A Practical Primer on Series Crossover Design
by
Ken Perkins

There have been a lot of questions lately regarding how exactly to design a series crossover by ear from the curious on the MAD and PE message boards. More people are hearing about these an want to know what the deal is and how you come up with them. The typical response is to point them the the known websites or publications for perusal. In response to this I've decided to share with the group how I come up with mine. This is only my experience and opinion and is by no means intended to be the rule or to contradict anyone else or their experience with these networks.

The biggest confusion has seemed to come from the term "zeta". As John Kresovsky describes it in his excellent papers, zeta is simply a description of the damping of the circuit. Butterworth values in the circuit give zeta = 1. Zeta less than 1 is an underdamped circuit, with a slight peaking overlap at or around the crossover frequency. The circuit will typically be more capacitive as it uses a smaller choke and larger capacitor than straight Butterworth. Zeta greater than 1 is an overdamped circuit, with a slightly sagging underlap at or around the crossover frequency. It is more inductive than Butterworth, using a larger choke and smaller capacitor. As would be expected, the lower zeta values will give a slightly more than 6 dB/octave rolloff due to the peaking. A zeta of .5 gives a nearly 12 dB/octave rolloff. A zeta .7 is nearly 9 dB/octave rolloff and as zeta goes up, the rolloff becomes even more gradual. Thus the need for a really beefy tweeter and a woofer that can play cleanly at the top limit of its range for high zeta. In my opinion, zeta should not necessarily be the target for the design. It's the result. The crossover frequency you choose is the most important consideration. When the right blend of drivers and crossover have come together, you can then determine what the zeta is.

First, don't do what I did. Research your driver choices extensively. I already had a woofer and tweeter purchased, and as it turned out they were not compatible with a first order series crossover at the frequency I had targeted. This resulted in a nightmare of trial and error that got me nowhere and I almost gave up. Since I liked the woofer I had and the tweeter seemed to be the bottleneck, I began to search for another tweeter. That did the trick. What I recommend is, unless you have alot of different woofers and tweeters laying around to try, ask questions and get the impressions of others regarding what's out there. Study the measurements for the drivers, whether they're your measurements, someone elses or as a last resort the manufacturer's measurements. Ask for advice or help if you have to in order to find drivers with well behaved, extended frequency response.

Once you've determined the drivers you want to try, figure out the padding to level match the tweeter. This doesn't have to be exact as this point. You'll tweak this later. Also, design a zobel for the woofer to get it's inductance neutralized and rising impedance flattened. Now determine where to cross the drivers. My experience is that the typical, fullrange two way will need to be crossed between 1800 and 2500 hz for and 8"/1" combo and 2000 to 3500 hz for a 6"/1" pair.

Next, build a pair of test circuits at each end of the damping extremes you want to try. Andy's adjustable spreadsheet is very helpful here, if not absolutely necessary. I build a .7 and 1.2 zeta to start with. With these two circuits, you have a base to work with that lets you modify values without throwing off the crossover frequency alot. Remember that large changes in the cap value will mean a complementary change in the inductor value to maintain a given crossover frequency. If you increase the cap you'll need to decrease the choke and vis-versa. Large changes will affect the damping. Don't get to crazy with this if your amp is sensitive to highly capacitive or inductive loads. Mine obviously isn't either way as I got very "creative" at times and didn't blow anything up. This is the trial-and-error stage that can get pretty repetetive and tedious at times and takes, seemingly, forever. If you can model these with software, you'll be ahead of the game. In a given tweaking session, don't make alot of large changes. This will throw you off, bigtime. Make small changes, no more than a couple a night and listen. Sometimes for days if necesary before changing something. I've hit combinations of values that blew me away at first, but on exteneded listening turned out bad. Sometimes the changes will be very bad or good and very obvious, sometimes hardly noticable. Now fully tweak the padding resistor and when you hit on the final crossover you like, get Andy's spread sheet again an figure the zeta from that. That's really all there is to it. You have to listen and listen very closely to what you're doing. Software cannot determine the final crossover, only your ears can.

Here are some general rules to follow. Some are known already, some are my own insight from experience. I've played with, all together, four woofers and about five tweeters of varying quality and have determined it's the drivers and patience that make them work, not magic.
 

You'll know you're close to the right frequency when the image "locks in". I don't want to sound vague and I don't know how, exactly to describe it but you'll hear what I mean. It will literally float between and around the speakers when right.

I'm done. Have fun and happy listening! That's what this is all about...

(C) 2001 Ken Perkins, All rights reserved