JBL MPC - Manual (Page 16)

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Short-Circuit Protection—MPC amplifiers use the proven Output Averaging™ short circuit
protection system. The circuit permits full output current even into resistive or reactive 2 Ω
load, but reduces the current safely by about 75% if the output is shorted.
Transient Muting—Turn-on and turn-off muting keeps transients—both from the amplifier
itself and from upstream equipment—from reaching the speakers when the amplifier is
turned on or off. The turn-on delay is approximately 3 seconds to allow the power supplies
and circuitry to stabilize. Turn-off muting occurs almost immediately after power is shut off.
Muting occurs whether power is turned on and off using the front panel power switch or
externally at the AC source.
Inrush Protection—It is common in some amplifiers for a large surge of current from the
AC power line to occur in the first seconds after the amplifier is turned on, during which the
depleted power supply capacitors charge. This inrush current can trip circuit breakers—
severely limiting the number of amplifiers that can be switched on at the same time—and can
also damage the internal workings of the amplifier. To guard against this, models MPC300,
300T, 600, and 600T feature an internal NTC (negative temperature coefficient) thermistor in
series with the power switch and the transformer primary to limit inrush current. The
thermistor initially has a high resistance, which then diminishes rapidly as it warms to avoid
power loss. Typically, the inrush current of such an MPC amplifier is about what you could
expect from a typical amplifier of 1 /
3
to 1 /
2
the MPC’s power rating. The smaller models,
MPC200 & 200T, have low inrush currents and do not require an NTC thermistor.
DC Fault Protection—When the fault protection circuitry of an MPC300, 300T, 600, or
600T senses a DC voltage on a channel output, it actuates a relay that shorts the output to
ground to help protect the load against damage. (The amplifier’s high pass filter, described
below, will itself block DC coming into the amplifier inputs.) The amplifier’s Output Averag-
ing protection circuitry limits current to further protect the output devices. If the DC
problem persists, that channel will go into thermal protection mode and should be serviced by
a qualified technician. The output stage of models MPC200 and 200T has AC coupling to
prevent it from passing DC.
Thermal Protection—Temperature sensors on the channel heatsinks and on the power
transformers are part of the thermal management circuitry. The sensors govern whether the
dual-speed fan runs at low or high speed. Below 55°C the fan runs at low speed; above it runs
at high speed. Above 85°C, the channel mutes for thermal protection.
Infrasonic / Subsonic and Saturation Protection—Built-in second-order 50 Hz
(MPC200T) and third-order 45 Hz (MPC300T and 600T) Butterworth high-pass filtering
helps prevent saturation in speaker transformers by sharply reducing the amount of ultra-low
frequency energy the amplifier delivers. However, some lower-quality speaker transformers
have responses that do not extend that low; if you have any of these in your distributed line
system, you should insert a corresponding high-pass filter in the audio path before the
amplifier(s) unless you are also using subwoofers. All models have a subsonic filter to prevent
passing DC or excessive subsonic energy.
Glossary
Distributed output, isolated output, audio transformer output, and constant voltage output all refer to the
terminals labeled “AUDIO TRANSFORMER” on the shrouded output connectors of the
MPC200T, 300T, and 600T models. Various terms for 200V, 140V, 100V, 70V, 50V, or 25V
systems are constant voltage speaker system, distributed speaker system, or an isolated system (if using
full-isolation transformers in the amp—as do the MPC amplifiers—and on the loudspeakers).
Direct output and low impedance output refer to the output terminals that drive low impedance loads,
such as 16 Ω , 8 Ω , 4 Ω , and 2 Ω speakers.