Power Supply Design
John Linsley HoodActive systems such as audio amplifi ers operate by drawing current from some voltage
source—ideally with a fi xed and unvarying output—and transforming this into a variable
voltage output that can be made to perform some useful function, such as driving a
loudspeaker, or some further active or passive circuit arrangement. For most active
systems, the ideal supply voltage would be one having similar characteristics to a large
lead-acid battery: a constant voltage, zero voltage ripple, and a virtually unlimited ability
to supply current on demand. In reality, considerations of weight, bulk, and cost would
rule out any such Utopian solution and the power supply arrangements will be chosen,
with cost in mind, to match the requirements of the system they are intended to feed.
However, because the characteristics of the power supply used with an audio amplifi er
have a considerable infl uence on the performance of the amplifi er, this aspect of the
system is one that cannot be ignored.
5.1 High Power Systems .............................................................................................
In the early days of valve-operated audio systems, virtually all of the mains-powered
DC power supply arrangements were of the form shown in Figure 5.1(a) , and the only
real choice open to the designers was whether they used a directly heated rectifi er, such
as a 5U4, or an indirectly heated one, such as a 5V4 or a 5Z4. The indirectly heated
valve offered the practical advantage that the cathode of the rectifi er would heat up at
roughly the same rate as that of the other valves in the amplifi er so there would not be an
immediate switch-on no-load voltage surge of 1.4, the normal HT supply output voltage.
With a directly heated rectifi er, this voltage surge would always appear in the interval
between the rectifi er reaching its operating temperature, which might take only a few
seconds, and the 30 s or so that the rest of the valves in the system would need to come
CHAPTER 5