Bose FREESPACE BUSINESS Service Manual page 9

2. Differential Amplifier
Audio input is at barrier strip P5, which is located on the I/O PCB Assembly (see the schematic
located in the back of this manual). This connects directly to the left and right differential
amplifier buffers on the Main PCB. This stage neatly accomplishes several functions: provides
rejection of common-mode interference (like hum) that might be picked up by the long audio
cable between the FreeSpace™ system and the customer's audio source; provides input
overload protection through diodes (D101-104); strips off radio frequency interference through
capacitors C203 and 204; provides DC blockage by capacitors C201 and 202; and provides
a 10 dB gain through op-amp U201 .
3. Compressor
U201 (pins 5,6,and 7), under normal audio conditions, operates as a conventional inverting op-
amp stage with a gain of 6 dB. However, when overload is present at the speaker outputs,
transistor U205 (pins 5,6 and 7) receives a turn-on signal from the compressor detect stage.
When this happens the transistor acts as a variable resistor to ground, and with resistor R207
forms a voltage divider network that attenuates the audio signal. The amount of attenuation
and timing action is carefully controlled by the compressor detect stage (discussed later).
4. Audio Sense
The FreeSpace system remains in standby mode until audio is detected at its inputs. Audio
sense occurs after the compressor gain stage where the left and right signals are summed
through resistors R222 and R224. The signal is preamplified by inverting op-amp summer
stage U203 (pins 1,2,and 3), and is presented at the pin 1 input of turn-on chip U204 (NJM2072).
Diodes D109 and D110 are necessary to prevent overload of the turn-on chip at high volume
levels. With audio input ≥0.3 mVrms, U204 pin 7 goes high, and through diode D111 turns on
transistor Q401. This triggers relay K1 to energize the main power transformer. Turn-on
(settling) time of the power amplifier section is approximately 80 ms.
Without audio input,U204 pin 7 will go low after approximately 90 seconds, and will de-energize
relay K1 and the main supply. This turn-off time is set by capacitor C217 .
5. Tone /Electronic Volume Control
A separate tone control PCB attaches to the Main PCB through a seven conductor ribbon cable
connector. Power is derived from the main board. The tone control is a standard Baxandall
circuit with +/- 6 dB bass and treble controls. Volume control function is accomplished through
the use of current-controlled amplifier IC U502 (CA3280). The gain of U502 is directly
proportional to the current flowing into pins 3 and 6 from transistor Q502. This current is
proportional to the voltage at the base of Q502. This voltage is adjusted by the external 10K
potentiometer that indirectly attaches across connector J502 through the I/O PCB barrier strip
terminal P5,pins 7 and 8. At minimum volume (0Ω between J502,pins 1 and 2),the voltage at
the base of Q502 should be .36 ± .1 VDC. At maximum volume (10 KΩ between J502,pins 1
and 2),the voltage at the base of Q502 should be 4.6 ± .1VDC. A schematic is located at the
back of this manual.
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