388 Chapter 13
be connected to this track, but it seems to be permissible to connect input bias
resistors that pass only very small DC currents.
● Put the input transistors close together. The closer the temperature match, the
less the amplifi er output DC offset due to VBE mismatching. If they can both be
hidden fromseeing the infra-red radiation from the heat sink (e.g., by hiding them
behind a large electrolytic), then DC drift is reduced.
● Most power amplifi ers will have additional control circuitry for muting relays,
thermal protection, etc. Grounds from this must take a separate path back to the
reservoir ground, and not the audio starpoint.
● Unlike most audio boards, power amps will contain a mixture of sensitive
circuitry and a high-current power supply. Be careful to keep bridge rectifi er
connections and so on away from input circuitry.
● Mains/chassis ground will need to be connected to the power amplifi er at some
point. Do not do this at the transformer center tap as this is spaced away from the
input ground voltage by the return charging pulses and will create severe ground-
loop hum when the input ground is connected to mains ground through another
piece of equipment.Connecting mains ground to starpoint is better, as the charging pulses are excluded, but
the track resistance between input ground and star will carry any ground-loop currents
and induce a buzz.
Connecting mains ground to the input ground gives maximal immunity against ground loops.
● If capacitors are installed the wrong way round the results are likely to be
explosive. Make every possible effort to put all capacitors in the same orientation
to allow effi cient visual checking. Mark polarity clearly on the PCB, positioned
so it is still visible when the component is fi tted.
● Drivers and the bias generator are likely to be fi tted to small vertical heat sinks.
Try to position them so that the transistor numbers are visible.
● All transistor positions should have emitter, base, and collector or whatever
marked on the top print to aid fault fi nding. TO3 devices also need to be identifi ed
on the copper side, as any screen printing is covered up when the devices are
installed.