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A Class A // Class AB amplifier rated 100 Watts when driving a 4 ohm loudspeaker.
This circuit developed out of my 30+ years of JLH class-A based investigations.
The original 'simple' 1969 JLH class-A design provided excellent first cycle
accuracy through mid and high frequencies (dynamic clarity) because there
were no stabilisation components nor a series output choke, whilst the NFB
error correction was established via the input emitter (some describe this
as current feedback).
However it did generate rather a lot of heat, the damping was 'soft' at
lower frequencies and the positive slew was weaker in the presence of
loudspeaker system generated back-EMF at higher audio frequencies. I was
eventually able to improve upon the original JLH circuit by;-
1) adding a 'helper' class-AB output stage to substantially increase power
and efficiency, but leave sufficient class-A bias for real-time control
maintenance through phase shifted zero current class-AB crossovers;
2) adding a differential input stage for zero output voltage control, but
also fit a 10nF base-emitter capacitor for differential voltage operation at
audio frequencies, though with leading emitter routed feedback to maintain
circuit stability and NFB control above 20kHz;
3) adding a current mirror to obviate power-up thump, but also fit a 220nF
base-emitter capacitor to ensure stable 'source only' operation above audio
Thus, when compared to conventional circuits, one complete high frequency
phase change has been removed from the closed feedback loop in order to
minimise need for any additional stabilisation components that would
otherwise render the circuit inductive at audio frequencies. The original
JLH hf (current feedback) stability and simultaneously phase coherent
class-A control are retained.
This circuit was intended for 2SC5200 operation, yet it has been
successfully constructed using other device types, including a single Sanken
2SA1216+2SC2922 pair in place of the paralleled ABs. Keep VAS, Zobel and
output related wires 5cm/2" away from input devices and wiring. Use a star
earth from the input location. Use star power distribution from each fused
10mF at the pcb. Use star output node connections. Parallel all large Cs
with smaller ones. Do not twist any extender e-b-c wires that might be used
out to heatsink mounted output stage devices. Mount the Vbe multiplier on
the output heatsink for automatic temperature compensation. Adjust the
class-AB pre-set to 50%, but adjust the class-A pre-set to be a short
circuit before the initial switch-on. I recommend 22 ohm per rail in place
of the fuses in case of error at initial power-up, but do not try to set any
bias with them in circuit.
100W-4ohm (conservatively rated) is 50W-8ohm. For 70W/35W use 30V rails. For
50W/25W, 25V. For 100W-8ohm with higher voltage rails try a 47pF Miller
connected C.dom to the VAS, it should compensate for device/stability
changes with increased voltage, yet not adversely affect the class-A
operation in maintaining crossover continuity. Always bias from zero current
if you increase the rail voltage. Insert a 0.22 ohm series output resistor
to maintain stability when driving purely capacitive loads, and don't forget
to bi- or tri-wire out to composite loudspeaker system sections and drivers
in order to obviate single cable developed dynamic voltage drops with
varying loudspeaker system generated back-EMFs, no matter how expensive your
loudspeaker cable might be.