to set the upper and lower points of sweep, and twiddle the foot-pedal within that range. I suspect you could also stick dual-ganged variable resistors in parallel with two lugs from the sweep pots and tune the taper. The fact that the actual value of the sweep pot is not critical is a real plus here. Now THERE is something you don't often get in a wah or phasor.  
 
There was a post I sent in wayyyyyyy back, about how to use parallel resistors to change the taper of a pot. I don't have it, but it may be worth digging up if you do.

 
Replies:
GFR I have an article that appeared in ... -- 9/9/1998 6:14 AM