BOSE AM-3P I Schematic

ACOUSTIMASS®-3 POWERED

SPEAKER SYSTEM

©

1992 Bose Corporation Service Manual

TABLE OF CONTENTS

Note: This service manual has been updated with information from service manual supplement
175032-S2. This supplement affects part number for R146 and R246 as used on the amplifier PCB.
Refer to 175032-S2 for more information.

1. SAFETY INFORMATION.......................................................................................... 2

2. SPECIFICATIONS.................................................................................................... 3

3. BLOCK DIAGRAMS.................................................................................................. 5

4. THEORY OF OPERATION....................................................................................... 6

5. DISASSEMBLY/ASSEMBLY PROCEDURES......................................................... 13

6. TEST PROCEDURES.............................................................................................. 19

7. PARTS LISTS AND EXPLODED VIEWS................................................................. 23

8. SCHEMATICS AND PCB LAYOUTS....................................................................... 39

CAUTION: THE ACOUSTIMASS®-3 POWERED SPEAKER SYSTEM
CONTAINS NO USER-SERVICEABLE PARTS. TO PREVENT
WARRANTY INFRACTIONS, REFER SERVICING TO WARRANTY
SERVICE STATIONS OR FACTORY SERVICE.

PROPRIETARY INFORMATION

THIS DOCUMENT CONTAINS PROPRIETARY INFORMATION OF
BOSE

®

CORPORATION WHICH IS BEING FURNISHED ONLY FOR
THE PURPOSE OF SERVICING THE IDENTIFIED BOSE PRODUCT
BY AN AUTHORIZED BOSE SERVICE CENTER OR OWNER OF THE
BOSE PRODUCT, AND SHALL NOT BE REPRODUCED OR USED
FOR ANY OTHER PURPOSE.

1

SAFETY INFORMATION

1. Parts that have special safety characteristics are identified by the symbol on
schematics or by special notes on the parts list. Use only replacement parts that
have critical characteristics recommended by the manufacturer.

2. Make leakage current or resistance measurements to determine that exposed
parts are acceptably insulated from the supply circuit before returning the unit
to the customer. Use the following checks to perform these measurements:

A. Leakage Current Hot Check-With the unit completely reassembled, plug
the AC line cord directly into a 120V AC outlet.(Do not use an isolation
transformer during this test.) Use a leakage current tester or a metering
system that complies with American National Standards Institute (ANSI)
C101.1 "Leakage Current for Appliances" and Underwriters Laboratories
(UL) 1492 (71). With the unit AC switch first in the ON position and then in
OFF position, measure from a known earth ground (metal waterpipe,
conduit,etc.) to all exposed metal parts of the unit (antennas,handle bracket,
metal cabinet,screwheads,metallic overlays,control shafts,etc.), especially
any exposed metal parts that offer an electrical return path to the chassis.
Any current measured must not exceed 0.5 milliamp. Reverse the unit power
cord plug in the outlet and repeat test. ANY MEASUREMENTS NOT WITHIN
THE LIMITS SPECIFIED HEREIN INDICATE A POTENTIAL SHOCK HAZARD
THAT MUST BE ELIMINATED BEFORE RETURNING THE UNIT TO THE
CUSTOMER.

B. Insulation Resistance Test Cold Check-(1) Unplug the power supply and
connect a jumper wire between the two prongs of the plug.(2)Turn on the power
switch of the unit. (3) Measure the resistance with an ohmmeter between the
jumpered AC plug and each exposed metallic cabinet part on the unit. When the
exposed metallic part has a return path to the chassis, the reading should be
between 1 and 5.2 Megohms. When there is no return path to the chassis, the
reading must be "infinite". If it is not within the limits specified, there is the poss­ibility of a shock hazard, and the unit must be repaired and rechecked before it
is returned to the customer.

2

SPECIFICATIONS

Dimensions Module: 7.75"H x 18.5"W x 8.5"D (20x47x22 cm)

Satellite: 4.5"H x 4.5"W x 3.5" D (11x11x9 cm)

Weight Module: 21 lbs. (9.5 kg.)

Satellite: 1.3 lbs. (0.6 kg.)

Packed System: 27 lbs. (12.3 kg.)

