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From: sjc@mordor.UUCP (Steve Correll)
Newsgroups: net.audio
Subject: Re: subwoofers and xovers
Message-ID: <3852@mordor.UUCP>
Date: Wed, 9-Oct-85 14:04:33 EDT
Article-I.D.: mordor.3852
Posted: Wed Oct  9 14:04:33 1985
Date-Received: Thu, 10-Oct-85 13:38:17 EDT
References: <1395@teddy.UUCP>
Distribution: na
Organization: S-1 Project, LLNL
Lines: 34

> > I would like to build a sub-woofer for my stereo system.  I know how
> > to make crossover networks for regular speakers, but I can`t figure
> > out how to make one work for a sub-woofer.  The catch is that I want
> > bass from both right and left channels to driver the sub-woofer. 
> > I can use caps to eliminate the low frequencies sent to my
> > regular speakers and chokes to eliminate the high frequencies for
> > the sub-woofer, but how do I combine the left and right low
> > frequency signals so they can be sent to one speaker? 
> > 
> >						 Thanks,
> >						Walt Barnes
> 
> Making a passive crossover network for subwoofer type frequencies 
> (50Hz - 250Hz) is a real bitch. The component values needed are
> astronomical. We need (say for a 200Hz cutoff frequency) capacitances
> on the order of 200 uFd and inductances in the neighborhood of 6 mH...

I second rdp's sensible observations on the difficulty of passive
crossovers for low audio frequencies. You might instead consider
building a little op-amp circuit to drive a mono amplifier to drive the
subwoofer.  One op-amp can easily sum the left and right outputs of a
preamp (or, with appropriate voltage dividers, the left and right
speaker outputs of a receiver) and two more op-amps can low-pass filter
the result.  The money you save by buying a single woofer (as opposed
to a dual-voice-coil woofer or a pair of woofers) will help pay for the
mono amp.

The National Semiconductor Audio/Radio Handbook (contact National Semi
at (408) 737-5000 or 2900 Semiconductor Drive, Santa Clara, CA 95051)
tells how to build active crossovers (page 5-1 of the 1980 edition) and
summing amplifiers (page 6-12).
-- 
                                                           --Steve Correll
sjc@s1-c.ARPA, ...!decvax!decwrl!mordor!sjc, or ...!ucbvax!dual!mordor!sjc