404 Chapter 13
or cabling to the power transistors if these are mounted off board. For this reason, the
transformer should be kept physically as far away as possible from even the high-current
section of the amplifi er PCB.
As usual with toroids, ensure that the bolt through the middle cannot form a shorted turn
by contacting the chassis in two places.
13.5.4 Wiring Layout
There are several important points about the wiring for any power amplifi er:
● Keep the and HT supply wires to the amplifi ers close together. This
minimizes the generation of distorted magnetic fi elds that may otherwise couple
into the signal wiring and degrade linearity. Sometimes it seems more effective
to include the 0-V line in this cable run; if so, it should be tightly braided to
keep the wires in close proximity. For the same reason, if the power transistors
are mounted off the PCB, the cabling to each device should be confi gured to
minimize loop formation.
● The rectifi er connections should go directly to the reservoir capacitor terminals
and then away again to the amplifi ers. Common impedance in these connections
superimposes charging pulses on the rail ripple waveform, which may degrade
amplifi er PSRR.
● Do not use the actual connection between the two reservoir capacitors as any
form of starpoint. It carries heavy capacitor-charging pulses that generate a
signifi cant voltage drop even if thick wire is used. As Figure 13.1 shows, the
starpoint is teed off from this connection. This is a starpoint only insofar as the
amplifi er ground connections split off from here, so do not connect the input
grounds to it, as distortion performance will suffer.13.5.5 Semiconductor Installation
● Driver transistor installation. These are usually mounted onto separate heat sinks
that are light enough to be soldered into the PCB without further fi xing. Silicone
thermal washers ensure good thermal contact, and spring clips are used to hold
the package fi rmly against the sink. Electrical isolation between device and heat