From: jreiser@scott.skidmore.edu (Jason Reiser... Asleep)
Subject: Re: Bridging Amps
Organization: Skidmore College, Saratoga Springs NY
Date: Mon, 25 Jan 1993 19:58:10 GMT

cr415@cleveland.Freenet.Edu (Adam S. Wang) writes:
> 
> 
>  
> I just bought a Rockford Fosgate Series 1 260sd amplifier.
> It is rated at 30 watts X 2 into 4 ohms per channel, or it
> is bridgeable to 120 watts into 2 ohms (stable).  The
> salesman told me that I can hook it up to two in-dash 4-ohm
> speakers and then bridge the output to my new Rockford
> Fosgate 4 ohm subwoofer tube in the back to get something
> like 120 watts into a 2 ohm load.  This sounded strage to
> me.  Can anyone explain exactly how much power I'm getting
> into which speakers, and what kind of impedance I'm getting?

Okay, lets start at start.  First of all, this amp is rated as you
stated as 30 watts into two channels at 4 ohms.  It is 2 ohm stable in
this stereo mode, and under perfect conditions, it could theoretically
put out 60 watts per channel into 2 ohms... expect something under
that though as the amp is not 100% efficient in this respect.  Now, if
we were to bridge the amp to run in true one channel mono, we must add
the watt/channel rating of both channels, and also add the impedance
ratings.  Thus, we have an amp that will put out 60 watts (30+30) into
8 ohms (4+4).  The amp will now only be FOUR ohm stable in this mono
configuration, NOT 2 ohms!  Any 2 channel amp that is x ohm stable in
stereo will be 2x ohm stable in mono.  
  
> Suppose the left-front speaker has leads A(+) and B(-), and
> the right-front speaker has leads C(+) and D(-).  The 
> salesman told me to connect the subwoofer leads to A and D.

What you are describing here is refered to as mixed mono
configuration, or often called 3-channel mode (a misnomer, but that's
okay).  

> 1.  How much power is going to the front left speaker?
> 2.  How much power is going to the subwoofer?
> 3.  What is the impedance of the left (or right)  channel at 
>     the amplifier?

The idea of mixed mono is that you will get the full rated 4 ohm
stereo rated power to a pair of full range speakers, and ALSO be able
to run a subwoofer in mono by sharing power from both channels as you
correctly described above.  This should get you 30x2 to the full range
speakers, and another 60 watts (or a number approaching this figure)
going to the subwoofer (assuming that the sub is 4 ohms).  Generally,
you are limited to 4 ohm minimum impedance on the main speakers, and 4
ohms on the sub as well.  These loads can, of course, be achieved with
multiple drivers as well, if so desired.  For example, instead of a
single 4 ohm sub, use a pair of 8 ohm drivers in parallel with each
other.  etc.

Hope that helps you out, Adam.

				- Jason
-- 
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\ Jason A. Reiser  \ Send E-Mail to jreiser@scott.skidmore.edu \
 \ Skidmore College \ Saratoga Springs, NY 12866 \ 518-581-6580 \  
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From: kirk@hpcc01.corp.hp.com (Kirk Lindstrom)
Date: Wed, 20 Jan 1993 21:04:46 GMT
Subject: Re: Suggestions on 6x9's
Organization: Shredding the water of SF Bay, HP-OCD