Bose FreeSpace E4 Series II, Service Manual

Page: 92 / 109

92
THEORY OF OPERATION
The Tripath’s modulation pattern has a no-signal center frequency of about 700 kHz. The
switching frequency varies downward as the signal level increases, reaching about 100 kHz just
before clipping. Near clipping, the switching pattern is further adjusted to provide soft clipping
behavior. This appears to be oscillation when viewed on a scope, but all of the artifacts are well
above the audio band, and the “fuzz” actually helps reduce the audibility of clipping. Choke L4
and capacitor C41 form a 2-pole low-pass filter at approximately 70 kHz to remove the switching
frequency from the audio output. C42, R84 and R85 are a Zobel network whose main function is
to damp the resonance of the L4/C41 filter with very high impedance or nonexistent speaker
loads. This network also does double duty: the voltage across R84 and R85 is rectified to
determine if the amplifier is being fed a signal with too much high frequency content. You’ll
notice that there is no analog feedback from the audio output. All of the feedback in the ampli-
fier is derived by comparing the actual FET switching transitions with the predicted transitions,
thus avoiding the inherent delay of the output LC filter. “Servo Amp” A1 provides DC feedback to
insure that the entire amplifier’s output DC offset remains very low. This is especially important
in distributed sound applications, where the speakers are connected through matching trans-
formers that have very low impedance at DC.
The voltages across R87 and R86 are sensed differentially and level-shifted within the Tripath
module (this is how excessive current i.e. a shorted load is determined). This over-current, or
main rail voltages that are too high or too low will cause the module to shut down very quickly
and output a logic 1 on the module’s “Fault” pin.
Protection
PIC microcontroller, U14, monitors amplifier functions including the Tripath’s “Fault” pin (and
numerous other inputs) and responds by turning the amplifier on and off by means of the
Tripath’s “Mute” pin. The speakers are also disconnected under U14s control via relay K1.
In order for the amplifier to become fully functional at start-up, the following conditions must be
true:
• The AC power must be OK.
• The heatsink temperature must be below 160 o F.
• The “Wakeup” line from the host processor must be high.
• The “Amplifier Off” command (I 2 C) from the host processor must be absent, i.e. set to “On”.
• The main rail voltages must be within prescribed limits (+100-190V).
• There must not be DC at the output (after a 1 second “servo setting time”).
• There must not be excessive high frequency content at the output.
In addition to this series of tests at start-up, the determination is made whether the unit is set for
70V or 100V output by measuring the + rail voltage <140V = 70V mode, >150V = 100V mode.
70V mode causes a 1 (+5V) output on U14s “Vshift” pin. This 1 causes a small amount of
current to be sourced/sunk into Tripath pins 37 and 38. These currents cause the Tripaths
internal Overvoltage/Undervoltage limits to shift downward corresponding to the + 130V rails vs.
+ 170V.
Once the amplifier is up and running, all of these parameters are monitored continuously, with a
few minor adaptations. If the heatsink temperature exceeds 160
o
F, the fan is switched from low
to high speed operation. If the power dissipation is still excessive, and the heatsink exceeds
210
o
F, the amplifier will be shut down until the temperature is under 160
o
F. Once the initial
voltage measurements are made, the Tripaths internal overvoltage/undervoltage sensing is
relied upon.