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Path: utzoo!linus!decvax!harpo!eagle!mhuxi!houxm!whuxk!wjm
From: wjm@whuxk.UUCP
Newsgroups: net.audio
Subject: 75 Ohms at the amp ??
Message-ID: <284@whuxk.UUCP>
Date: Sat, 1-Oct-83 11:53:59 EDT
Article-I.D.: whuxk.284
Posted: Sat Oct  1 11:53:59 1983
Date-Received: Tue, 4-Oct-83 01:19:59 EDT
Lines: 25

There are several problems with Dave Martindale's approach of using 75 ohm coax
terminated with a 75 ohm resistor at the amplifier as speaker wire.  First,
(I'm assuming the resistor is in parallel with the amp output - if it were in
series almost all the power would go into the resistor rather than the speakers)
the coax is going to see the combination of the resistor and the amp's output
impedance (about 1/4 ohm for most modern solid-state amps) as its load and
that is about the same as the amp's output impedance - 1/4 ohm - so much for
an impedance match.  Second, the resistance per foot of RG-59U is about the
same as #24 wire, since the center conductor diameter is the same .02"
(RG-59 is the standard coax used for TV and FM tuner leadins and cable TV drops)
Admittedly, one could use RG-11U (the heavier stuff - about .4" in diameter)
but even that only has the resistance of #18 which is marginal for all but the
lowest power amplifiers with short speaker runs.  At audio frequencies, the
most important circuit parameter is the RESISTANCE of the wire, which is why
I use #14 SJ cord (available from your friendly hardware store or electrical
supply house) since I have fairly short speaker lines.  I agree with rabbit!jj
that #12 should be used on longer lines.
This is why the results of the "Stereo Review" speaker tests make sense.
The concept of putting a mono amp next to the speakers is not a bad idea, and
has been used by many manufacturers.  Its only problem is that you have to
run high impedance lines to the speakers, where the capacitance of the line
is more significant than with the low impedance power-amp speaker lines and
which are more susceptable to hum.
       Bill Mitchell
       (whuxk!wjm)