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# Re: home stereo impedance matching

Be careful, by placing a 4 Ohm resistor in series, the
average power into the speakers for the same voltage
setting on the amp is reduced by a factor of roughly 4
(hard to say exactly because the impedence of the speakers
is a complicated fuction of frequency.) You may end up
turning up the the volume to compensate. This can end up
doubling the load on the output transistors because half
the power is wasted on the resistor. This is a safe
practive ONLY if you are sure that you will be playing at
low volume.

Bob at PC

On 2/17/2004 at 1:11 PM Shapiro, Mark wrote:

If you are going to drive the speakers at low power levels
compared t=
o
the rated power output of the amplifier, they can be
connected to the=
8
ohm output without too much of a problem.

If you are going to use more of the rated power of the
amplifier, the=
n
the load should be matched to the output impedance to
prevent
overheating the output transistors. Adding 4 ohm
resistors in series
will work provided that the resistors have an adequate
power rating.

Dr. Mark H. Shapiro
Professor of Physics, Emeritus
California State University, Fullerton
Phone: 714 278-3884
FAX: 714 278-5810
email: mshapiro@fullerton.edu
web: http://chaos.fullerton.edu/Shapiro.html
travel and family pictures:
http://community.webshots.com/user/mhshapiro=20
=20
=20

-----Original Message-----
=46rom: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu] On
Behalf Of Matt Harding
Sent: Tuesday, February 17, 2004 12:44 PM
To: PHYS-L@lists.nau.edu
Subject: home stereo impedance matching

I have a pair of old (relatively) Bose 501 speakers that
are marked a=
=3D
s 4
Ohms, that I would like to use with a new receiver that
only offers 8=
=3D
-16
ohm connections. From what I've been able to establish,
higher end
speakers tend to be of the 4 Ohm variety due to greater
power associa=
=3D
ted
with lower resistance. =3D20
What types of problems can I expect if I use 4 Ohm
speakers with my 8
Ohm system?
If my only problem will be reduced performance, then I'm
not terribly
concerned. If I will cause permanent damage to my new
=3D
nk
I'll leave the speakers in storage.
In the past, my father had wired a 4 Ohm resistor (rated
at 15 Watts)=
=3D
in
series with the speaker so that the amp would see 8 Ohms.
Is it that
easy?
Cheers,
Matt

"An expert is a man who has made all the mistakes which
=3D
n a
very narrow field."
=3D20
- Niels Bohr

-----Original Message-----
=3D46rom: Forum for Physics Educators
[mailto:PHYS-L@lists.nau.edu] O=
n
Behalf Of Justin Parke
Sent: Tuesday, February 17, 2004 1:54 PM
To: PHYS-L@lists.nau.edu
Subject: Re: rolling

I just discovered that the latest edition of Halliday,
Resnick, and
Walker has the correct answers (i.e. .5 L and L) in the
back of the
book!

Justin

In a message dated 2/4/2004 8:11:12 AM Eastern Standard
Time,
FIZIX29@AOL.COM writes:

I need some help with the following question from
Halliday et al
(6th=3D3D
ed.),
ch. 12 question 5:

"A woman rolls a cylindrical drum, by means of a board
on top,
throug=3D3D
h the
distance L/2, which is half the board's length. The
drum rolls
smoot=3D3D
hly, and
the board does not slide over the drum.
a) What length of board has rolled over the top of
the drum?
b) How far has the woman walked?"

The answers in the book are L and 1.5 L. I am not
sure I underst=
=3D
and
=3D3D
what is
meant by "what length of board has rolled over the top
of the dru=
=3D
m."
=3D3D
It seems
to me that it should be L/2. If the questions means
how
far has the =3D3D
board
moved with respect to the ground then I agree it is L.

Answers in algebraic form (plus verbal explanations)
are
preferable t=3D3D
o
strictly verbal arguments.

Justin

Justin Parke
Oakland Mills High School
Columbia, MD