author: Queralt date: 000719
approved:
num:
52.0006 version: 01.00
FUNCTIONING DESCRIPTION
title:
EP05-99
E
C
LER
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 22Ω 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 75Ω parallel
resistors to C=47µ/50V.
Sheet 2 of 2
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