WhitakerAudio 60 W Stereo Power Amplifier, Руководство пользователя и по сборке

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option, or reset the timer interval, two small (0.25-inch) holes will need to be drilled on one side
of the chassis.
In keeping with the 1960s era technologies of the 20 W, 40 W, and 60 W amplifiers, the
circuit design for the Automatic Protection Option board is adapted from classic circuits
contained in the 1964 editions of the RCA Transistor Manual and the GE Transistor Manual .
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.
For users who want the ability to defeat the auto shutdown feature, an optional toggle switch
is provided, typically mounted on one side of the chassis. This switch as two positions— auto off
enable / auto off disable . A separate push-button is provided to reset the time interval for the auto
shutdown system, if desired.
A schematic diagram of the Automatic Protection Option is given in Figure 1. 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). 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 approximately 5 hrs. With the circuit components
chosen for this implementation, a range of about one hour to three hours can be selected through
setting of R209 (Delay Adjust). The timing interval is initiated by applying power to the circuit.
At the end of the timing interval, which is determined by the value of R201+R209 and C201,
Q201 (2N494C) triggers a controlled rectifier, Q203.
Note that the nominal time delay interval is different between the 20 W version (four to six
hours) and 40/60 W version (one to three hours), the difference being the value of C201 (1,000
μ
F
in the 20 W version and 470
μ
F in the 40/60 W version). The assumption here is that the lower
power amplifier would normally be used for longer periods of time.
1. The circuit description is adapted from: GE Transistor Manual , the General Electric Company, Syracuse, NY, 1964.