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Parametric EQ circuit

9/2/1999 4:06 PM
Anthony	Parametric EQ circuit Does anyone know where I can get a simple parametric EQ schematic that is designed for electric guitar. The only ones I can find on the web are at www.rane.com and those are more pro audio designs. All I need is a simple circuit with typical boost/cut and sweepable frequency controls, preferably in the 500-2khz range. Finding parametric EQ circuits is unbelievably difficult for some reason. I know for a fact that a few companies make such pedals but for whatever reason no one has ever posted the schems. The longer I play, the more convinced I am that a parametric EQ pedal may be the most useful addition to my setup and may make an overdrive/distortion pedal not needed.

9/2/1999 4:06 PM

Anthony

Parametric EQ circuit

Does anyone know where I can get a simple parametric EQ schematic that is designed for electric guitar. The only ones I can find on the web are at www.rane.com and those are more pro audio designs. All I need is a simple circuit with typical boost/cut and sweepable frequency controls, preferably in the 500-2khz range. Finding parametric EQ circuits is unbelievably difficult for some reason. I know for a fact that a few companies make such pedals but for whatever reason no one has ever posted the schems. The longer I play, the more convinced I am that a parametric EQ pedal may be the most useful addition to my setup and may make an overdrive/distortion pedal not needed.

9/2/1999 6:35 PM
Mike Burgundy	May not be all, but; Korg par. eq at Aron´s," target="_blank">http://members.tripod.com/~triparon/">Aron´s, Studiomaster at Stellan´s, and there was also an inteseting idea floating around this board on using a phaser as notch-filter. Hope it helps.

9/3/1999 3:14 AM
Mark Hammer	There is a schematic floating around (although I think it is in Postscript format) of a 4-band quasi-parametric that Jules Rykebusch designed for PAiA (which they sells as a kit in rackmount form). I remember seeing it on the U of A archive. I used an old DOS utility called Freedom of Press to print out my copy, but others tend to use Ghostview and work in a windows environment. Worse comes to worse, I can scan my copy and let someone post it on an archive as a GIF. The only really exotic parts are dual ganged pots, but these are standard Radio Shack parts. The design is fairly simply, and easy to intuit mods for if you stare long enough. The posted design uses an AC wall wart to generate +/-DC, so you'd need to use two 9V batteries at least. I can't imagine the current drain would be that heavy with just two sections instead of 4. For guitar, I suspect that a 2-band quasi-parametric will be more than sufficient to create many different tone textures. The "quasi-parametric" designation is used to refer to EQ's which lack all 3 of the "parameters" available to a "true" parametric EQ. Generally speaking, this implies an EQ where you can sweep the centre frequency, and boost or cut, but you can't adjust the Q. Adjustable Q is really most useful in those circumstances where you want to turn an EQ into a highly selective notch filter for feedback cancellation purposes; i.e., for sound reinforcement. This is in contrast to what most players tend to use EQ devices for: introducing resonances, or goosing the signal over broad bands. When it comes to fussy Q-control electric guitarists can generally live without it. Some studio buffs have found that true parametrics can make interesting Wah's, since there is generally more control over Q than is normally found in even hot-rodded wah's, and the variable boost or cut can produce interesting textures. If you can find an old volume pedal or other pedal, where you can use the rack-and-pinion system with a retrofitted dual-ganged pot (which most, if not all, para EQ's use), you can get yourself a nifty wah out of it.

9/3/1999 3:14 AM

Mark Hammer

There is a schematic floating around (although I think it is in Postscript format) of a 4-band quasi-parametric that Jules Rykebusch designed for PAiA (which they sells as a kit in rackmount form). I remember seeing it on the U of A archive. I used an old DOS utility called Freedom of Press to print out my copy, but others tend to use Ghostview and work in a windows environment. Worse comes to worse, I can scan my copy and let someone post it on an archive as a GIF.

The only really exotic parts are dual ganged pots, but these are standard Radio Shack parts. The design is fairly simply, and easy to intuit mods for if you stare long enough. The posted design uses an AC wall wart to generate +/-DC, so you'd need to use two 9V batteries at least. I can't imagine the current drain would be that heavy with just two sections instead of 4.

For guitar, I suspect that a 2-band quasi-parametric will be more than sufficient to create many different tone textures.

