Geofx parametric eq article.....

[josh777]

Does anyone know if the simple parametric eq schematic in this article has a high enough q for zappa-esque tones and effects? I was reading about the eqs he installed in his guitar, and it seemed an interesting thing to try and breadboard. Better still, does anyone have the schematic of the eqs he actually used??

Many thanks

[Paul Perry (Frostwave)]

""They were identical parametric filter circuits," explains ["Midget" Sloatman, Zappa's chief guitar technician in the early 1980s]. "One of the filters was set for the bass frequencies from about 5oHz to 2kHz, and the other one was set for the top end, from about 500Hz up to 20kHz." The filters had a variable resonant frequency ("q") independent from the EQ gain. "You could find a tone and get right on top of it, tweak it. and nail it," says Sloatman. The Q ranged from .7 to 10, or a very wide dynamic range to a very narrow one, and was adjustable via a 1/4" screwdriver notch on the face of the guitar. This allowed Frank to control his feedback characteristics in any hall. He could basically tune his guitar to the room, find out how the room responded to the amplifier, and dial it up so he could have maximum control of the feedback. That was the whole point behind the equalizers. But Frank also played a lot with his left hand, and in order to hear the nuances-the string presence-he'd have to bring the treble up, which is another thing he liked about the filters. He could pick high frequencies anywhere from 4k to 8k and bring out the nuances of the strings, so you could hear what his fingers were doing, even if he wasn't picking every note." "

[R.G.]

If you're trying to get a special high-Q resonance, you might want to go for a different circuit. The state variable filter like Anderton used in his Super Tone Control can be used to great effect. It takes a dual pot to do tuning, but other than that, it's a great way to get high Q and tunability of gain, frequency and Q independently.

The bootstrapped Twin T can get to high Qs, but it gets touchy there. It's great for general peaky resonant sounds, and is simpler than a SV filter to make. But for really high Qs, and especially high Q with high gain, go SV.

[josh777]

Thanks very much, I was looking at the state variable ones, I'll ave a crack at Anderton's ones. The reason i asked is because i thought there must have been a rediculously simple alternative as zappa fitted them into his guitar.

Also. wikipediaing the state variable filters, has anyone tried the uaf42 chip on guitar?? It needs 18v but looks pretty much like the anderton one in one chip, which is great for space.