TOA a-503a Supplementary Manual
17
TOA Electronics Amplifier Guide
Troubleshooting 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.
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.
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.
Table 1: Power Tap to Impedance Conversion
Power Tap
Impedance (Ohms)
(Watts)
25 V
70 V
0.25
2500
20000
0.5
1250
10000
1
625
5000
2
313
2500
3
208
1667
4
156
1250
5
125
1000
8
78
625
10
63
500
12
52
417
15
42
333
20
31
250
30
21
167
60
10
83
75
8
67
100
6
50
120
5
42
150
4
33
180
3.5
28
200
3.1
25
220
2.8
23
300
2.1
17
400
1.6
13