The
simplified circuit [Fig.2] shows that each sub-amplifier
consists of two voltage gain stages. The first stage consists of a complementary
two–stage common emitter [Q1-7] whose gain is about x2.3. The second stage is a current
mirror stage [Q13-14] which drives the voltage across a load resistor tied to 0V. the gain
of this stage is about x200. Thus the overall open loop voltage gain is of the order of
x460 and so, as the closed loop gain is x26.7, the reduction due to negative feedback is
x17.2 or about 24db. The input amplifiers are powered from ±
15V supply rails derived from R14-D1 and R21-D2. The current through the first stage [Q1]
is held constant, at about 0.36mA by the floating regulator stage [Q2-3] which provides
temperature compensation . The gain of this stage is set by emitter resistor R6 which
provides some local negative feedback. The second stage [Q4] is loaded by two series
cascode transistor [Q5-6], the first having its base tied to ground and the second having
its base tied to the –15V rail. Thus the maximum collector- voltage swing on Q4 is
greatly reduced , so reducing the effect of the base-collector capacitance [Miller effect]
which would reduce this stages high frequency bandwidth. In summary , the presence of Q5
and Q6 improves the bandwidth and linearity. The load on Q6 is one half [Q12] of the
current mirror and can be visualized as a resistor in series with a forward –biased
diode. The second half of the current mirror is a common-emitter stage [Q16-17], a simple
voltage amplifier except that its collector current equals [or ‘’mirrors’’] the
collector current of the other half [Q12].This stage is made up of two transistor in
parallel which share the current. This arrangement was found to improve the linearity of
the stage. The other sub-amplifier [Q7 to Q14] works in exactly the same way but with
opposite polarity. The output stage uses the conventional Darlington emitter follower
arrangement, but with three parallel pairs of driver and output transistors. A transistor
Q15 is wired across the bases of the pre-driver transistors [Q18-19], providing a bias
voltage to set the standing current in the output stage. Q15 is mounted on the heatsink
with the aim of keeping this current constant regardless of temperature. The TR2 trimmer is used to set the value of this current.
The output DC offset voltage is set to zero by TR1 trimmer, in the input stage. In theory there should be no DC
offset at the output but, because of component tolerances and consequent mismatching ,
there always is. TR1 is arranged to make the current in the first stage of one
‘’sub-amplifier’’ either higher or lower than in other and so null out any
residual offset. A simple low-pass filter is created by an RC networks at the input R2,C2
to reduce the bandwidth of the signal below that of the open loop amplifier and thereby
eliminate the generation of any transient intermodulation distortion.
The output stage is
quite substantial, using a total of six 250W power transistors. Fairly 'old- fashioned'
power transistors have been used [MJ4502/802 family] in preference to some of the higher
performance devices now available. They have been chosen because the die used to mount the
semiconductor junction is of a large area ; the device is quite rugged and can handle high
currents. The short term current capability of the output stage is, in fact, of the order
of 90A, somewhat in excess of the current capability of the wiring. The rest of
construction is equally massive with a steel chassis supporting six very large heatsinks.
However , construction is straightforward provided that the builder has strong arm muscles
and circuit alignment simple- there are but two adjustments- quiescent current and DC
offset voltage nulling.The coil L1 is wound onto the
body of R41. This is not a critical procedure- about 17 to 20 turnoff enameled copper wire
should do nicely. Particular care should be taken in mounting the power transistors. Good
quality insulating washers and bushes should be used and a generous smearing of thermal
paste is essential. These transistors should be bolted to the heatsinks very tightly to
ensure good thermal contact at all temperatures. One final point regarding construction.
Once the amplifier has been completed and tested, it should be switched on and allowed to
reach its normal operating temperature [about 20 minutes]. The amplifier should then be
switched off and all the screws tightened up. Differences in thermal coefficients of
expansion can result in some of the screws becoming slightly loose, particularly those
holding the heatsinks to the top and bottom covers.
My own proposal,
for whoever it decides makes the amplifier, is him it manufactures in two separate boxes,
one for each channel, [monoblock manufacture ], separating thus and the power
supply's. This will facilitate too much the mechanic and electrical manufacture. For
all the manufacture, is required certain relative experience, in these sectors. The sound
result is sure that it will vindicate, that tries, one and proportional performance
amplifiers in the trade, costs exceptionally expensively. ETI 8/81
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