Page 1
SA-2 and SA-3 Theory of Operation
At full volume the freque ncy response of the syst em is basica lly flat, over the frequency range from 10 Hz to
15 kHz. At l ower volume levels, the dynamic equalizer prov ides up to 16 dB of bass boost at 50 Hz. A graph of
frequency response vs. volume is sh own in Figure 2.
Microcontrol ler:
Control of the product occurs via a serial data lin k (BoseLink) from the music cen ter, communicated through
one of the wires of the D IN cable. Th e microcontroller automatically detect s whether it is conne cte d to an
existing (CD-5, CD-20, MRI, AV28) or new (AV1 8, AV38) mu sic cent er, b y recognizin g either the original (1-
way) smart speaker prot ocol, or new er 2-way smart speaker BoseLink pr otocol.
In addition t o communicating back a nd forth with the music center, the microcontroller does the fol lowing:
Determines power mode (Standby or ON).
Sets volume level and Mute.
Reads DIP switch sett ings (for room address, et c)
Detects pre sence of au dio at RCA input jacks ( on SA-3).
Detects pre sence of external potenti ometer (at J6 input).
Monitors temperature of power supply bridge rectifier
Monitors power supply voltage
Detailed Theory of Ope ration:
Power Supp ly:
The primary side of the power supply consists of the fuse in series with t riac switch Q1 in series with the
primary coil of the transf ormer. In parallel with th e triac is a l arge film capacitor (C11 or C27) and resistor (R1 0)
which provides a small t rickle curre nt to the tra nsfor mer wh en the triac i s off (open circuit). Thi s small amount
of current is enough to keep the microcontroller ( U4) powere d. This mode is cal led Standby . Wh en an ON
command is received via BoseLink, from the mu sic cent er, p in 10 of the micro goes t o logic hig h and activates
opto-coupler triac driver U7 which then turns on t riac Q1, and the power supply becomes fully energized.
The transfor mer secondary power s upply is comprised of bri dge rectifier BR1 and electrolytic filter capacitor s
C7 and C10. The voltage for LSA-2 is nominally about 36 VDC, and for LSA\
-3 it is ab out 48 VDC. LSA-3 uses
a 2 nd set of power supply rails from BR2 and electrolytic cap acitors C8 a nd C9 at about 24 VDC. These parts
are not used on LSA-2.
The low voltage electron ics are pow ered by +8 VDC and -“8 VDC voltage regulators U8 and U9 (respectively).
For LSA-2 the input of t he regulator s are conne cted to the BR1 power supply through diodes D 6 and D7. On
LSA-3 these diodes are not used, an d the regulat ors are conn ected direct ly to the BR2 supply. In Standby
mode, the +8 VDC and -“8 VDC supply rails are turned OFF by transistor s 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 po wered by a 5 VDC regul ator (U10). The input of this regulato r is conne cted to the
output of a l ow drop-out discrete 26 VDC regulator, comprised of transist ors Q10, Q1 1, and Q12. In Standby
mode this voltage is mu ch less, usu ally between 9 and 16 VDC.
Differential I nput Am plifier:
Channel 1 and 2 inputs are applied t o unity gain differential a mplifier U1. The differen tial amplifier rejects
common-mo de noise tha t may be picked up on th e long DIN cable wires.
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Page 2
SA-2 and SA-3 Theory of Operation
Volu me Con trol:
Local volume control co nnection ja cks (J6). All ows you to control the volume in a room using a wall-mounted
control. Curr ently the volume control adjustment i s not availa ble to end consumers. Contact your Bose® dealer
or call Bose Customer Service for inf ormation on install ing thi s feature. See the conta ct list insi de the back
cover of the Owners Gui de. The out put of U1 is AC coupled to the volume control ch ip U3. This is an analog
resistor lad der type vol ume control, incremented in steps of 1 dB. This I C is controll ed by I2C b us, Data and
Clock line s ( pins 6 and 7 ) from the microcontrol ler (U 4). At vo lume levels below -“20 dB (the volume control has
a range between -“90 and 0 dB of gain); dynami c EQ act ion (bass boost) occurs from the op-amp circuit
connected t o pins 12 an d 19. This ci rcuit simulat es a parallel resonant RLC circuit at about 50 Hz that provides
progressivel y greater relative boost to frequencie s below 200 Hz, as the volume level decreases.
Co mpressor:
The output of the volume control i s AC coupled th rough C104 and C204 to amplifier/compressor stage U6
(pins 1,2,3 a nd 8,9,10). This is a diff erential inverting amplifi er with gain of 6.3 (16 d B). At normal and low
volume level , current-co ntrolled amplifier U5 is tu rned OFF. At high (clip ping) volume l evel transist or Q4 turns
ON and supplies current to U5, which reduces th e gain of the circuit.
Specifical ly, if either the l eft or right speaker outp ut reaches a positive voltage amplitude within 3 volts of the
positive power supply rail, then comparator output U6 pin 14 switches lo w and charges capacitor s C18 and
C19 to a negative voltage level. Thi s v oltage is converted to a current at the colle cto r of Q4, which then
increases the gain of U5, and decrea ses the overall gain at th e output of U6 pins 1 (l eft) and 8 (ri ght). C18 is a
hold capacitor. Once charged, its voltage stays const ant for at least 20 ms, until C19 discharge s (becomes
more positive) enough to turn on dio de D12. C19 st arts di scharging immediately, through R38, when the
speaker out put voltage is less than t he clipping level.
Power Amplifier:
The LSA-2 uses the TD A7294 (100V) power amplifier. It is a class AB linear amplifi er IC. The amplifier is
muted when pins 9 and 10 are at log ic low. It i s unmuted whe n they are at logic high (greater than 3.5 VDC).
The mute state is contro lled by the microcontrol ler U4 pin 1 1. LSA-2 does not use t he class-G switching
transistors (Q154, Q155 , Q254, Q25 5) and asso ciated 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 conf iguration wit h switching
transistors a nd diodes mentioned ab ove. At low volume, t he amplifier runs off the low er set of voltage rails,
supplied thr ough D152 and D153 (left channel). At a specifi c threshold, depending o n the amplitude of the
audio signa l, diode D150 conducts and turns on a string of tr ansistors (Q 151, Q153, Q154 on the positive half
cycle, Q150, Q152, Q15 5 on the neg ative half cycle) that in creases the supply voltage at IC pins 13 and 15.
When the voltage at the se pins i ncrease, diode s D152 and D153 become reverse biased and t he low voltage
supply is sh ut off and th e amplifier draws current only from th e higher voltage supply. Overall this operation
results in higher efficien cy, because most of the time the amplifier IC is o perating fro m the low voltage suppl y
rails.
Therm al Pro tection:
There are two forms of thermal protect ion empl oyed. One relates to the tem perature of the brid ge rectifier(s),
the other to the temperature of the e nclosure.
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