Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!ames!lll-winken!dalek!sjb
From: sjb@dalek.UUCP (Seth J. Bradley)
Newsgroups: sci.electronics
Subject: Re: subwoofer crossover ..... how ?
Message-ID: <441@dalek.UUCP>
Date: 19 Feb 89 16:48:03 GMT
References: <2413@uokmax.UUCP>
Reply-To: sjb@dalek.UUCP (Seth J. Bradley)
Distribution: usa
Organization: Bradley Marketing, San Jose, CA
Lines: 32

In article <2413@uokmax.UUCP> mcmiller@uokmax.UUCP (Michael C Miller) writes:
>Maybe this is a stupid question but here goes.... I want to build
>a sub-woofer for my stereo but I need some help designing the thing..
> I can handle the low pass filters, (they are built and work well ), but 
>I want to drive a single 15" speaker , not one per channel...
> Does anyone know of a passive summing circuit that will handle 120 watts
>per channel ? I've heard that tying the two filter outputs would be bad
>as one channel may try to "drive " the other and blow the amp.

You are correct in that you cannot just connect the two outputs together.
I only know of three methods to implement a mono subwoofer:

1.  Use an active crossover, sum the outputs and use a separate amp
for the subwoofer.  Not super cheap (you can get a decent amp for
a little over $200 and a crossover/bridge can be built for around
$60) but it is definitely the best.  You get many advantages, like;
sharper cut-off slopes, better damping, lower system I.M.  This is
what I use.

2.  Use a dual voice coil subwoofer.  Not much help if you already
have the woofer!  Most manufacturers use this technique, as it is
the cheapest.

3.  Use 1:1 isolation transformers between the outputs of the
crossovers and the woofer.  Audio grade isolation transformers
are VERY hard to find (I've never been able to find them) and
rather expensive.  Dahlquist made/makes? an outboard crossover/
summer that used this technique.  If you hunt around real hard
you might be able to find one used.
-- 
Seth J. Bradley     UUCP: uunet!{lll-winken|ubvax}!dalek!sjb
		Internet: lll-winken.llnl.gov!dalek!sjb