Driver 1-5-1/4" Woofer
Complement 2-2-1/4" Twiddlers

Finish Module: Black or white,scratch-resistant,satin-finished

vinyl

Satellites: Black or white,painted polymer finish

PERFORMANCE SPECIFICATIONS

Amplifier Power Woofer (single): 50 watts into 5Ω resistive load,

≤0.2% THD, from 40-200 Hz

Input Impedance System: 5.9kΩ ± 4%

Distortion @ 1W <0.1% THD (all channels
Output Noise <180µVrms A weighted for L/R channels
Output Hum <500µVrms Unweighted for bass channel
Channel @1kHz >40 dB

Separation @10 kHz >30 dB
Port Tuning Small Chamber: 110 Hz

Frequency Large Chamber: 43 Hz
Input Sensitivity 0.6 Vrms input @1kHz produces rated power in L/R channel

Turn-On Delay 80ms ± 40 ms

(Auto)

Satellites (each): 20 watts into 4Ω resistive load,

≤0.2% THD, from 200Hz-15kHz

Satellites: 4Ω
Woofer: 5Ω

outputs (at full volume)

Turn-Off Delay 100 sec. ± 20 sec.
(Auto)

Acoustic Output: 97 dB SPL in IEC standard room @ 50W

3

Frequency Response Curves

Figure 1. Bass Channel Frequency Response Curve

Figure 2. Left/Right Channel Frequency Response Curve

4

BLOCK DIAGRAM

Figure 3. AM-3P Amplifier PCB Assembly Block Diagram

Figure 4. AM-3P Equalizer PCB Assembly Block Diagram

5

AM-3P THEORY OF OPERATION

NOTE: The following discussion references the AM-3P EQ and Amplifier schematics and

block diagrams. The block diagrams,Figures 3 and 4, can be found on page 5 and the
schematics are located in the back of this service manual.

GENERAL

The AM-3P powered Acoustimass loudspeaker system is designed to be used with the BOSE
Lifestyle Music Center (or other line level audio source) to form a simple, yet complete, home
audio system. The AM-3P is based acoustically on the AM-3 Acoustimass (unpowered)
loudspeaker system. The AM-3P, like its big brother, the AM-5P, offers many advantages over
a separately powered AM-3 system. These include:

- Automatic turn-on/ turn-off (mute) of the amplifier output stage

- Automatic (BOSE patented) dynamic equalization

- Bi-amplification for better power distribution to speakers

- Active equalization for smoother frequency response

- Amplifier short-circuit and DC offset fault protection

- Local volume/sensitivity control

- Bass/treble room compensation controls

- Differential input stage (to reject hum)

- Dynamic compressor to prevent amplifier output overload distortion

BLOCK DIAGRAM DESCRIPTION

NOTE: In the discussion of L/R (left/right) channels,only the right channel is discussed. The left

channel operation is identical.

1. Power Supply

A single, universal, 115/230V EI core power transformer is used to power the system. It has
been specially designed for minimum magnetic flux leakage and stand-by power consumption.
The transformer primary remains energized (always on) except when the power switch is in the
off position.

The two primary windings of the transformer are wired in series for 230V operation, and in
parallel for 115V operation, depending on the position of the customer accessible voltage
select switch. If the system is accidentally energized at 230V with the switch in the 115V
position, the replaceable fuse (F1 located on the Equalizer PCB assembly), will open the
circuit within 2 minutes. This will be the only damage to the system.

6

A non-serviceable thermal fuse is located inside the transformer to limit the maximum
temperature to 145 °C under abnormal conditions. However, in most cases the external fuse
(F1) should fail before the internal transformer thermal fuse. The thermal fuse can be checked
by measuring the continuity between pins 1 and 4 (black and white wire) of the primary
connector. This connector is located on the Equalizer PCB.

2. Differential Input Stage

Audio input applied at the phono jack (J8) connects directly to the right (left) differential amplifier
buffer. This stage accomplishes several functions:

- Provides rejection of common-mode interference (such as hum) that might be picked up
by the long audio cable between the AM-3P and lifestyle music center

- Provides input overload and ESD protection through diodes (D101-104)

- Removes radio frequency Interference through C103,125,127

- Provides DC blockage by C101,102

- Provides a 6 dB gain through op-amp U101.

3. Compressor/Treble Stage
U102 (pins 1,2,3), with potentiometer VR2 at the center position, operates as a conventional

inverting operational amplifier stage with a gain of 6 dB. However, when voltage overload is
present at the speaker outputs, transistor array U1 (pins 1,2,3) receives a turn-on signal from
the compressor detect stage. Under this condition, the transistor acts as a variable resistor
to ground, and with R107, forms a resistor divider network that attenuates the audio signal.
The amount of attenuation and timing action is carefully controlled by the compressor detect
stage (to be discussed later).

Potentiometer VR2 adjusts the amount of treble boost or cut. In the full counterclockwise
position, the wiper (pin 4) shorts to pin 1. The maximum boost and cut frequency response
graph is shown in Figure 2 on page 4.

