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These are different things. Wire size now becomes a function of
the geometry involved. For example, if all 100 loudspeakers are
connected up daisy-chain fashion in a continuous line, then 14.14
amps flows to the first speaker where only 0.1414 amps are used
to create the necessary 10 watts; from here 14.00 amps flows on
to the next speaker where another 0.1414 amps are used; then 13.86
amps continues on to the next loudspeaker, and so on, until the
final 0.1414 amps is delivered to the last speaker. Well, obviously
the wire size necessary to connect the last speaker doesn’t need
to be rated for 14.14 amps! For this example, the fanatical installer
would use a different wire size for each speaker, narrowing the
gauge as he went. And the problem gets ever more complicated if
the speakers are arranged in an array of, say, 10 x 10, for instance.
Luckily tables exist to make our lives easier. Some of the most
useful appear in Giddings{3}as Tables 14-1 and Table
14-2 on pp. 332-333. These provide cable lengths and gauges for
0.5 dB and 1.5 dB power loss, along with power, ohms, and current
info. Great book. Table 1 below reproduces much of Gidding’s Table
14-2{4}.
Rane Constant Voltage Transformers
Rane offers several models of constant-voltage transformers. The
design of each is a true transformer with separate primary and secondary
windings—not a single-winding autotransformer as is sometimes encountered.
The MA 6S transformers are sold individually, and are designed to
mount on a separate 3U rack space mounting panel sold separately.
For the U.S. market, a tapped secondary is provided offering a choice
of 70.7 V or 25 V output voltages.
The model TF 170 is rated 100 watts, while the model TF
370 is rated for 300 watts. For the European market a 100 watt,
100 V secondary is available as model TF 110. The mounting
kit for all these models is ordered as a KTM 6 and mounts
(6) 100 W or (3) 300 W or any combination thereof. See the KTM
6 data sheet (links to pdf file, file size 58 k) for more
information.
The MA 3 transformers are also sold individually and mount directly
inside the unit. The choices are TF 407 rated 40 W, 70.7
V and TF 410 rated 40 W, 100 V.
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References
{1} Langford-Smith, F., Ed. Radiotron Designer’s
Handbook, 4th Ed. (RCA, 1953), p. 21.2.
{2} Earley, Sheehan & Caloggero,
Eds. National Electrical Code Handbook, 5th Ed. (NFPA, 1999).
{3} See: Giddings, Phillip Audio
System Design and Installation (Sams, 1990) for an excellent
treatment of constant-voltage system designs criteria; also Davis,
D. & C. Sound System Engineering, 2nd Ed. (Sams, 1987) provides
a through treatment of the potential interface problems.
{4}Reproduced by permission of the
author and Howard W. Sams & Co.
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