Protection Features - Martin Audio MA1.3 User Manual

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CONTENTS PRINT GUIDES
8

PROTECTION FEATURES

Each MARTIN AUDIO MA Series amplifier has many advanced protection
features that will protect both the amplifier and the speakers connected to
it, should a fault condition arise. Under normal use these features are
inaudible. All protection circuits are independent.
Output limiter
An output limiter is included to avoid amplifier clipping. When an amplifier is
severely overdriven, its output waveform is clipped (its peaks are squared off) -
reducing the crest factor. In extreme cases, the waveform can approach that of
a square wave. An amplifier is normally capable of producing far more power
under these conditions than its normal undistorted rated power output. The
limiter works by monitoring the output and comparing the distortion produced
between the input and output of the amplifier. If the distortion exceeds 1%THD
for any reason (voltage or current clipping), the limiter reduces the input signal
proportionally. Note that, if the signal is distorted or clipped before it reaches the
amplifier, the clip limiter will not be activated.
Under normal operation the clip limiting is inaudible. The limiter can be turned
On or Off by depressing the relevant clip limit switch (2). Some manufacturers
of loudspeaker processors do not recommend the use of clip limiters in
amplifiers, as they tend to upset the tracking of the processor's limiters.
Apart from this one exception, MARTIN AUDIO recommend leaving the clip
limiters switched "on" (button depressed).
As a by-product, when the amplifier comes out of a protect condition, the output
level has a slow rise time - the effect is like turning the gain up slowly.
Thermal protection
When the amplifier is driven very hard into a low impedance load, the
cooling fans will run at high speed. If these operating conditions continue,
the Protect indicator(s) will illuminate indicating that the amplifier is about to
go into thermal shutdown.
After five seconds the amplifier will go into thermal protect, by muting the
input signal. After 15-20 seconds the amplifier will have cooled down
enough for the amplifier to come out of shutdown and operate as normal. If
the load conditions remain unchanged the thermal protection will be re-
initiated. Thermal protection starts when the heatsink reaches a
˚
temperature above 90 C.
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.
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Martin Audio – MA1.3 Power Amplifier