Additionally the 3 power amplifier channels in L125 share a common pair of
power MOSFET
lifters
(the part of the block diagram shown as U_Lifter_C)
which control the amount of output voltage fed to the collectors of the power
transistors in the 3 power amplifiers.
The Centre channel Driver Stage
starts with the balanced to unbalanced
converter IC700B (one half of an NJM2114) which rejects common mode
noise on the input when grounded to AGND_FF via the handbag link
CON700. Its output feeds one half of the stereo high voltage driver IC
(LM4702C - IC701) via the low pass network R706 and C704. The LM4702 is
used in a non-inverting configuration and provides all the voltage gain of the
system, set by R713 and R704. Pins 11 and 12 of IC701 feed the inputs of the
negative and positive halves of the O
utput Stage
. The dominant pole
compensation is set by C705, C713/R714 provides some second order
feedback in the audio band to provide a higher open loop gain at high audio
frequencies and thus reduce hf distortion. R715 and R716 (without TR700
which is a “no fit”) provide some extra voltage to IC701’s negative supply at
high negative output voltages via a bootstrap arrangement in the output stage
to prevent premature clipping of the negative half of the output.
Note that the
heatsink of IC701 is connected to its negative rail – this must NOT be
accidentally shorted to ground!
Good power supplying decoupling of IC701 is essential and is provided by
C701, C708, C709, C712 and C714.
Note that the amplifier is DC coupled throughout. IC702B (one half of a
TL072) has a very high input impedance and forms a ground referenced
inverting integrator with R712 and C711; their time constant is approximately
1.5 seconds. Its output is fed back to the positive input IC701 via the
attenuator R711 and R706 to keep the DC output of the amplifier close to
zero. It can also correct moderate DC offsets appearing at the input of
IC700B. Should these become excessive, or should a circuit fault cause
significant DC at the loudspeaker output, then the error voltage at the output
of IC702B will feed through to the system microprocessor via D700 (which
works in conjunction with the similar diodes in the other power amplifier
channels as a wired OR gate) to generate a system shut down signal.
NB. R702 will also mute the power amplifier electronically in the absence of
the +/- 12V supply.
The Centre channel Output Stage
comprises complementary triples in a
classic emitter follower configuration, with enhancements to ensure near class
A operation at power levels of up to about 10 watts. Both the positive and
negative halves are essentially identical.
The pre-driver and driver transistors are connected to the high voltage rails
Vcc and Vee (/-59V). However because this is a Class G design the
output transistors TR406A and TR409A normally run at half these voltages
(+Vlift and –Vlift, /- 30V) connecting to the +Vlo and –Vlo supplies via
Shottky power diodes D100 and D101 shown on the main block diagram.
When the amplifier is required to deliver more than about +/- 25V peak