USB-80 Mixer Amplifier
Owners Manual
7
70V Distributed Speakers
These high impedance speakers are typically used in application requiring a large number of speakers, or
when extremely long speaker wire runs are necessary. In this application, all speakers are connected in
parallel, and may be wired in a "daisy chain" configuration. It is not necessary to "home run" wire from
each speaker back to the amplifier. Shown below is a very typical arrangement of (14) 70V speakers
connected to the USB-80 amplifier.
Typically, speakers having attached 70V transformers will have multiple wattage taps available for
connection. For example, the very popular MCM Custom Audio ceiling speaker,
Model #555-9018
, includes
taps for 0.325W, 0.625W, 1.25W, 2.5W, 5W and 10W. The popular outdoor speaker
Model #80-492
includes a rotary selector for selection of 5W, 10W, 20W and 30W.
In the above case, any number of speakers can be connected to a 70V amplifier output, as long as the
total wattage of all speaker taps does not exceed the total RMS wattage rating of the amplifier. For
example, if tapped at 1.25 watts, a total of (32) Model #555-9018 ceiling speakers could be connected to
the 555-9750 amplifier. If tapped at 5W, a total of (8) Model #80-492 outdoor speakers could be
connected.
A second benefit of this type of arrangement is the ability to vary sound level in different areas. If higher
volume is desired in some areas than others, simply select a higher tap in those areas. This is completely
suitable as long as the total of all speaker taps does not exceed the rated output of the amplifier.
As mentioned above, these speakers are much higher impedance than conventional consumer high fidelity
speaker systems. For example, a 70V speaker, tapped at 1W will have a nominal impedance of 5,000
Ω
. If
tapped at 5W, the same speaker would have an impedance of 1,000
Ω
. Because of this, loss from long
speaker wire runs is greatly reduced.
70V Distributed Speakers (cont)
The following chart shows typical maximum lengths allowable in 70V systems
Output power per wire run
Wire
Gauge
5W
10W
15W
30W
50W
100W
200W
16
10,000 7,000 4,600 2,300 1,400 700
350
18
9,000 4,500 2,800 1,400 830
415
205
20 5,500
2,700
1,800
900
540
270 135
22 3,400
1,700
1,100
550
330
115 60
24 2,100
1,000
700
350
210
105 50
Maximum distance of cable run (ft.)
For example, it is desired to locate a 10W paging horn some distance from an office in a large warehouse.
Considering wiring requirements, the length of the speaker cable will need to be approximately 4,000 ft.
As shown above, 18 gauge wire will be suitable for this installation.
