Specifications

VHF protection

If a signal of more than 12kHz, at full power, is detected at the amplifier

outputs and lasts for more than five seconds, the VHF protection mutes the

input signal. (This is indicated on the front panel (3) labelled “Protect”). After

five seconds the outputs will un-mute and return to normal operation, unless

the output signal has remained unchanged in which case the VHF protection

will re-initiate. This circuit is inaudible in normal use, however it can be turned

off. If the VHF protection is required to be turned off i.e. for studio monitors,

please consult your supplier, as this is a non-user adjustment.

Short circuit protection

All MARTIN AUDIO MA Series amplifiers are completely short circuit

protected. The protection circuit permits very high peak currents, but still holds

the output devices within their safe operating area. If a short circuit is

maintained, the channel affected will eventually go into a thermal protect cycle

until the short circuit is removed.

Adaptive Fuse Saver, AFS™ limiter

The AFS™ limiter circuit continuously monitors the mains current. If the

average mains current exceeds 16 amps, as it can do during sustained high

power sine wave testing, the AFS™ limiter will restrict the mains current to a

safe level, consequently reducing the amplifier output.

If the AFS limiter circuit is activated, this will be indicated by the AFS LED on

the front panel. The AFS limiter will not activate during normal use, even

under sustained heavy clipping.

AC mains voltage protection

If the AC mains voltage is outside the operational window (over or under

voltage), the power supply will automatically shut down. When the mains

voltage is above the minimum start voltage and below its maximum operating

voltage the amplifier will restart.

It takes a couple of seconds to self check (this is known as the "soft start" or

"slow start" sequence), the gain will gradually be increased back to the

previous set level and the fans will blow at high speed before going into "idle".

The two green "ON" LED's then indicate that the power supply and output

stage is operating correctly.

D.C. protection

There are two types of DC protection:

• Fuses on the supply branches of each channel.

• A DC crowbar protection circuit that shorts the output if more than 10 volts is

detected on the outputs. Both these circuits come into effect once a DC

level is detected.

(See figure 14).

Martin Audio – MA4.2 Power Amplifier

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