SA-2 and SA-3 Theory of Operation
General Information:
The Lifestyle
non-powered environmental speakers or Bose non-powered accessory speakers to your Lifestyle
Bose proprietary Integrated Signal Processing technology, featured in the amplifier, provides full, rich stereo
sound, even when the speakers are playing at low volumes. By using the Lifestyle
your system, you can enjoy Bose quality sound and Lifestyle
swimming deck, garage, or utility room.
The Bose
repair package for installation instructions) and Bose
require an additional RF remote control in order to operate the second zone. This is because the dip switches
located in the remote control must be set differently to control zone 2 (see repair package for settings).
Lifestyle
control.
Note: The circuit operation described below is for the left channel. The right channel operation is identical. The
designators inside the brackets are schematic sheet grid locations.
Refer to schematic for the following information.
The rear panel of the Lifestyle
DIN. These jacks, along with an audio input cable, part number 272902-220 supplied with the
amplifier, allow the use of the different Lifestyle
installation, see the repair package for the installation to the system you are working with.
®
Bose
link is the proprietary networking technology that allows a Lifestyle® system to integrate with other Bose
products to provide sound in secondary rooms.
Introduction:
The Lifestyle
printed circuit board. The LSA-2 is a depopulated version of the SA-3. The differences between the two
products can be summarized as follows:
Feature LSA-2 LSA-3
Power output 40W/ch 100 W/ch
Amplifier topology Class AB Class G
Auxiliary RCA input No Yes
Mono/Stereo switch No Yes
General Theory of Operation:
Refer to the system block diagram (Figure 1).
Audio Chain:
Audio Input is received at the DIN connector in the form of two separate analog stereo (left and right) channels
(channel 1 and channel 2) from the music center. The LSA-3 has a third audio input, via the left and right RCA
jacks. Channel selection, volume level, and dynamic equalization occur at the volume control IC U3 (the ST
TDA7309). Output from this IC is fed to a differential pre-amplifier stage, and then to the power amplifier IC.
®
SA-2 or SA-3 stereo amplifier provides you with a simple solution when you want to add Bose®
®
®
system convenience in your yard, patio,
®
Lifestyle® SA-2 and SA-3 amplifiers are a zone 2 accessory to the Bose Lifestyle® systems (see
®
systems that use a PMC/MRI (such as the Lifestyle® 40 and 50) do not require an additional remote
®
amplifier utilizes two different types of audio input jacks, RCA and
®
Stereo Amplifier 2 and the higher power Lifestyle® Stereo Amplifier 3 product use the same
®
speakers. Lifestyle® Systems that use a music center
®
music centers. Different Lifestyle® systems require different
stereo amplifier to expand
®
system.
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SA-2 and SA-3 Theory of Operation
At full volume the frequency response of the system is basically “flat”, over the frequency range from 10 Hz to
15 kHz. At lower volume levels, the dynamic equalizer provides up to 16 dB of bass boost at 50 Hz. A graph of
frequency response vs. volume is shown in Figure 2.
Microcontroller:
Control of the product occurs via a serial data link (BoseLink) from the music center, communicated through
one of the wires of the DIN cable. The microcontroller automatically detects whether it is connected to an
existing (CD-5, CD-20, MRI, AV28) or new (AV18, AV38) music center, by recognizing either the original (1way) smart speaker protocol, or newer 2-way smart speaker BoseLink protocol.
In addition to communicating back and forth with the music center, the microcontroller does the following:
Determines power mode (Standby or ON).
Sets volume level and Mute.
Reads DIP switch settings (for room address, etc)
Detects presence of audio at RCA input jacks (on SA-3).
Detects presence of external potentiometer (at J6 input).
Monitors temperature of power supply bridge rectifier
Monitors power supply voltage
Detailed Theory of Operation:
Power Supply:
The primary side of the power supply consists of the fuse in series with triac switch Q1 in series with the
primary coil of the transformer. In parallel with the triac is a large film capacitor (C11 or C27) and resistor (R10)
which provides a small “trickle” current to the transformer when the triac is off (open circuit). This small amount
of current is enough to keep the microcontroller (U4) powered. This mode is called Standby. When an ON
command is received via BoseLink, from the music center, pin 10 of the micro goes to logic high and activates
opto-coupler triac driver U7 which then turns on triac Q1, and the power supply becomes fully energized.
The transformer secondary power supply is comprised of bridge rectifier BR1 and electrolytic filter capacitors
C7 and C10. The voltage for LSA-2 is nominally about 36 VDC, and for LSA-3 it is about 48 VDC. LSA-3 uses
nd
a 2
set of power supply rails from BR2 and electrolytic capacitors C8 and C9 at about 24 VDC. These parts
are not used on LSA-2.
The low voltage electronics are powered by +8 VDC and –8 VDC voltage regulators U8 and U9 (respectively).
For LSA-2 the input of the regulators are connected to the BR1 power supply through diodes D6 and D7. On
LSA-3 these diodes are not used, and the regulators are connected directly to the BR2 supply. In Standby
mode, the +8 VDC and –8 VDC supply rails are turned OFF by transistors Q13 and Q14 respectively. This is to
conserve power and to prevent low voltage poor operation of the input op-amp circuit.
