384 Chapter 13
13.1.3.1 Advantages
● The amplifi er PCB can be constructed so as to form a complete operational
unit that can be tested thoroughly before being fi xed into the chassis. This
makes testing much easier, as there is access from all sides; it also minimizes
the possibility of cosmetic damage (scratches, etc.) to the metalwork during
testing.
● It is impossible to connect the power devices wrongly, providing you get the right
devices in the right positions. This is important for such errors usually destroy
both output devices and cause other domino-effect faults that are very time-
consuming to correct.
● The output device connections can be very short. This seems to help stability of
the output stage against HF parasitic oscillations.13.1.3.2 Disadvantages
● If the output devices require frequent changing (which obviously indicates
something very wrong somewhere) then repeated resoldering will damage the
PCB tracks. However, if the worst happens, the damaged track can usually be
bridged out with short sections of wire so that the PCB need not be scrapped;
make sure this is possible.
● The output devices will probably get fairly hot, even if run well within their
ratings; a case temperature of 90°C is not unusual for a TO3 device. If the
mounting method does not have a degree of resilience, then thermal expansion
may set up stresses that push the pads off the PCB.
● Because the heat sink will be heavy, there must be a solid structural fi xing
between this and the PCB. Otherwise the assembly will fl ex when handled,
putting stress on soldered connections.13.1.4 Single- and Double-Sided Printed Circuit Boards
Because of their lower cost, single-sided PCBs are the usual choice for power amplifi ers;
however, the price differential between single- and double-sided plated-through-hole
(PTH) PCBs is much less than it used to be. It is not usually necessary to go double-sided
for reason of space or convoluted connectivity, as power amplifi er components tend to