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From: rdp@teddy.UUCP
Newsgroups: net.audio
Subject: Re: Crossovers - Throw the book away?
Message-ID: <1452@teddy.UUCP>
Date: Fri, 18-Oct-85 11:12:56 EDT
Article-I.D.: teddy.1452
Posted: Fri Oct 18 11:12:56 1985
Date-Received: Mon, 21-Oct-85 04:56:50 EDT
References: <808@houxa.UUCP>
Reply-To: rdp@teddy.UUCP (Richard D. Pierce)
Organization: GenRad, Inc., Concord, Mass.
Lines: 48

In article <808@houxa.UUCP> 593aac@houxa.UUCP (S.JOHNSON) writes:
>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.
>
In most cases, they don't work at all!

>
>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.
>
One of the most ignored characteristics of wide band drivers (woofer-midranges)
is the fact that the on-axis response oftyen shows a tendency to rise
with rising frequency, due to the fact that the radiation angle is reduced,
but show an integrated power response that drops with rising frequency.
A network can compensate for this, but the choice of response is a compromise.
Also (my favorite bitch), cones made from paper have the disconcerting
tendency to change these characteristics with changing humidity!

>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.

I just spent the last ten minutes searching my office for the notebook
holding about 100 AES and JAS reprints, and, to my anger, discovered
that someone has walked off with it. DAMN!

But, I will try to relate this as best I can from memory. In the late '70s
an article appeared in the Audio Engineering Society about inductors for
use in crossovers. In it the author, A. N. Thiele (yes, that Thiele) 
described a novel and simple method for designing and producing coils
for crossover use. The name of the article is "AIr Core Inductors for
Audio Use), or something very much like it. Without the article in hand,
it is difficult to summarize things, but the method described allows
accurate production of coils where all parameters (inductance, resistance,
etc.) are controllable as needed by the user. When I find the b*****d
who took my articles, I will post more information.

Dick Pierce