Output Filter - JBL A6000GTi Service Manual

137159-1 Rev. 1 A6000GTi Service Manual
Now, back to the description of output circuit flow: after the FETs, the signal is then routed
to the BCA filter inductors (Lp and Ln). 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 (Q301, Q302, Q303, Q304) turn on and off at the same time as
the negative FETs (Q305, Q306, Q307, Q308). 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 (D301-A, D301-B, D302-A, D302-B) 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
amplified 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 two individual filters: a 62.5kHz filter (L800,C823-C831) and
a 125kHz filter (L801, C832-C836). The signal passes through these two filters to
eliminate both residual 125kHz and 62.5kHz signals. The audio is then sent out the 6-wire
cable to the output terminator board. The output terminator board connects the output
signal to the amplifier output terminals.
2003 Crown Audio, Inc.
Theory Of Operations 3-4