4. High Frequency (HF) Equalization

High frequency equalization and crossover is achieved through three cascaded active EQ
stages. These stages consist of U103,U104 and U105. The purpose of equalization is two­fold:

- It provides very sharp low frequency attenuation below 200 Hz (the crossover frequency)
to prevent overload of the L/R speakers

- It provides finely tailored correction in the speaker pass-band so that overall frequency
response of the system is acoustically correct.

7

The U103 stage is a combination 2nd order 200 Hz high-pass filter and 5 dB 2 kHz dip (notch)
filter. The U104 stage provides the 5 dB peak at 200 Hz and the sharp band-reject between
100 and 200 Hz. Finally, the U105 stage is a combination 2

nd

order 20 kHz low-pass filter and

2 dB 350 Hz notch filter.

5. Bass Control

The bass channel signal is derived from the sum of the left and right channels through resistors
R129 and R229 and through inverting op-amp summer U2. Potentiometer VR3 simply adjusts
the gain of the stage between -6 dB to +6 dB. In the flat (center) position of the control ,the gain
is 0 dB. Adjusting the bass by varying the gain of the overall bass channel has been determined
to be acoustically appropriate to compensate for various placement options of the bass module.

6. Automatic Dynamic Loudness

In order to compensate for the ear’s loss of bass response at low listening levels, a BOSE
patented (4,739,513) automatic loudness circuit is employed. This circuit automatically senses
the volume level of the incoming audio signal and properly adjusts the amount of low frequency
bass boost. When the volume level is high, the frequency response of the circuit is flat. When
the volume level is decreased (at the music center), the low frequency gain is increased in the
region between 50 and 150 Hz (see the bass channel frequency response graph on page 4).
This loudness contour, and the time constants associated with it, have been precisely
determined through psycho-acoustic testing and should not be confused with other conven­tional loudness schemes.

Circuit operation is as follows: Left and right audio output at pins 7 and 1 of U101 are summed
together by resistors R132 and R232. The AC signal at pin 3 of level detect op-amp U2 is
negative-peak detected and is presented across emitter resistor R9. The peak detected signal
has a (approximate) 4 second hold time and a 4 sec/10 dB release time constant which is
developed within the feedback loop of U2 (pins 1,2,3). The voltage across R9 is converted to
a current source through transistor Q3, and flows into pin 5 of transconductance amplifier U4.
U4 is placed in the feedback loop of op-amp U3 to create an inverse voltage controlled
bandpass stage such that increased current into pin 5 of U4 causes the gain of the band-pass
stage to decrease. The 55 Hz band-pass filter consists of resistors R10,11 and 15 and
capacitors C6 and C7. At frequencies above 200 Hz, the stage acts as a unity gain follower
(flat frequency response) through resistors R16,17, and 18. The over-all loudness contour
response is visible at U3 pin 1.

7. Low Frequency (LF) Equalization

Low frequency bass channel equalization and crossover is accomplished in two active filter
stages. The first stage consists of op-amp U3 (pin 5,6,7),capacitors C9 and 10 and resistors
R19,20, and 21. It creates the sharp band-reject attenuation below 50 Hz. The second stage
consists of op-amp U5 (pin 1,2,3,), capacitors C13 and 14, and resistors R24,25,26 and 27. It
acts as a combination 2

nd

order 50 Hz high-pass and 200 Hz low-pass filter. The combined

response of these two stages is shown in the bass channel frequency response graph in Figure

1 on page 4.

8

8. Auto Turn-On/Off Mute

An improved version of auto turn-on/off is used in the AM-3P. A separate 3.5 mm stereo mini­jack (J7) is used to receive the turn-on signal from the serial data output of the Lifestyle music
center. When the 3.5 mm plug is inserted, the audio sense circuit is disabled and the amplifier
outputs will only unmute if the DC control signal is present at J7 pin 3. This action occurs
through the built-in DPDT switch in J7 which connects J7 pin 3 directly to the Q9 mute circuit.
Therefore, the system is not subject to random false turn-on (from electrical noise, etc.) when
used with the Lifestyle music center.

When the AM-3P is used with a source other than the Lifestyle music center,the stereo mini­plug is not inserted into the jack, and therefore mute transistor Q9 is directed to the output of
the audio detect comparator U6 (pin 5,6,7). This comparator uses hysteresis to achieve a
positive turn-on of at least 90 seconds before the system will revert to stand-by (mute) mode
after removal of the audio signal.

