WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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determined by the value of R201+R209 and C201, Q201 (2N494C) triggers a controlled rectifier,
Q203.
Charging resistor R201+R209 must be small enough to supply the minimum trigger current of
Q201 plus the leakage current of the capacitor when the emitter of the unijunction is biased at its
peak point voltage. This would place a limit on practical time delays of only 6 or so seconds. As
shown in the figure, a relaxation oscillator (Q202, 2N491) is included to reduce the minimum
trigger current requirement more than 1000 times by pulsing the upper base of Q202 with a 0.75
V pulse. The negative pulse causes the peak point voltage to drop slightly and if the voltage level
at C201 is greater than this, the unijunction will trigger. R202 is optimized for best stabilization
over the required temperature range. The maximum time delay that can be achieved by this circuit
is mainly dependent upon the maximum values that can be obtained for R201+R209 and C201
consistent with the low leakage requirement. Without diode D201, R201+R209 is limited to 15
megohms for an accuracy of 0.5% at 25 degrees C, but with D201, R201+R209 can be increased
considerably beyond that.
Transistors Q204 and Q205 comprise the voltage sensing circuit. The values of R214 and
R215 are determined by the operating voltage being measured. As a general guideline, R214 +
R215 = 800 × voltage being measured. The voltage sensing circuit is designed to latch on after the
proper voltage has been detected. Using the values of R214 and R215 shown in the parts list (240
k
Ω
total), the circuit will latch on at about 300 V dc.
Because the sensing circuits require time to settle (and the B+ voltages require time to build
up to operating value), a time delay is needed prior to enabling the protection devices. This delay
is accomplished by R211 and C207, which triggers SCR Q206 through zener diode D208. Using
the component values given in the parts list, a delay of approximately 30–60 seconds is provided.
The schematic diagram of the 20 W Stereo Amplifier shown in Figure 1.1 a illustrates the
power management system as a black box device with two input terminals and two output
terminals. The system is best thought of in this way. When all operating conditions are satisfied,
the power management PWB functions as a piece of wire connecting the ac input power to the
rest of the amplifier. When an out-of-tolerance condition is observed, or the time-out function is
initiated, the load is disconnected from the ac line input.
Strictly speaking, the power management system is not required for operation of the power
amplifier. The board can be wired-around for testing. A critical design requirement for any