Noise and Grounding 391● Unwanted AC currents entering the amplifi er on the signal ground, due to
external ground loops, must be diverted away from the critical signal
grounds, that is, the input ground and the ground for the feedback arm. Any
voltage difference between these last two grounds appears directly in the
output.● Charging currents for the power supply unit (PSU) reservoir capacitors must be
kept out of all other grounds.Ground is the point of reference for all signals, and it is vital that it is made solid and kept
clean; every ground track and wire must be treated as a resistance across which signal
currents will cause unwanted voltage drops. The best method is to keep ground currents
apart by means of a suitable connection topology, such as a separate ground return to
the starpoint for the local HT decoupling, but when this is not practical it is necessary
to make every ground track as thick as possible and fattened up with copper at every
possible point. It is vital that the ground path has no necks or narrow sections, as it is no
stronger than the weakest part. If the ground path changes board side then a single via
hole may be insuffi cient, and several should be connected in parallel. Some CAD systems
make this diffi cult, but there is usually a way to fool them.
Power amplifi ers rarely use double-insulated construction and so the chassis and all
metalwork must be grounded permanently and solidly for safety. One result of permanent
chassis grounding is that an amplifi er with unbalanced inputs may appear susceptible to
ground loops. One solution is to connect audio ground to chassis only through a 10- Ω
resistor, which is large enough to prevent loop currents becoming signifi cant. This is not
very satisfactory as:
● The audio system as a whole may not be grounded solidly.
● If the resistor is burnt out due to misconnected speaker outputs, the audio
circuitry is fl oating and could become a safety hazard.
● The RF rejection of the power amplifi er is likely to be degraded. A 100-nF
capacitor across the resistor may help.A better approach is to put the audio-chassis ground connection at the input connector so
that in Figure 13.1 , ground-loop currents must fl ow through A–B to the protected earth at
B and then to mains ground via B–C. They cannot fl ow through the audio path E–F. This