Bose FreeSpace 6 Manual page 9

3.2 IO2 PCB Schematic Sheet 6
The remote volume control function is implemented on this sheet with a dual VCA, IC U201,
and its buffer, U202. (An alternate part could be used in U201A, but this is a manufacturing
alternative, not a field service alternative.) U201 is controlled by a current sourced into the
signal VCA, coming from Sheet 7. When VCA is about 2mk, gain is at its maximum, which is
unity (0dB). When the current into VCA is zero, the gain is essentially off (-80dB or so).
S4 and its related components are factory set gain trims.
3.3 IO2 PCB Schematic Sheet 7
Transistors Q202, Q203 and related components transform the variable resistance of the
remote volume control into the current source that feeds signal VCA, on Sheet 6.
U205 is a conventional bass and treble control, while U203 is an equalizer stage intended to
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provide the EQ adjustment needed when FreeSpace 6 systems are configured with Surface-
mount loudspeakers (as opposed to the Flush-mount FreeSpace 6 speakers).
4. Power Amplifier PCB
4.1 PA PCB Schematic Sheet 8
U301, part of U300, and U302 implement a acoustically correct low frequency compensation
for more natural bass. The other two sections of U300 process the signal CD-DIA ("clip-
detect-diagnostic", see Sheet 10) into a control signal that is sent through P2A, remote volume
control circuit on Sheet 7 . CD-DIA becomes active when the power amplifiers are driven into
clipping. This in turn causes the remote volume control circuit to be turned down so that clip-
ping ceases.
4.2 PA PCB Schematic Sheet 9
U303 combines the low frequency compensation signal with the full range signals and then
equalizes them to compensate for room acoustic and speaker driver characteristics.
4.3 PA PCB Schematic Sheet 10
U304 and U307 are power amplifiers with differential outputs. Both amplifier outputs can
deliver 16 watts into the nominal 2 ohm load of a pair of FreeSpace 6 speakers. The CD-DIA
pins are pulled low when the amplifiers begin to clip. This is used to control a compressor
implemented on Sheet 9 and Sheet 7.
D306 develops the DC power supply for the power amplifier. Q305 and associated compo-
nents constitute a shunt voltage regulator which only conducts in the unlikely event of both a
high power line voltage and a low or no signal output case. It holds 18V maximum which is
presented to U304 and U307.
D305, U305, and U306 are the +/-15V supply for all the circuits except for the power amplifier.
The power transformer secondary plug can be inserted two ways; one way will handle a nomi-
nal 120VAC main, the other a nominal 240VAC main. The two orientations of this plug are
noted on the PCB.
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