Transformer Isolation; Speaker Level Wiring; Minimizing Line Loss - Toa A-503A Manual

TOA Electronics Amplifier Guide
Balanced circuits also protect the system against noise from ground loops. This is because the signal
carried on the balanced pair represents a complete,"floating" or independent circuit, and is not con-
nected to ground as a reference.

Transformer Isolation

Another way of protecting against ground loops is to use a transformer at one or both ends of the line.
The transformer works by converting the signal from electric energy into magnetic energy, then back
to electric energy. Since it is not a direct electrical connection, the transformer does not complete the
circuit that would create the ground loop. But it still passes the audio signal unchanged. Low-cost
transformers should be avoided, since they can add distortion and limit frequency response. But good
quality transformers have a transparent audio quality and can give a high degree of assurance that
ground loops will not occur. In balanced applications, where the floating circuit already protects
against ground loops, the transformer adds protection against equipment failure that could occur if
one side of the audio pair were shorted to ground. Here again, because it is not a direct connection,
the transformer does not complete the circuit, and the output stage is protected. This is an important
benefit in high powered speaker applications.

Speaker Level Wiring

Noise pickup is not usually a problem for speaker cables, because the voltages used to drive speakers
are much greater than the voltage levels of induced noises. The main concerns for speaker wiring are
adequate durability for the installation environment, adequate spacing from mic- and line-level
wiring to avoid feed-back loops (do not put speaker and mic lines in same conduit), and adequate
wire size to minimize line loss.

Minimizing Line Loss

Line loss occurs in speaker wiring in two ways, both related to the resistance of the wire. First, the wire
will dissipate some of the power as heat. This power is wasted. Second, the wire will increase the total
line resistance, causing the line to draw less power from the amp. This power is not wasted, but is just
unused. Either way, it is best to keep line losses down to a minimum — preferably less than 1 dB.
One of the great benefits of 70.7 volt distributed line systems is that they are not affected by losses
due to speaker line resistance to the same degree that low impedance or 25 volt line systems are. In
most typical installations, if 18 gauge speaker wire is used, line loss will be less than 1 dB. If the total
speaker load on the line is greater than 120 watts, or if the cable runs exceed 200 feet, consider using
heavier gauge wire, as indicated in Appendix A, Table 1. Line losses are greater in 25 volt line systems.
Appendix A, Table 2 shows the wire size to use for a given load and distance on a 25 volt line. An 8
ohm load will be very susceptible to line losses when the cable length exceeds about 100 feet.
16