154 Chapter 5
overcurrent (s/c) and thermal overload protection, coupled with a very high degree
of output voltage stability, and coupled with a typical 60-dB input/output line ripple
rejection. They are available most readily in 5 V and 15V/ 15V output voltages
because of the requirements of 5-V logic ICs and of IC op-amps, widely used in
preamplifi er circuits, for which 15-V supply rails are almost invariably specifi ed.
Indeed, the superlative performance of contemporary IC op-amps designed for use in
audio applications is such an attractive feature that most audio power amplifi ers are now
designed so that the maximum signal voltage required from the pre amp is within the
typical 9.5-V rms output voltage available from such IC op-amps.
Higher voltage regulator ICs, such as the LM337T and the LM317T, with output voltages
up to 37 and 37 V, respectively, and output currents up to 1.5 A are available but
where audio amplifi er designs require higher voltage-stabilized supply rails, the most
common approaches are either to extend the voltage and current capabilities of the
standard IC regulator by adding on suitable discrete component circuitry, as shown in
Figure 5.5 , or by assembling a complete discrete component regulator of the kind shown
in Figure 5.6.
In the circuit arrangement shown for a single channel in Figure 5.5 , a small-power
transistor, Q1, is used to reduce the 55- to 60-V output from the unregulated PSU to a
level that is within the permitted input voltage range for the 7815 voltage regulator IC
(IC2). This is one of a pair providing a 15-V DC supply for a preamplifi er. A similar
15-V regulator IC (IC1) has its input voltage reduced to the same level by the emitter–
follower Q4 and is used to drive a resistive load (R7) via the control transistor, Q5. If
the output voltage, and consequently the voltage at Q5 base, is too low, Q5 will conduct,
current will be drawn from the regulator IC (IC1), and, via Q4, from the base of the pass
transistor, Q2. This will increase the current through Q2 into the output load and will
increase the output voltage. If, however, the output voltage tends to rise to a higher level
than that set by RV1, Q5 will tend toward cutoff and the current drawn from Q2 base will
be reduced to restore the target output voltage level.
Overcurrent protection is provided by the transistor Q3, which monitors the voltage
developed across R4 and restricts the drive to Q2 if the output current is too high.
Safe operating area conformity is ensured by the resistor R3, which monitors the
voltage across the pass transistor and cuts off Q2 base current if this voltage becomes too
high.