Parametric EQ Schem

Magnus Danielson magda at
Fri Mar 7 00:53:09 CET 1997

>>>>> "g" == gstopp  <gstopp at> writes:

 g>      Good question! Made me dig thru my stuff....
 g>      I found an app note in Electronotes (AN-13) that shows some simple 
 g>      fixed shelving tone controls, bass and treble, plus a bandpass 
 g>      boost/cut circuit. According to the text they are taken directly out 
 g>      of Jung's "Op-Amp Cookbook".
 g>      I'm not sure how to make these tuneable, probably need some dual pots.
 g>      I've seen two different designs for graphic EQ's (don't worry this is 
 g>      leading somewhere) - one, each fixed band has a boost/cut control with 
 g>      unity in the middle, and two, each band is a fixed bandpass whose 
 g>      level is varied from zero to max. The former design is a bunch of 
 g>      boost/cut circuits, and the latter design is really a fixed-filter 
 g>      bank and "zero cut/boost" is actually just the band turned up halfway. 
 g>      The latter design is what you will find in a Moog 914 Fixed Filter 
 g>      Bank module, for example.
 g>      Now a parametric EQ has tuneable sections - if I were to build my own 
 g>      parametric, I would probably build it out of several standard 
 g>      state-variable filters in parallel and just use level controls on 
 g>      each. At the low end I'd tap off a lowpass, in the middle bands I'd 
 g>      use the bandpass tap, and at the high band I'd use the (you guessed 
 g>      it) highpass output. This EQ would have frequency, level, and Q 
 g>      controls for each band. I think the big difference between a 
 g>      synthesizer filter and this design would be that I would go out of my 
 g>      way to make accurate panel markings for the frequency pots, whereas on 
 g>      a synth I'd probably not use scaled markings at all (I always tune 
 g>      VCF's by sound anyway).

I have built a state-variable fully parametric EQ once with great
success, it sound just great. I used a standard summing filter for the
boost/cut pot mixing. I can recommend this approach and it is being
used by Klark Teknik and T.C. Electronics in thier parametric EQ

In principle this is an version of an equalent to those dirt-cheap
graphical equaliser cursuits which uses inductor-capacitor-resistor BP
networks at the middle tap of the boost/cut slider/pot that we have
seen so much of...

 g>      One problem with building manually-tuneable state-variable filters is 
 g>      that you need to use dual pots for the frequency controls. One way 
 g>      around this is to use OTA's and tune them with a single CV. If you did 
 g>      that you'd have a voltage-controlled parametric EQ and you'll be 
 g>      tempted to add all kinds of CV inputs (sweep, spread, etc.) so beware.

Please tell me what the big fuzz about double pots is all about... I
can imagine diffrencies between the pots, but what else?
 g>      One other thing to note would be that each state-variable bandpass 
 g>      filter will have a non-controllable bandwidth (well it is affected by 
 g>      Q, but that's another complication). In order to have mid-bands with 
 g>      frequency *and* bandwidth adjustments, you'd need to make the band out 
 g>      of a pair of filters in series, one highpass and one lowpass, and as 
 g>      you can see the number of parts in this design is getting up there.

Hello! Stop it rigth there...

I used a 2-pole standard state-variable filter and let the BP output
be used for the summer, I could easilly vary the bandwidth using the
so called Q-pot. The fact is that the Q value is the inverse of the
bandwidth divided by the nominal centrum frequency...

The bandwidth is really set by the distance of the poles from the
jomega axis in the s-plane map.
 g>      Once you get this far it's just a little step to add an LFO to each 
 g>      band's CV input to make a vocal-effect animator.... ah sorry guys, I'm 
 g>      going a little overboard here.:)

Please do... but leave a normal parametric design behind or I will
have to do that ;-)


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