Pre-Driver, Driver Stage And To-3 Outputs; Output Filter And Feedback Paths And Output Loading Information; Filter Stage - Crown XTi series Service Manual

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voltage stresses on Q105 and Q107. The temperature compensated servo transistor Q109 is
located under the same heat sink ( + ) and near the LM75. The bias set string with D174, R180,
R144 and R179 run about 400ua and are not under that heat sink. With the pot R144 centered an
bias on the collector to emitter voltage of Q109 measured across C109 should land about 1.9 to 2
volts. C109 provides filtering and holdup charge. Q108, Q106, R135 and R142 set the servo and
pre-driver (Q112 and Q113) currents (see note 2). C108 is slew rate limiting. Bias can be
measured across the two TO-220 heat sinks and may be set ( at room temp) to about 0.8v +/-
.05V. See note 4.
d) Pre-driver, driver Stage and TO-3 outputs.
Pre-dirvers Q112 and Q113 , when bias is on are biased on with R147 and R148 (see note 5) to
about 1.8ma via drivers Q114 and Q115 whose currents are about 6 to 7ma. Their collectors are
tied to the TO-220 sinks (see note 6). In any product these sinks at idle should not produce much
heat to the touch. About 0.5 watts each at idle. The TO-3 devices are normally off, class B mode.
Diodes D180, D181, D182 and D191 across R150 reduce common mode currents of the output
devices (see note 6).
e) Output filter and feedback paths and Output loading information
The Output de-coupling inductor L100 and Rs and Cs around it form the filter. The amplifer
feedback lines are in pcb foil and with 1 ohm connecting resistors for remote sense location
depending on the product and output connector location. On a product with both speakons and
binding posts , the feedback connections are referenced off the binding posts (SPECS). High side
low frequency feedback is R84. High side high frequency feedback is C65. Low side low passed
remote sense feedback is thru R87 (note 7).
On the larger Tracker based units (note 8) the use of a load of at least 24 ohm (suggested for
THD1 test) , maybe 32 ohm (not higher) is needed due to a tracker based gate drive charge pump
in order to get proper tracker voltage drive to the amplifier rails otherwise positive rail clipping
might occur causing DSP compression off of the clip_not function.
Suggested amplifier loading for initial system verification is 24 ohms and starting with the
THD1 test specified at MFT. Units that start and crowbar immediately for an unknown reason
should be without a load and then re-started.
Non tracker based units (1000s) have a full VI limiter and can drive inductive loads at low
levels. Loads down to 1 ohm can be driven as well but due to the VI term the allowed output
voltage peak collapses to less than 25v and thus for the non-tracker based units use of a 1 ohm
load will not give the a peak current limit you might expect off a data sheet 2 ohm power
number.
Tracker based units have no V term in the limiter so a 1 or a 2 ohm (better) load may be used to
witness the true peak current limit. Tracker based can drive 45 degree inductive loads but a pure
inductor or torture load is not really recommended at this time.
On any products , driving into less than 8 ohms at high voltage levels and at and above 15khz
long enough ( longer than a music based signal) is very likely to invoke the clip based
compression off the DSP.
Driving into a short of thru a cable to a short with a non-tracker unit will result in a full collapse
of its current limit and the display may say "SHORT" until either the drive or output short is
removed.
Forcing a tracker based unit into a short with a continous sinewave is not recommended at this
time. Driving a tracker based unit thru a long cable into a short may result in a display of
"SHORT" until it is removed or invoke the DSP compression.

2) Filter Stage

a) Input cheb differential filter from codec to amplifier.
b) Amplifier high pass filters
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U10 forms a differential to single ended chebychev 22khz low pass filter for the CODEC
to the amplifier differential front end thru R138 and C124. Under normal signal
conditions the absolute expected peak output of U10 pin 1 should be less than +/-5v.
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