Specifications

5
1 A
1 A
1 A
1 A
Amp: 800 watts
@ 0.08 ?Ω
8W
8W
8W
8W
8Ω
8v
100 A
Spkr 1
Spkr 2
Spkr 99
Spkr 100
Total load impedance 0.08≈ Ω
“Natural” voltages
Some power amps designed for powering direct low-impedance speaker loads have power ratings that make
them suitable for driving distributed lines, too. An 8-ohm load draws 625 watts at 70.7 volts, so an amp rated
for 600 to 650 watts into 8 ohms is often termed a “natural” for driving a 70-volt line. This relationship works
for other line voltages, too, although they are rare for 100 volts and higher:
25V 75–80 watts @ 8Ω 150–160 watts @ 4Ω 300–320 watts @ 2Ω
70V 600–650 watts @ 8Ω 1200–1300 watts @ 4Ω 2400–2600 watts @ 2Ω
100V 1200–1300 watts @ 8Ω 2400–2600 watts @ 4Ω 4800–5200 watts @ 2Ω
140V 2400–2600 watts @ 8Ω 4800–5200 watts @ 4Ω
200V 4800–5200 watts @ 8Ω 9600–10400 watts @ 4Ω
Transforming voltages and impedances
Imagine driving a system of 100 8-ohm speakers at a low power (say, 8 watts each) with a single amplifier, like
you might need to do in an office building’s paging system. How would you do it? Connect them all in parallel,
perhaps with 00 AWG cable to wire them all together, and find a power amp that can do 800 watts into 0.08
ohms, which comes out to 8 volts and 100 amperes?
That’s not practical, not the least because no such amplifier exists!
Or would you use a 70-volt amp with a rating of 800 watts or better, and put a transformer on each speaker to
provide the desired power level? Then the amp has to put out 70 volts at 11.4 amperes, for an equivalent load
of 6.13 ohms. That’s much more reasonable.
A speaker transformer steps the line voltage down to a lower level to drive the
speaker. In doing so, it also steps up the speaker impedance, so that the line itself
sees the speaker/
transformer combina-
tion as a relatively high
impedance.
For example, the
transformers in this
example have an
8.75:1 voltage step-down, converting 70 volts from the distributed line to 8
volts for the speaker. Into an 8-ohm speaker, that will produce 8 watts.
The ratio of the impedance step-up is equal to the square of the voltage ratio
in the other direction. Therefore, the 8-ohm impedance of the speaker driver
will be multiplied by a factor of 76.56, resulting in the line seeing a theoretical
impedance of 613 ohms. (The actual figure will be somewhat less because
of the transformer’s insertion loss.)
The importance of this phenomenon is that the high impedances allow you
to connect many speakers—25, 50, 100, etc.—in parallel on the line, which
you would not be able to do with speakers alone in a practical way.
1 A
800-watt
70V amp
8W
8W
8W
8W
8Ω
70v
8v
613Ω
70v
11.4 A
Spkr 1
Spkr 2
Spkr 99
Spkr 100
1 A
8W
8Ω
70v
8v
613Ω
Sp
Total load impedance 6.13≈ Ω