146 Chapter 5
In view of the normal lack in much pop music of any identifi able reference sound source,
such as would be provided by the orchestral or acoustic keyboard instruments in classical
music forms, a variety of descriptive terms has emerged to indicate the success or
otherwise of the amplifi er system in providing attractive reproduction of the music. Terms
such as “exciting,” “giving precise image location,” “vivid presence,” “having full sound
staging,” “blurred,” or transparent are colorful and widely used in performance reviews,
but they do not help the engineer in his attempts to approach more closely to an ideal
system performance—attempts that must rely on engineering intuition and trial and error.
5.5 High Current Power Supply Systems ....................................................................
In order for the power supply system to be able to provide high output currents for short
periods of time, the reservoir capacitor, C3 in Figure 5.1(b) , must be large and have a
low ESR value. Ideally, the rectifi er diodes used in the power supplies should have a
low conducting resistance, the mains transformer should have low resistance windings
and low leakage inductance, and all the associated wiring, including any PCB tracks,
should have the lowest practicable path resistance. The output current drawn from the
transformer secondary winding, to replace the charge lost from the reservoir capacitor
during the previous half cycle of discharge, occurs in brief, high current bursts in the
intervals between the points on the input voltage waveform labeled 1 and 2, 3 and 4, 5
and 6, and so on, shown in Figure 5.1(c). This leads to an output ripple pattern of the
kind shown in Figure 5.1(d). Unfortunately, all of the measures that the designer can
adopt to increase the peak DC output current capability of the power supply unit will
reduce the interval of time during which the reservoir capacitor is able to recharge. This
will increase the peak rectifi er/reservoir capacitor recharge current and will shorten the
duration of these high current pulses. This increases the transformer core losses, both the
transformer winding and the lamination noise, and also the stray magnetic fi eld radiated
from the transformer windings. All of these factors increase the mains hum background,
both electrical and acoustic, of the power supply unless steps are taken—in respect of the
physical layout, and the placing of interconnections—to minimize it. The main action
that can be taken is to provide a very large mains transformer, apparently excessively
generously rated in relation to the output power it has to supply, in order that it can cope
with the very high peak secondary current demand without mechanical hum or excessive
electromagnetic radiation. Needless to say, the mains transformer should be mounted as
far away as possible from regions of low signal level circuitry; its orientation should be