Parametric EQ Schem

[gstopp at fibermux.com]

     Good question! Made me dig thru my stuff....
     
     I found an app note in Electronotes (AN-13) that shows some simple 
     fixed shelving tone controls, bass and treble, plus a bandpass 
     boost/cut circuit. According to the text they are taken directly out 
     of Jung's "Op-Amp Cookbook".
     
     I'm not sure how to make these tuneable, probably need some dual pots.
     
     I've seen two different designs for graphic EQ's (don't worry this is 
     leading somewhere) - one, each fixed band has a boost/cut control with 
     unity in the middle, and two, each band is a fixed bandpass whose 
     level is varied from zero to max. The former design is a bunch of 
     boost/cut circuits, and the latter design is really a fixed-filter 
     bank and "zero cut/boost" is actually just the band turned up halfway. 
     The latter design is what you will find in a Moog 914 Fixed Filter 
     Bank module, for example.
     
     Now a parametric EQ has tuneable sections - if I were to build my own 
     parametric, I would probably build it out of several standard 
     state-variable filters in parallel and just use level controls on 
     each. At the low end I'd tap off a lowpass, in the middle bands I'd 
     use the bandpass tap, and at the high band I'd use the (you guessed 
     it) highpass output. This EQ would have frequency, level, and Q 
     controls for each band. I think the big difference between a 
     synthesizer filter and this design would be that I would go out of my 
     way to make accurate panel markings for the frequency pots, whereas on 
     a synth I'd probably not use scaled markings at all (I always tune 
     VCF's by sound anyway).
     
     One problem with building manually-tuneable state-variable filters is 
     that you need to use dual pots for the frequency controls. One way 
     around this is to use OTA's and tune them with a single CV. If you did 
     that you'd have a voltage-controlled parametric EQ and you'll be 
     tempted to add all kinds of CV inputs (sweep, spread, etc.) so beware.
     
     One other thing to note would be that each state-variable bandpass 
     filter will have a non-controllable bandwidth (well it is affected by 
     Q, but that's another complication). In order to have mid-bands with 
     frequency *and* bandwidth adjustments, you'd need to make the band out 
     of a pair of filters in series, one highpass and one lowpass, and as 
     you can see the number of parts in this design is getting up there.
     
     Once you get this far it's just a little step to add an LFO to each 
     band's CV input to make a vocal-effect animator.... ah sorry guys, I'm 
     going a little overboard here.:)
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Parametric EQ Schem
Author:  Christopher_List at Sonymusic.Com at ccrelayout
Date:    3/6/97 11:39 AM


     
     
     
Hi List -
     
I've been quite for a while - actually working on music (for a change) and 
busting my hump here at work. I still haven't sent the images to Metalphoto 
for the new faceplates, but soon, I promise....
     
Anyway, my question is:
Parametric EQs - are they hard to build? 
Is there some trick to how they work?
Why does it seem like they are rare in modulars and schems are equally 
rare?
Seems like they shouldn't be THAT complicated...
Anyone know where I can find a schem or two on the www? 
I looked a little but came up with nothing.
     
- Thanks,
Chris