Noise and Grounding 383The PCB layout is only one component of this, as cross talk must be both emitted and
received. In general, the emission is greatest from internal wiring due to its length and
extent; wiring layout will probably be critical for best performance and needs to be
fi xed by cable ties, etc. The receiving end is probably the input and feedback circuitry
of the amplifi er, which will be fi xed on the PCB. Designing these sections for maximum
immunity is critical to good performance.
13.1.2 Rail Induction Distortion
The supply rails of a Class-B power-amp carry large and very distorted currents. As
outlined previously, if these are allowed to cross talk into the audio path by induction, the
distortion performance will be severely degraded. This applies to PCB conductors just
as much as cabling, and it is sadly true that it is easy to produce an amplifi er PCB that is
absolutely satisfactory in every respect but this one, and the only solution is another board
iteration. The effect can be completely prevented, but in the present state of knowledge I
cannot give detailed guidelines to suit every constructional topology. The best approach
is to minimize radiation from the supply rails by running theV and V rails as close
together as possible. Keep them away from the input stages of the amplifi er and the output
connections; the best method is to bring the rails up to the output stage from one side,
with the rest of the amplifi er on the other side. Then run tracks from the output to power
the rest of the amp; these carry no halfwave currents and should cause no problems.
Minimize pick-up of rail radiation by keeping the area of the input and feedback circuits
to a minimum. These form loops with the audio ground and these loops must be as small
in area as possible. This can often best be done by straddling the feedback and input
networks across the audio ground track, which is taken across the center of the PCB from
input ground to output ground.
Induction of distortion can also occur into the output and output-ground cabling, and even
the output inductor. The latter presents a problem as it is usually diffi cult to change its
orientation without a PCB update.
13.1.3 Mounting of Output Devices
The most important decision is whether to mount the power output devices directly on the
main amplifi er PCB. There are strong arguments for doing so, but it is not always the best
choice.