Troubleshooting Guide; Load Troubleshooting; Power Tap To Impedance Conversion Chart - Toa A-503A Manual

TOA Electronics Amplifier Guide

Load Troubleshooting

Shorted speaker lines and mis-matched loads are among the most common causes of sound system
failure. Being attentive to the condition, configuration and installation of the speakers and wiring are
the first line of defense against these common problems. But alas, the best laid plans do sometimes
go awry, and when this happens, the installer/troubleshooter's best friend is a speaker line impedance
meter such as the TOA ZM-104. Mastering this relatively simple measuring device can save hours of
valuable field service time per job when tracking down existing problems, and most importantly, can
help avoid call-backs by identifying mis-matched loads before the system is ever turned on.
When installing a system, it is prudent to check each branch line with the meter before bringing them
together at the amplifier's output terminals. A final test of the impedance of the full load should be
made before connecting it to the amplifier. If the system is already in place and load problems are
suspected, the process is reversed: First, check the load at the amp. If the impedance is below the
amplifier's rated impedance (or the effective power tap total is above the amplifier's rated power out-
put), then check each branch line to see which one (or more) has a lower impedance than it should.
Keep tracing this path, following the lowest impedance (or the impedance farthest below its expect-
ed value), until you find the culprit. This may be either an improperly tapped speaker/ transformer, a
speaker without a transformer, a shorted line, or even a shorted speaker voice coil or transformer.

Troubleshooting Guide

Power Tap
(Watts)
0.25
0.5
1
2
3
4
5
8
10
12
15
20
30
60
75
100
120
150
180
200
220
300
400
Table 1: Power Tap to Impedance Conversion
Impedance (Ohms)
25 V 70 V
2500 20000
1250 10000
625 5000
313 2500
208 1667
156 1250
125 1000
78 625
63 500
52 417
42 333
31 250
21 167
10 83
8 67
6 50
5 42
4 33
3.5 28
3.1 25
2.8 23
2.1 17
1.6 13
17