WhitakerAudio JC Whitaker 20 W Stereo Integrated Audio Amplifier, Manual

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20 W Stereo Integrated Audio Amplifier
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 trigger 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 Integrated 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
protective system is that it must be considerably more reliable than the circuits it is protecting.
Every effort has been made, through system design and component selection, to make certain this
is the case.
Note that because the power management PWB supplies V+ operating voltage to the
preamplifier board, the amplifier cannot be used without the board operating properly.
1.4 General Considerations
The front panel of the amplifier is shown in Figure 1.4. The back panel is shown in Figure 1.5.
This amplifier is complex and includes a number of components and sub-systems. In order to
simplify construction and maintenance, the following component numbering scheme is used: