Ecler MPA4-150, Service Manual

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52-0006-0101 EP05-99 anglès.doc 2 of 2
Moreover, the amplifier also includes an ANTICLIP system.
When the amplifier reaches its clipping level, the OpAmp becomes unable to keep the system
under control, and as a consequence ±V peaks appear at its output (15V power supply). This
peaks are used to be rectified and sent to an optocupler (led-resistor) which modifies its
impedance as a function of those peaks' amplitude. The resulting impedance is part of a voltage
divider, together with the amplifier's input impedance. So, as the optocoupler increases its
impedance, the amplifier's input signal level decreases until the system becomes stable.
Also a dual-function temperature control circuitry is provided:
- Temperature-depending control of the cooling fan speed, whose voltage supply is variable
between 7 and 14 Vac.
- Amplifier shutdown when temperature exceeds approximately 90ºC.
The circuitry is formed by LM35D-type IC, which acts like a thermal probe, an amplifier, thermal
probe level comparator and a 7805-type voltage regulator.
The amplifier is responsible for the cooling fan speed control. The comparator triggers a relay,
which cuts off the MOSFETs' bias current by shunting a 22
Ω
resistance to the BF-type
transistors' load resistors. This way, the output signal of the amplifier is effectively cutted off.
The STAND-BY circuit.
This circuit keeps the safety relay closed for about 10 seconds, thus the MOSFET's bias current
is cutted off during this period, until the whole system reaches again a voltage-stable situation.
Due to this, hearing annoying transients and noises during start up through the loudspeakers is
avoided. This delay is obtained by a RC-cell, where R=287K, and C=47µF/50V. During start up,
this RC-cell's voltage smoothly rises until the 40106-type Trigger-Schmitt trigging level is
reached, and the amplifier starts functioning. C=47µF resets or discharges when the unit is
turned off. During a short period of time, a BC817-type transistor acts like a switch, connecting
two 75
Ω
parallel resistors to C=47µ/50V.