Audio Engineering

398 Chapter 13

C–F–G–D as before. Now, however, the internal design of unit 2 is such that the ground
current fl owing through F–G also fl ows through G–G  before it encounters the ground
wire going to point D. G–G  is almost certain to be a PCB track with higher resistance
than any of the cabling and so the voltage drop across it can be relatively large and the
hum performance correspondingly poor. Exactly similar effects can occur at signal
outputs; in this case the ground current is fl owing through F–F .

Balanced inputs will have no effect on this; they can cancel out the voltage drop along
F–G, but if internal hum is introduced further down the internal signal path, there is
nothing they can do about it.

The correct method of handling this is shown in Figure 13.6. The connection to mains
ground is made right where the signal grounds leave and enter the units and are made as
solidly as possible. The ground current no longer fl ows through the internal circuitry. It
does, however, still fl ow through the interconnection at F–G, so either a balanced input or
a ground-canceling output will be required to deal with this.

13.3.5 Balanced Mains Power

There has been speculation in recent times as to whether a balanced mains supply is a
good idea. This means that instead of live and neutral (230 and 0 V) you have live and the
other live (115 V–0–115 V) created by a center-tapped transformer with the tap connected
to neutral (see Figure 13.7 ).

Unit 1

Chassis

L

D

N

E

C

Audio cable

Unit 2

Chassis

F

B

G

Figure 13.6 : The correct method of dealing with ground currents; they are diverted away

from internal circuitry.