Newsgroups: rec.audio.car

Jason Reiser gives a good explaination:
>cr415@cleveland.Freenet.Edu (Adam S. Wang) writes:
>>  
>> I just bought a Rockford Fosgate Series 1 260sd amplifier.

deleted.

>> 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.  
>
Thanks.  I tried going to half circuits myself to answer the question and
couldn't figure out where all the numbers in the question were coming
from (good question, BTW).
  
>> 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).  
>
>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.
>
>\ Jason A. Reiser  \ Send E-Mail to jreiser@scott.skidmore.edu \
>----------

This may help me solve the "problem" of having my rear speakers about 2
db more efficient than my fronts (problem in that I have to use the fader
to direct more power to the fronts).  From what you are saying, I could add
a 4 Ohm sub to my rear which will lower the overall efficiency of the amp
which is really what I want.  My only major concern is the sub could lower the
supply voltage some and show up as added distortion in the rear speakers.
Since the better speakers are in the front with my set-up, this MAY be ok.

I wonder if the efficiencey drop is linear?  Hummm.  Probably not, the
upper limit could be set by the supply capacitors and/or current
limiting.  Also, I wonder how one goes about matching sub efficiency
with a given speaker...Probably with adjusting the gain of the amp which
you can't do with mixed mono or adding series resistance to a speaker (a
bad idea).

Ramble, ramble......Probably get the "best sound" by sticking with one
amp (or half of a 260sd or 1/4 of a 460) per speaker set.
Correct?

Kirk out 
Newsgroups: rec.audio.car