9. Compressor Detect

The output of each left/right amplifier is half-wave detected through diodes D108 and 208. This
voltage is divided down by R53 and R54 and presented at the emitter of Q12. Power supply
voltage is similarly divided down by R50 and R51 and presented at the base of Q12. Diodes
D13 and D14 simulate the saturation voltage drop that occurs in the power amp stage.
Therefore, when the audio output of any amplifier comes within 2 volts of the instantaneous
power supply voltage, Q12 will conduct.This signals the compressor (limiter) to act. Current
from Q12 charges hold capacitor C35 and releases capacitor C36. Voltage at C35 is buffered
by transistor U1 (pins 5,6,7) and decreased down to a logarithmic voltage by resistor R67 and
diode U1 (pins 12,13,14). This voltage is then presented to the base of compressor transistors
U1 (pins 1,2,3) and U1 (pins 8,9,10) which are described in the compressor section.

The DC control voltage present across capacitor C35 is derived through the use of an improved
hold and release characteristic, resulting in lower compressor distortion. In conventional
limiters, this DC control voltage is derived by a simple series diode and parallel RC network to
ground. Unfortunately, for low frequency overload, this results in substantial ripple voltage
present on the DC gain control voltage. This results in the all too common problem of mid-range
voices or instruments being badly “modulated” by low frequency overload. This problem is
solved by having two separate capacitors, one that “holds” the smooth DC voltage just long
enough to prevent low frequency ripple (approximately 40ms) and the other (C36) which quickly
releases the hold capacitor (through diode D17) at the proper time to maintain overall 100ms
release time.

9

10. Power Amp (Amplifier PCB)

Four identical topology class B, power amplifier stages are used to create the three power
amplifier channels. One each is used for the left and right power amplifiers (2 x 20W into 4
Ohm), and two are used in bridged mode for the bass channel to create 50W into 5 Ohm.
Operation of the left channel (representative) is as follows:

A positive signal at op-amp input U106 pin 3 causes op-amp supply current to flow into pin
7 (and out through pin 6) through Q103. Q103 is used simply to buffer the +20V high voltage
supply down to a safe +7.4V for the op-amp. This current also flows through R132 and diode
D105. R132 and D105, together with driver transistor Q105 and R134 form a current mirror
which multiplies the current through Q103 by the ratio R132/R134. This increased current flows
through driver transistor Q105, and into the base of output transistor Q107. The hFE current
gain of output transistor Q107 (about 100) provides final current amplification before reaching
the speaker output.

The power amplifier topology is complementary.Therefore,operation of the amplifier on
negative half cycles is identical to positive cycle operation, except for component reference
designations.

11. Short-Circuit Protection

Left and right power amp stages have short circuit protection. This is necessary to prevent
damage to the amplifier from customer misuse that might occur, for instance, if the customer
were to accidentally short the speaker cable wires that connect to the cube speakers. If this
happens, all three amplifiers will momentarily mute for about 3-4 seconds and continue to mute
until the abnormal condition is removed.

Short circuit protection is achieved with the use of a .1

Ω resistor (R146 for left channel) in series

with each amplifier output. Under normal conditions, peak current to the 8 Ω speakers should
never exceed 3.5A. If current flow momentarily exceeds 5.0A, then Q109 will conduct and
trigger monostable multivibrator circuit U5 (pins 5,6,7)on the Equalizer PCB. Pin 7 will go high
(for about 3 seconds) and through diode D12 will trigger mute transistor Q10. Turn-on of Q10
causes all three amplifiers to mute. Referring to the left channel power amplifier, actual muting
occurs by turning on transistors Q110, 101, and 102 which cause both driver transistors (Q105
and Q106) to turn off. When the driver transistors are off, there can be no current flow through
the output transistors, which results in no current flow to the speaker.

10

12. DC Offset Protection

If any one of the three amplifier channels should fail, it is likely that a large DC voltage will occur
at the output of that channel. If this occurs, the DC offset detect circuit will cause the AM-3P
amplifier power supply to shut down. It will remain shut-down until the unit is unplugged from
the power source. This provides a safe indication to the customer that the unit requires service.

Detailed operation is as follows: The amplifier outputs are summed by resistors R149,249,318,
and 418, and (low pass) filtered by capacitor C29. For a sustained DC offset, C29 will charge
either negative or positive. A positive offset will trigger Q7 and a negative offset will trigger Q8.
This in turn causes Q6 to conduct and latches Q5 permanently on. Q5 then shorts the emitter
of Q4 to ground (off). Q4 is the transistor that controls turn-on of 2P1T relay K1 (on the Amp
PCB) that is used to connect +/- 20V supply rails to the power amplifiers. Thus, the amplifiers
are de-energized.

11

NOTES FOR FUTURE REFERENCE

12