The "quasi-parametric" designation is used to refer to EQ's which lack all 3 of the "parameters" available to a "true" parametric EQ. Generally speaking, this implies an EQ where you can sweep the centre frequency, and boost or cut, but you can't adjust the Q.

Adjustable Q is really most useful in those circumstances where you want to turn an EQ into a highly selective notch filter for feedback cancellation purposes; i.e., for sound reinforcement. This is in contrast to what most players tend to use EQ devices for: introducing resonances, or goosing the signal over broad bands. When it comes to fussy Q-control electric guitarists can generally live without it.

Some studio buffs have found that true parametrics can make interesting Wah's, since there is generally more control over Q than is normally found in even hot-rodded wah's, and the variable boost or cut can produce interesting textures. If you can find an old volume pedal or other pedal, where you can use the rack-and-pinion system with a retrofitted dual-ganged pot (which most, if not all, para EQ's use), you can get yourself a nifty wah out of it.

9/6/1999 2:33 AM
Rick	Re: your comments about using a parametric EQ as a wah pedal. I've done it twice and it really does sound good. A buddy of mine who's ears I trust more than my own actually prefered one of them to my Vox Clyde. Incidently the old plastic Ibanez wah used a dual pot and had a Q control. I wonder if it was just an EQ in wah clothing?

9/7/1999 3:05 AM
Mark Hammer	The dual-ganged pot, and Q control DOES suggest a state-variable filter, rather than a traditional wah circuit. Years ago, I had a state-variable filter (the Super Tone Control from Anderton's circuit collection) wired up as a wah, and it was decent. If this is what Ibanez did, hats off to them for marching to a different drum. After reading RG's treatise on pots, and the other one on wah's, I got to thinking about whether a dual-ganged pot might (or at least could be rigged to) provide a more desirable taper for wah's. Consider that a dual 50k pot is simply a 6-terminal 100k pot chopped in half. If you jumper two opposite outside lugs together, that connection can function like the wiper, while the two wipers can function like the outside lugs. AS RG, Jack, and others have noted, you CAN provide other pot tapers by paralleling well-chosen resistor values between the wiper and either outside lug. The problem is that the total load of the combined pot+resistor is not identical through its travel and this can pose problems in some circuits. If one is using two cascaded pots however, the total load should remain constant (assuming reasonably decent matching between pots). Now, I say this at the same time as being too damn lazy to pick up my meter and plot it out with the dual 50k sitting a mere 3 feet away. Am I onto something here or just spreading vicious rumours? So, which terminals of the dual-ganged pot were used on this Ibanez wah?

9/7/1999 3:05 AM

Mark Hammer

The dual-ganged pot, and Q control DOES suggest a state-variable filter, rather than a traditional wah circuit. Years ago, I had a state-variable filter (the Super Tone Control from Anderton's circuit collection) wired up as a wah, and it was decent. If this is what Ibanez did, hats off to them for marching to a different drum.

After reading RG's treatise on pots, and the other one on wah's, I got to thinking about whether a dual-ganged pot might (or at least could be rigged to) provide a more desirable taper for wah's. Consider that a dual 50k pot is simply a 6-terminal 100k pot chopped in half. If you jumper two opposite outside lugs together, that connection can function like the wiper, while the two wipers can function like the outside lugs. AS RG, Jack, and others have noted, you CAN provide other pot tapers by paralleling well-chosen resistor values between the wiper and either outside lug. The problem is that the total load of the combined pot+resistor is not identical through its travel and this can pose problems in some circuits. If one is using two cascaded pots however, the total load should remain constant (assuming reasonably decent matching between pots). Now, I say this at the same time as being too damn lazy to pick up my meter and plot it out with the dual 50k sitting a mere 3 feet away. Am I onto something here or just spreading vicious rumours?

So, which terminals of the dual-ganged pot were used on this Ibanez wah?

9/7/1999 9:01 AM
Mike Burgundy	Re: Parametric EQ Ibanez "wah" -circuit? I remember those Ibanez Wahs. Looked like (built like) crap, but sounded terrific. Anyone gotta schematic? Pretty please?

9/7/1999 8:23 PM
Gus	Robin T. site has a schematic of a svf converted to a wha

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