trying to find a bandpass filter design article

[Chico]

I have been interested in filter design lately.  I built one gyrator based eq for guitar that turned out pretty good for what it is (a great resource for gyrator based graphic eq can be found at www.geofex.com ).  However, after working with gyrators for a while, their advantages and shortcomings become quite apparent.

In doing some research, I stumbled across a fascinating article
see

http://www.forsselltech.com/Evolution%20of%20an%20EQ%20Design2.pdf

This article suggests that a parametric EQ can be used to replace typical gyrator circuits where the parametric is composed of allpass filter sections arranged to form a bandpass filter.

[I know that there are other ways to build a parametric EQ- I am currently building a prototype based upon an article that I downloaded from RANE - The article was the subject of a previous post here]

However, this idea is something that I have never seen before.  As such, I quest to learn about it.

The allpass based bandpass is referred to as a CAPS - Constant Amplitude Phase Shift Circuit.

Apparently, there is a detailed design article by Steve Dove

Steve Dove, “Designing a professional mixing console, parts 7,8,9” Studio Sound Magazine, April, May, June, 1981.

However, after Googling around I have come to learn that this magazine has been out of print since 2001, and I cannot find any information on this article.

Any of you have a copy that they could share?  Anyone know about CAPS or where I can find design equations?

Thanks in advance for your help

Tom

[niftydog]

texas instruments web site (and other major semiconductor manufacturers) has a whole bunch of application notes about filters.

[Chico]

Niftydog:
thanks for the tip.  Unfortunately, I think that I have scoured all the obvious spots for related information.

The idea here is that two allpass filters and a buffer can be used to form a fully parametric filter.  One advantage of this "CAPS" filter is that it seamlessly integrates with the single amplifier topology common to most gyrator style eq circuits.  Thus, I thought that this would be a handy circuit to stuff into my toolbox.  

In playing with the circuit, I have learned a great deal and will continue to experiment with it.

Best regards

Tom