The microcontroller is powered by a 5 VDC regulator (U10). The input of this regulator is connected to the
output of a low drop-out discrete 26 VDC regulator, comprised of transistors Q10, Q11, and Q12. In Standby
mode this voltage is much less, usually between 9 and 16 VDC.
Differential Input Amplifier:
Channel 1 and 2 inputs are applied to unity gain differential amplifier U1. The differential amplifier rejects
common-mode noise that may be picked up on the long DIN cable wires.
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SA-2 and SA-3 Theory of Operation
Volume Control:
Local volume control connection jacks (J6). Allows you to control the volume in a room using a wall-mounted
control. Currently the volume control adjustment is not available to end consumers. Contact your Bose® dealer
or call Bose Customer Service for information on installing this feature. See the contact list inside the back
cover of the Owners Guide. The output of U1 is AC coupled to the volume control chip U3. This is an analog
resistor “ladder” type volume control, incremented in steps of 1 dB. This IC is controlled by I2C bus, Data and
Clock lines (pins 6 and 7) from the microcontroller (U4). At volume levels below –20 dB (the volume control has
a range between –90 and 0 dB of gain); dynamic EQ action (bass boost) occurs from the op-amp circuit
connected to pins 12 and 19. This circuit simulates a parallel resonant RLC circuit at about 50 Hz that provides
progressively greater relative boost to frequencies below 200 Hz, as the volume level decreases.
Compressor:
The output of the volume control is AC coupled through C104 and C204 to amplifier/compressor stage U6
(pins 1,2,3 and 8,9,10). This is a differential inverting amplifier with gain of 6.3 (16 dB). At normal and low
volume level, current-controlled amplifier U5 is turned OFF. At high (clipping) volume level transistor Q4 turns
ON and supplies current to U5, which reduces the gain of the circuit.
Specifically, if either the left or right speaker output reaches a positive voltage amplitude within 3 volts of the
positive power supply rail, then comparator output U6 pin 14 switches low and charges capacitors C18 and
C19 to a negative voltage level. This voltage is converted to a current at the collector of Q4, which then
increases the gain of U5, and decreases the overall gain at the output of U6 pins 1 (left) and 8 (right). C18 is a
“hold” capacitor. Once charged, its voltage stays constant for at least 20 ms, until C19 discharges (becomes
more positive) enough to turn on diode D12. C19 starts discharging immediately, through R38, when the
speaker output voltage is less than the clipping level.
Power Amplifier:
The LSA-2 uses the TDA7294 (100V) power amplifier. It is a class AB linear amplifier IC. The amplifier is
muted when pins 9 and 10 are at logic low. It is unmuted when they are at logic high (greater than 3.5 VDC).
The mute state is controlled by the microcontroller U4 pin 11. LSA-2 does not use the class-G switching
transistors (Q154, Q155, Q254, Q255) and associated drive circuitry. Instead, they are jumpered out by
jumpers JP1, JP2, JP3, JP4). It also does not use power diodes D152, D153, D252, D253.
LSA-3 uses the higher voltage TDA7293HS (120V) power amplifier in a class-G configuration with switching
transistors and diodes mentioned above. At low volume, the amplifier runs off the lower set of voltage rails,
supplied through D152 and D153 (left channel). At a specific threshold, depending on the amplitude of the
audio signal, diode D150 conducts and turns on a string of transistors (Q151, Q153, Q154 on the positive half
cycle, Q150, Q152, Q155 on the negative half cycle) that increases the supply voltage at IC pins 13 and 15.
When the voltage at these pins increase, diodes D152 and D153 become reverse biased and the low voltage
supply is shut off and the amplifier draws current only from the higher voltage supply. Overall this operation
results in higher efficiency, because most of the time the amplifier IC is operating from the low voltage supply
rails.
Thermal Protection:
There are two forms of thermal protection employed. One relates to the temperature of the bridge rectifier(s),
the other to the temperature of the enclosure.
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SA-2 and SA-3 Theory of Operation
The enclosure temperature is monitored by thermistor RT3, located close to the screw hole between the power
amplifiers. When the temperature exceeds 60 degrees C, transistor Q7 begins to conduct and this slowly
reduces the voltage at pin 12 of comparator U6. This has the effect of making the compressor circuit activate
more frequently, at a lower threshold voltage, and the maximum output power is reduced.
The bridge rectifier (BR1) temperature is monitored by NTC (negative temperature coefficient) thermistor RT2.
As the temperature of RT2 increases, the voltage at the collector of Q6 increases. This voltage is present at
the A/D input of microcontroller U4 pin 9. At 110 deg C or greater, the voltage at U4 pin 9 exceeds 3 volts and
the microcontroller will decrease volume by 1 dB every 20 seconds. If the voltage exceeds 4 volts (fault
condition) the microcontroller will turn the unit OFF into standby mode. After 1 minute, if the voltage is less than
4 volts, the unit will turn back ON.
On LSA-3 there is a second bridge rectifier, BR2, monitored by thermistor RT1. The operation of the thermal
protection is the same as BR1 described above.
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