Audio Engineering

Power Supply Design 143

voltage. The main reason for this is that the stored energy in a capacitor is defi ned by the
relationship:

ECVc ^122

where E c is the stored energy, in joules; C is the capacitance, in farads; and V is the
applied voltage. This means that there is as much energy stored in an 8- μ F capacitor,
charged to 450 V, as there is in a 400- μ F capacitor charged only to 64 V. Because the
effectiveness of a decoupling capacitor in avoiding the transmission of supply line
rubbish, or a power supply reservoir capacitor in limiting the amount of ripple present
on the output of a simple transformer/rectifi er type of power supply, depends on the
stored charge in the capacitor, its effectiveness is very dependent on the applied voltage,
as is the discomfort of the electrical shock that the user would experience if he or she
inadvertently discharged such a charged capacitor through his or her body.

5.2 Solid-State Rectifi ers.............................................................................................

The advent of solid state rectifi ers—nowadays almost exclusively based on silicon bipolar
junction technology—effectively caused the demise of valve rectifi er systems, although
for a short period, prior to the general adoption of semiconductor rectifi ers, gas-fi lled
rectifi ers, such as the 0Z4, had been used, principally in car radios, in the interests of
greater circuit convenience because, in these valves, the cathode was heated by reverse
ionic bombardment so that no separate rectifi er heater supply was required. The diffi culties
caused by the use of these gas-fi lled rectifi ers were that they had a relatively short working
life and that they generated a lot of radio frequency (RF) noise. This RF noise arose
because of the very abrupt transition of the gas in the cathode/anode gap of the rectifi er
from a nonconducting to a conducting state. The very short duration high current spikes
this caused shock excited the secondary windings of the transformer—and all its associated
wiring interconnections—into bursts of RF oscillation, which caused a persistent 100- to
120-Hz rasping buzz called modulation hum to appear in the audio output.

The solution to this particular problem was the connection of a pair of capacitors, shown
as C1 and C2 in Figure 5.1(a) , across the transformer secondary windings to retune any
shock-excited RF oscillation into a lower and less invasive frequency band. Sometimes
these modulation hum prevention capacitors are placed across the rectifi ers or across
the mains transformer primary winding, but they are less effective in these positions.