Output Stage - Crown Pulse 2X1100 Supplementary Manual

Overall voltage feedback of the amplifier is derived through R243 and R241. R242 and C20 provide local feedback around the
Class A section only to define the dominant pole of the amplifier. C126 connected in series with R241 gives 100% DC
feedback to minimize any DC offset at the output. The resultant feedback signal is applied to the base of TR77.
The collector currents of TR76 and TR77 are fed via D76 and D75 to R260 and R272 respectively. Hence, in the quiescent
state, R260 and R272 should each exhibit a voltage drop of 1.35V or so.
Under normal conditions the signals at the bases of TR76 and TR77 will be identical. However, under fault conditions, such as
a DC offset at the output, the base voltages will become offset also. For example, in the event of a large DC offset of +50V at
the output, a positive DC voltage will appear at the feedback point and hence at the base of TR77. Although this would, in
theory, be the full +50V, owing to C126 being rated
at only 16V, the voltage will, in practice, be somewhat lower. However, the important issue is that the voltage is positive. In the
event the voltage is negative this indicates that the feedback network is faulty (most likely R243 itself). The voltage at TR77
base being positive whilst the base of TR76 is close to 0V will then reverse bias TR77 base-emitter hence turning off the
transistor. Hence, no voltage should appear across R239 and R272 while double the normal voltage will appear across R238
and R260 (150mV and 1.3V respectively). Should this not be the case, it indicates a fault in the input stage itself.
The output of the input long-tailed-pair (i.e. the voltages at the anodes of D76 and D75) are fed to a second long-tailed-pair
TR80 and TR81. The bias current for this stage, is set by resistor R261 thus; D76 drops approximately the same voltage as the
base-emitter junction of TR80. The same can be said of D75, and the base-emitter junction of TR80. This sets a current of
about 5.75mA, split between TR80 and TR81. C137 and C138 provide a little Miller Feedback around TR80 and TR81
respectively. These capacitors can be important to the stability of the amplifier but do not define the dominant pole. It should
also be noted that either of these capacitors becoming "leaky" (difficult to measure in circuit) will result in a DC offset at the
output. The collector of TR81 drives the output stage in conjunction with the collector of TR67 while the collector of TR80
drives current mirror TR66/TR67 via R212. In the quiescent state R212 will show a voltage drop of around 52V, and the current
mirror emitter resistors R188 R189 and will show equal voltage drops of 145mV. Hence, for the same +50V DC offset,
described earlier, one would expect no voltage drop across any of R212, R188 or R189, indicating that the feedback is
attempting to correct the fault. Likewise, for a negative DC offset one would expect these voltages to be twice their usual value.
If this is not the case then the second stage (TR80-TR67) is at fault. The collectors of TR81 and TR67 are joined to form the
output of the class A driver by the Vbe multiplier - R128, R127 and TR71 (mounted on the heatsink) bypassed at AC by C124 -
which sets the output stage bias. The bias voltage across the Vbe multiplier should range between 2.4V (heatsink warm) and
2.5V (heatsink cold). Bias voltages outside this range indicate a fault with the Vbe multiplier and/or a fault in the second long-
tailed pair (TR80 - TR81, R261, R212, R188, R189). For example, too small a bias voltage could be caused by: R261 being
high, R189 being high, R127 being low, TR71 being faulty etc. Too high a bias voltage is rare, and would, most likely, be
caused by a faulty transistor or resistor in the Vbe multiplier circuit.
C132 is very important for ensuring HF Stability. A faulty capacitor in this position will usually cause excess distortion and in the
case of anything less than 100pF can reveal a very spiky instability.

Output Stage

The output stage consists of a symmetrical Siklai follower - TR89-TR59, R189, R29A, R35A, R56A and C21A - generating the
high current drive required for the parallel connected symmetrical follower output stage TR57, TR73, TR79, and TR93, R231,
R244, R248, R257. V-I limiting is controlled by TR90, TR68B, R36A-R43A, C1A, C2A, R212, R25A-R27A, R30A, R33A, R55A,
D7A-D9A, D11A, ZD76-ZD6A. As the output stage is symmetrical, the positive half only will be described (Q13A-Q16A, R44A-
R47A, C2A, TR68A, R36A-R39A, R25A, R26A, R30A, R55A, D8A, D11A, ZD76, ZD5A).
Output stage protection is accomplished by a three-slope V-I limiting circuit which has limiting characteristics chosen to
emulate the Safe operating area of the output stage transistors at their maximum operating temperature.
The V-I limiting works by controlling TR68A: when the base-emitter voltage of TR68A exceeds about 0.65V then TR68A turns
on and steals current, via D8A, from the input of the output stage and thereby limiting the output. So, V-I limiting is controlled
by controlling the base-emitter voltage of TR68A.
Each output device has its own current sharing resistor - R44A-R47A - the voltage across which is proportional to the current
flowing in the output device. These voltages are sampled and summed by R36A-R39A. C2A ensures stability when V-I limiting
is activated.
The voltage across the output devices is sampled by R25A and R26A (R30A and ZD5A limit the voltage range to reduce off-
load distortion) and this, summed with the output current derived signals from R36A-R39A, controls TR68A for output voltages