From: worley@a.cs.okstate.edu (WORLEY LAWRENCE JA)
Subject: Re: Request Subwoofer Advice
Organization: Oklahoma State University
Date: Mon, 4 Jan 93 17:54:21 GMT

From article <1993Jan3.210145.1@pearl.tufts.edu>, by edente@pearl.tufts.edu:
> I'd appreciate some advice on setting up my new Mirage PS12-90 Subwoofer.
> I'm using my Pioneer AV Receiver and a pair of KEF C95 main speakers.
> The best way for me to install the subwoofer, I beleive, would be to run a
> loop through my receiver's lowlevel inputs and outputs, preamp outputs of
> receiver to subwoofer, subwoofer to power amp inputs of receiver. No pro-

So are you running from the rcvr output 30' interconnect cables TO the sub,
and then another 30' interconnect BACK to the rcvr?  If so, then sure, I can
see how this would be a problem.  You might reconsider how you have the
interconnects hooked up.  I would suggest only using one cable to the sub, and
none back from it.  It is likely that the subwoofer circuitry is inducing some
hum on the low-level line back to the rcvr, so you should eliminate that loop.

> Instead of the above setup, I'm using the second choice - using the speaker
> inputs to interrupt the run from receiver to speakers.  So the run goes
> speaker outputs from receiver to speaker inputs on subwoofer to speaker
> inputs of main speakers.  This is ok, but have couple of other questions:

Sounds fine.

> The subwoofer (powered) has a low pass filter dial to adjust: 50Hz - 150Hz.
> I just can't hear the optimum setting myself. Should I just set it halfway?

The way we set x-over settings (I work at a car stereo shop (5 yrs)), is to
set the knob to the HIGHEST possible frequency that does not make the sub
sound "boomy" or "muddy".  Also, you want to eliminate as much mid-range
as possible from coming from the sub.  One way to do this is to disconnect
the main spks, adjust the x-over so that voices and musical instruments are
not highly audible.  Also, placement of a sub can make a BIG difference.  YOu
should experiment with where the sub is located.  Try corners, different 
distances from wall etc.

> The phase switch, set at 0 or 180 deg. is another aural setting I'm supposed
> to make, but I can't hear the difference.  I just have leaden ears, I guess.
> Should I just leave it at 0?  The I have not been able to reach the Mirage
> people to ask them.

Well, this is simple.  Maintain correct polarity from the rcvr spkr outputs
to the inputs of the sub, and to the main spkrs (+ and -).  Put on some music
with good LOW bass content.  Turn up the volume, and have a helper stand 
equidistant from the sub/main speakers.  Flip the phase switch back and forth.
Your helper should be able to easily tell which position has more bass (less
cancellation).

> Last question.  I know I'm supposed to use 10 or 8 guage w/ a run of this
> length (30 ft.), but I've been using 12 gauge lamp cord - Am I really
> missing much? 

It depends on the amount of power your amp is capable of producing, and how
loud you listen to it.  If it is say, over 75W / channel and you like to 
crank-it-up a lot, or if the music you listen to has a lot of bass content,
then you certainly could be losing some.  Remember that as your length of
run increases, it decreases your effective wire guage.  That is, a piece
of 12 guage 30' long can handle the same current (with same voltage loss) as
a piece of say 16 guage 1' long.  In your case, using 8 guage for the 30'
run may be equivalent to using 12 guage only 3' long.  Again, it depends
on the amount of power (current) you are running through the wires. 

> Oh, one final note - my KEF main spearkers are 4 ohm impedence.  Does this
> affect my subwoofer settings?

It shouldn't.  From the way you describe it, the spkr outputs just run
"through" the sub box, so it shouldn't affect it's performance at all
(assuming the sub box (which I assume is amplified) doesn't have an
effect on the overall impedance that the rcvr sees).

BTW, long spkr wire runs like you are describing increase the impedance
significantly, and small diameter(high guage) wire just makes it worse.

-Jason