Ecler MPA4-400, Service Manual, Page: 5 / 72

52-0017-0100 EP14-02.doc 2 of 2
Moreover, as the helper-transistor's base-emitter current is temperature-dependent, the
controlling circuitry (basically the control-transistor) compensates the safe operation area
(SOA) drift due to temperature.
If the MOSFET's drain-source voltage (Vds) drops too low, a second circuitry actuates to
alter the control-transistor's triggering level, obtaining a SOA-like curve section and a
current stage, which can be adjusted adequately in order to maintain the MOSFET's power
consumption as close as possible to its SOA.
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.