WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

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20 W Stereo Audio Amplifier
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 power to the
preamplifier power supply PWB, the unit cannot be operated without the board functioning
properly.
1.2.1 Power Management Expansion Board
The power management expansion board adds additional functionality to the power management
system, specifically:
• The auto-off feature is modified to switch the amplifier off about one hour after no output
is detected at the speaker terminals. With the basic power management system, the auto-
off feature is initiated after a fixed time period, not based on whether audio is being
reproduced by the speakers. The expansion board changes that.
• The bias voltage is monitored to make certain it does not drop below –20 V dc. If the bias
voltage is too low, it can result in shortened tube life; in extreme cases, it can cause
component damage. The expansion board monitors the bias voltage and removes power
from the amplifier if the voltage is outside recommended operating limits.
• A chassis interlock is provided that will shut the amplifier down if the bottom plate is
removed. This feature can be defeated for servicing.
A schematic diagram of the power management expansion board is shown in Figure 1.4.
Samples of the right and left channel speaker terminals are taken from the speaker PWBs (at
the Head1 terminal). Each input is applied to a voltage divider with a trimmer potentiometer
feeding a two-stage buffer amplifier. The trimmer sets the sensitivity of the buffer stage. The
outputs of the right and left buffer amplifiers are rectified and applied to a voltage amplifier,
which drives relay RYL401. A filter capacitor is included in the base circuit of Q403 to average
the buffer amplifier outputs, and prevent rapid cycling of the relay. The normally-open relay
contacts are used to initiate a Timer Reset function on the power management PWB. In this way,
so long as audio is present at the speaker terminals, the Timer Reset contacts are held closed. This
prevents the auto-off system from timing out. However, once no output is detected at the speaker