Bose FREESPACE BUSINESS Service Manual page 11

The basic concept behind Class G operation is simple. It takes advantage of the fact that music,
even at loud volume settings, has only brief moments where peak voltage from the amplifier
is needed. Unfortunately, with conventional Class B amplifiers, this peak voltage is always
supplied to the output transistors and most of the time is wasted as heat. The result is that larger
and more expensive power supplies and heatsinks are needed (or power output transistors are
operated hotter with less reliability).
With Class G however, the amplifier operates off of two different power supply voltages. When
the music is low the amp runs off of the low voltage supply, thus conserving power. When
musical peaks occur, the amplifier "intelligently" switches to the higher voltage supply. On
average, this voltage "conservation" translates to efficiency almost double that of class B.
Detail operation of the amp is as follows: First, it is important to understand that a Class G amp
is really just a class B amp with special circuitry added to it to allow switching between two
different power supply voltages. In the case of the bass amplifier, this switching action is
accomplished on positive peaks by transistors Q335, Q334 and Q333, and on negative peaks
by transistors Q342,343, and Q344.
A positive signal at op-amp input U303 pin 3 causes op-amp supply current to flow into pin 7
through transistor Q336. Q336 is used simply to buffer the +40V high voltage amp supply down
to a safe +15V for the op-amp. This current also flows through resistor R353 and diode D325.
R353 and D325, together with driver transistor Q332 and R355, form a current mirror which
multiplies the current through Q336 by the ratio R353/R355. This increased current flows
through driver transistor Q332, through diode D330, and into the base of output transistor Q337.
The hFE current gain of Q337 (about 40) provides final current amplification before reaching
speaker output.
Class G switching action occurs as follows: the voltage at the anode of D330 tracks one diode
drop above the output voltage of the amp. When this voltage exceeds the nominal +20V power
supply rail transistor Q335 conducts. This in turn causes transistors Q334 and Q333 to turn
on. When Q333 turns on, it acts as a switch and connects the +40V supply present at its emitter
to the collector of output transistor Q337. When this occurs, the +20V supply, normally fed
through diode D329, is turned off because the +40V causes the diode to be reversed biased.
When the music voltage drops below +20V, the reverse happens. The +40V supply is turned
off, and the +20V supply is turned on again.
The power amplifier topology is complementary. Therefore, operation of the amplifier on
negative half cycles is identical to positive cycle operation except for component reference
designations.
10. Short Circuit Protection
The left and right power amp stages have short circuit protection. This is necessary to prevent
damage to the amplifier from possible customer misuse. For example, the customer could
accidentally short the speaker cable wires that connect to the cube speakers. If this happens,
all three amplifiers will momentarily mute for about 3-4 seconds, and will continue to mute until
the abnormal condition is removed.
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