(equivalent to about 30Watts rms into 8 ohms) then the lifters (fed from Vcc
and Vee) are progressively powered on to maintain a constant 5V or so
across the collector-emitter junctions of the output transistors.
Note that on L125 the lifter outputs are shared between all three power
amplifiers for reasons of economy and space – whereas the lifters on the
other 4 channels on L129 are only shared between two power amplifiers
each.
Since the worst thermal stress on the lifters occurs at output powers
somewhat above 30W rms equivalent into 8 ohms, we do not recommend
testing the amplifier for extended times with continuous signals in the 30 - 50
Watts range with all three channels (C, SL and SR) running simultaneously
and driving into low impedances. This is especially true of square wave
signals
! The thermal sensor IC400 is located close to the lifter MOSFETs in
order to monitor this condition.
The power transistors TR406A and TR409A have built in thermal
compensation diodes (TR406B and TR409B) which form part of the biasing
network. The thermal sense biasing transistors TR401 and TR416 are thus
mounted adjacent to (and ideally in intimate contact with) the driver transistors
TR403 and TR414 so that as they warm up the bias remains relatively stable.
(In practice it rises somewhat, but predictably so, as the drivers warm up).
The pre-drivers, TR400 and TR415, are in a DC feedback loop with TR401
and TR416, so no thermal drift in bias occurs from these. Bias is set by
RV400; D403 and R421 ensure that no catastrophic increase in bias will take
place if RV400 fails open circuit.
Optimum bias at quiescent operating temperature is measured across the two
0.1 ohm emitter resistors forming R408 – it is typically 15mV at the pins of
C400. When setting up from cold a good starting point to achieve this is to first
turn RV400 to minimum (i.e. fully anticlockwise) and then slowly turn it up to
6mV.
The transistors TR405A and B and their associated networks provide
comprehensive two slope safe operating area (SOA) protection for the output
devices. When the prescribed combination of voltage and current across
TR406A or TR409A is exceeded, the relevant protection transistor conducts
and shorts out the base drive to the associated pre-driver, thus limiting the
dissipation in the output device. The output of each channel of the LM4702 is
current limited to about 5-10mA so no damage can occur to it under these
conditions. Note that if an output device fails short circuit then the 7 Amp fuse
in its collector will fail – the driver transistors TR403 and TR414 will then try to
take over and so are protected by 1 ohm fusible resistors in their collectors.
Note that the heatsinks of the drivers TR403 and TR414 are “live” – an
accidental short to ground here with a test probe will probably blow the
associated 1 ohm fusible resistor.
Diodes D402 and D405 protect the output transistors from being reverse
biased in the presence of inductive load “spikes”. Diodes D401A/B form part
of a wired OR gate system with the other power amplifier channels on L124
and are used to drive the lifters connected to the collectors of TR406A and
TR409A. R439 and C411 are part of a bootstrap network for the negative rail
of IC701 and its associated pre-driver transistor TR415. R417 and C406 are