About The Design; Power Supply Theory Of Operation - DYNACO PWRAMP80 Assembly Manual

About the Design

Power Supply Theory of Operation

The power supply schematic is shown in Figure 15.
Stepped down AC voltage from the transformer enters on the left side of the schematic
through pins X1 and X2. These connect to D5-D8, which form a full-wave bridge that
turns the AC input into pulsating DC. C9, located externally to the power supply PCB,
filters the pulsating DC, turning it into VCC, a largely constant DC voltage. VCC is
nominally 72 Volts when the amplifier modules are connected but idle. VCC contains a
little bit of AC ripple, approximately 100 mV peak-to-peak (assumes 120 mA load
current and C9=10,000 µF).
C3 contains the high frequency switching currents from the diodes into a small loop that
doesn't include the power transformer, limiting interference into adjacent devices like
FM tuners.
R12 taps off some of the 72 volts and provides, with C11, filtered low voltages that run
the preamp section of the SCA80.
So, what does all the rest of the stuff in the power supply do? It controls the speaker
relays, hopefully in a way that eliminates pops and clicks on turn-on and turn-off. These
circuits have the following major parts:
1. Regulated Supply for the turn-on circuits
2. DC voltage good detection
3. AC voltage is present detection
4. Time Delay
5. Relay Driver
Regulated Supply
For the moment, consider the sub-set of the circuit shown in Figure 12.
R2 and D2 form a regulated 10V supply that makes turn-on operation timing consistent
and independent of the input voltage.
Time Delay and Relay Driver
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