Crown ce 4000, Service Manual

Page: 13 / 140

Theory of Operation 3-3
130485-1 Rev. B
CE 4000 Service Manual
©2002 Crown Audio, Inc.
Now back to the description of output circuit fl ow:
after the FETs, the signal is then routed to the BCA
fi lter inductors (Lp and Ln) and the current sense
resistors (R383/R361 and R384/R362). The combining
point (B1) is then sent to the Output Filter.
With no audio modulation, the PWM pulse train that is
sent to the gates of each set of FETs is the same and
the duty cycle is approximately 50%. This means that
for no audio input, the positive FETs (Q106, Q107,
Q110 and Q111) turn on and off at the same time
as the negative FETs (Q108, Q109, Q112, Q113. As
mentioned before, the Lp and Ln inductors control
the rate of rise of the current during this time so that
this does not act like a short during the couple of
microseconds that they are on. The time that the FETs
are on ramps current up in the inductors which stores
energy in them. This energy is released through the
diodes (D140-D143) when the FETs are turned off.
The Vp and Vn nodes swing between –Vcc and +Vcc
with a duty cycle of approximately 50%.
Operating the output stage in this way causes the
current in the Lp and Ln inductors to completely
cancel at the B1 node. This drastically reduces the
filtering requirements necessary after the output
stage.
If an audio signal is present at the input of the
amplifier, the widths of the pulses at Vp and Vn
change but the amplitude of the pulses stays at 2
* Vcc. Positive audio signals will cause the positive
side of the output stage to be on more than 50%
of the time while the negative side will be on less
than 50% of the time. The total “on-time” will add to
approximately 100%. Also, the on-time of the two
polarities will overlap so that when the negative side
is on, the positive side is also. For negative-going
signals, the action is reversed (swap positive and
negative). The result at the summing junction B1 is an
amplifi ed version of the input signal, with some noise
superimposed on it. The B1 signal is then routed
to the Output Filter.
3.1.6 Output Filter
The output filter is made up of three individual
filters: a 250 kHz filter (L101/C128), a 500 kHz
fi lter (L102/C131/C397), and a fi nal 250 kHz fi lter
(L104/C136-C139). The signal passes through these
three fi lters to eliminate both residual 500 kHz and
250 kHz signals. The audio signal is then connected
to the output PWA through buss bars.
3.2 Protection and Control Circuitry
3.2.1 Current Monitor
Audio output current levels are monitored by the use
of transformer T100. A small primary winding is in
series with the output current and the secondary
develops a voltage across R140 proportional to the
output current of the amplifi er. This output current
information is used for two purposes:
1. Negative feedback
2. Current feedback information for the modulators.
Figure 4.1 BCA Operation