The cover of the amplifier houses the model MPPS-II 60/90 power pack. It provides a regulated

24 Vdc output at 1.8 amps maximum over an ac input voltage range from 38 Vac to 90 Vac rms.

The input voltage waveshape is either a squarewave or a sinewave. The surge protector and the

EMI filter protect the power pack and amplifier from transient spikes. An integral fast-acting

zener diode provides additional surge protection.

The preregulator is a fixed frequency switching regulator that presents a near perfect power

factor to the input line and provides overvoltage and overcurrent protection. A precision output

regulator provides a precise output voltage and is also protected against overcurrent and short

circuits. If a short circuit is detected, the power supply shuts down and the regulator initiates a

sequence of line test pulses at approximately one-half second intervals. The regulator continues

these pulses for the duration of the short circuit but returns to normal operation when the fault

is cleared. 


The MPPS-II also contains a two position LO/HI selector that sets the start-up voltage for 38 Vac

or 55 Vac. The unit is shipped with the selector in the LO position which is the standard

configuration. The selector should be switched to the HI position only for a 90 Vac system. This

sets the start-up voltage at 55 Vac. Because this is only 5 V below 60 Vac, it is not practical to

switch to HI in a 60 Vac system. There is no damage to the amplifier if the selector is not

changed from the standard LO setting. However, changing the selector ensures that the dc

supply does not turn on until the proper input voltage, 38 Vac or 55 Vac, is reached. This

prevents excessive loading of the system power supply during turn-on after a system shutdown.