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From: 593aac@houxa.UUCP (S.JOHNSON)
Newsgroups: net.audio
Subject: Crossovers - Throw the book away?
Message-ID: <808@houxa.UUCP>
Date: Wed, 16-Oct-85 17:14:34 EDT
Article-I.D.: houxa.808
Posted: Wed Oct 16 17:14:34 1985
Date-Received: Fri, 18-Oct-85 00:21:17 EDT
Organization: AT&T Bell Labs, Holmdel NJ
Lines: 52

I read with interest a recent posting about crossover design circa 1952.
Bob, you're certainly on the right track by using film caps instead of
electrolytics, but unless you're very lucky, the design right out of the
book is not going to work very well.

The problem is that the book usually makes some simplifying assumptions:
the drivers present constant, purely resistive loads, and that the
acoustic frequency response of the drivers is perfect. In addition, most
experts in the field consider that 18db per octave is the gentlest
crossover slope to use with practical drivers (12 db networks put a nasty
notch at the crossover freq. unless you reverse the tweeter's phase, and
then you get some rather strange phase effects!) (6db networks are
theoretically perfect for ideal, physically coincident drivers but
don't work so well with most (all?) real drivers).

So in the past, most designers have either just used the book values,
or have empirically come up with other designs. Neither is acceptable
from my view. What is the answer???

1) Make each driver look like a constant, resistive load to the
crossover. By the way, assuming that the impedance is 8 ohms 
because the manufacturer says so just won't cut it. It's almost
certainly 6.2 or 5.1 or 9.7, etc. For the most part the Zobel network will
do this! It involves a capacitor and resistor across the driver in the
case of woofers  to compensate for the voice coil inductance
( no attempt to compensate for the fundamental resonance)
and a slightly more complex network for mids and tweeters to compensate
for the motional impedance at resonance as well as the voice coil inductance.
Note that ferrofluid often damps the fundamental resonance enough to simplify
matching problems for mids and tweets.

2) Don't assume that the frequency response of the driver is perfectly flat.
Tweeters do tend to rolloff at 1 Khz or 2 Khz, etc. Add the acoustic
rolloff to the electrical rolloff of the crossover. For example if the
tweeter is rolling off at 12 db per octave at 2000, a 6db electrical
network at 2000 will yield an acoustic crossover of 18 db per octave.
The other approach is to use a sharp rolloff rate (18 or 24db) per octave
far from where the driver rolls off and ignore the driver's rolloff.

Using these techniques, book designs can be made to work. And very well
indeed!

One last point, don't wind your crossover coils with small gauge wire.
The resistance, in many cases, will degrade performance appreciably
by throwing off your design. I use 18 guage in small coils, and 10
guage in my largest coils. Film caps are better than non-polar
electrolytics, never try to make your own non-polars by wiring
standard electrolytics back to back.

		Steve Johnson

(The above opinions are definitely my own)