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 90ºC.
Martin Audio – MA4.2 Power Amplifier
E
NGLISH
17
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