WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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operation could result in damage to one or more devices. This circuit monitors the plate
voltage supply at power-up and shuts down the amplifier if the B+ voltage is below
normal.
These protection mechanisms are in addition to the soft-start circuit and line voltage circuit
breaker already provided on the amplifier. The power management board also serves as a power
supply (+35 V dc) for the preamplifier power supply board through a switched output from
RYL205 in the high voltage detection circuit of Q204/Q205.
The power management PWB mounts inside the chassis near the rear panel. In keeping with
the 1960s era technologies of the 20 W Stereo Amplifier, the circuit design is based on discrete
transistor circuits.
Operation of the protection circuit is quite simple. If an out-of-tolerance condition is detected,
the amplifier will be shut off. To reapply power to the amplifier, push the front panel power
switch off and then on again.
For users who want the ability to defeat the auto-shutdown feature, a toggle switch is provided
on the front panel. This switch as two positions— Auto Off / Normal .
A schematic diagram of the power management system is given in Figure 1.3. Relay-based
logic is used to latch power to the amplifier on and integrate the various protection mechanisms
into the control loop. The protection architecture will keep the amplifier in the off state until the
input ac power is cycled by operating the front panel power switch (off and then on) or by
unplugging and then reconnecting the ac power cord.
Varistors are included across the 120 V ac relays to eliminate transient disturbances. Diodes
are used across the coils of the 24 V dc relays to snub transient energy when the coil is de-
energized.
The time delay circuit consists of Q201 through Q203 and associated components
1
. The basic
circuit gives a time delay from a few seconds to several hours. With the circuit components
chosen for this implementation, a range of about one hour +/– 15 minutes can be selected through
setting of R209 (Delay Adjust). Clockwise rotation of R209 increases the delay time. The timing
interval is initiated by applying power to the circuit. At the end of the timing interval, which is
1. The circuit description is adapted from: GE Transistor Manual , the General Electric Company, Syracuse, NY, 1964.