Theory Of Operation - Bose FreeSpace 6 Manual

1. OVERVIEW
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The FreeSpace 6 business music system delivers high-fidelity music and paging in commer-
cial environments. The FreeSpace 6 components have been engineered to work together as a
system. The electronics have separate compressors for music and paging sources that de-
crease the dynamic range of the audio signal, thereby increasing the audibility of musical
passages and paging anouncements. A protective compressor in the power amplifier prevents
audible overload distortion if the user attempts to play the system too loud.The system has the
following features:
• High-fidelity sound over a large area
• Active equalization, music compression, and system overload protection
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• Opti-voice
• Remote control of volume and source selection
The circuit descriptions that follow are grouped according to the PCB section and sheet num-
ber of the schematic.
2. IO1 PCB
2.1 IO1 PCB Schematic Sheet 1
Music Input A is routed through a differential input amplifier (U4) which accepts balanced
sources or unbalanced sources. The amplifier output is passed through a gain trim pot and
presented to the Music A/B selector. Music Input B is fed directly to a gain trim pot and then to
the Music Input A/B selector.
Music Selector A/B is a 2P2T analog switch, implemented with a special switching op amp
(U11,U12)). The op amp is controlled by the signal SELECTB.
The Music Selector Interface is a two transistor circuit that normally selects Music Input A. With
the Remote Selector switch open, transistor Q2 is on, which turns on D4, the Source A LED,
pulling SELECTB low. This turns off Q1, keeping its LED off. When the Remote Selector switch
is closed, Q2 is turned off, allowing Q1 and D3 (the Music B LED) to turn on. This moves
SELECTB high and turns off the Source A LED. When SELECTB is high, current is no longer
sunk from U11 and U12's SW pin, and they select Music Input B.
2.2 IO1 PCB Schematic Sheet 2
On their way to the compressor, the left and right music signals pass through a stereo/mono
switch, S1. After the compressor (U7 and part of U6), the music signals leave this sheet. The
compressor is enabled by a logical function of SELECTB, Music Compressor A, and Music
Compressor B enable switches. The logic (U8) determines that the compressor will be enabled
only for a music source if that source is the one currently selected and if it has been enabled
for compression via its switch (S2 or S3).
The compressor offers a maximum gain of about 20dB for very low level signals and operates
at a 2:1 slope. It slowly raises the level of quiet musical passages, and quickly reduces the
level of loud passages. The two channels are cross coupled to maintain a centered stereo
image. By selecting the rectifier input source as a DC current, the compressor is disabled (U9).

THEORY OF OPERATION

circuitry for high quality paging capability
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