In Part 1, I provided an overview of my MB1 amplifier and discussed its audio path in detail. However, there is more to a successful power amplifier than just the audio path design. Proper power supplies are required and there should be a monitoring/protection function to prevent, or at least minimize, damage to the amplifier and speaker in the event of abuse or component failure. The complete power amplifier is a system. Good system design involves making design trades just as is done with the basic amplifier design, but at a higher level. The mechanical design also provides a contribution to success. Let's start here with the power.
AC Switch
AC power enters the chassis through an IEC inlet filter mounted on the rear panel. The hot side is protected by a 5A panel-mounted slow-blow fuse. On/Standby switching is provided by the AC Switch circuit card assembly (CCA) (Figure 6). It contains two high-current (16A) power relays controlled by the Low Voltage Regulator/Protection CCA.
When 12V is applied to the RLY1 inputs, the output of timer U1 goes High, pulling in RLY1. This connects 120V AC to the main power transformer though an off-card 12Ω, 50W power resistor. The resistor reduces the initial turn-on surge to 10A rms maximum. After a delay of ~320ms, 12V is applied to the RLY2 inputs. RLY2 pulls in and bypasses the series resistor. There is a secondary surge but it is less than the initial one. After another ~330 ms, U1 times out and its output goes Low. RLY1 drops out, disconnecting the resistor as it is no longer needed. This reduces power dissipation in the 12V regulator and protects the power resistor in the event of a failure of RLY2 or its driver circuitry. R1/C1 and R2/C2 are snubber networks to reduce contact arcing and electromagnetic interference (EMI) emissions. VAR1 is a surge protector.
High Voltage Power Supplies
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