Bose FREESPACE BUSINESS User manual

©1993 Bose Corporation Service Manual
FREESPACE™BUSINESS
MUSIC SYSTEM

1
TABLE OF CONTENTS
1. SAFETY INFORMATION.......................................................................................... 2
2. SPECIFICATIONS.................................................................................................... 3
3. BLOCK DIAGRAM.................................................................................................... 6
4. THEORY OF OPERATION....................................................................................... 7
5. DISASSEMBLY/ASSEMBLY PROCEDURES......................................................... 13
6. TROUBLESHOOTING TIP FOR HUM AND RFI REMOVAL................................... 18
7. TEST PROCEDURES.............................................................................................. 19
8. PARTS LISTS AND EXPLODED VIEWS................................................................. 23
9. SCHEMATICS AND PCB LAYOUTS....................................................................... 47
CAUTION: THE FREESPACE™ BUSINESS MUSIC 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.

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 poss-
ibility of a shock hazard, and the unit must be repaired and rechecked before it
is returned to the customer.
2

SPECIFICATIONS
Dimensions Module: 9.1"H x 12.6"W x 20.1"D (23.1x32x51.05 cm)
Satellite: 3.75"H x 4.5"W x 4.5" D (9.5x11.4x11.4 cm)
Weight Module: 37 lbs. (16.8 kg.)
Satellite: 1.24 lbs. (0.56 kg.)
Driver 1-8" Woofer
Complement 4-2.24" Twiddlers™
Finish Module: Black or white,scratch-resistant,satin-finished
vinyl
Satellites: Black or white,painted polymer finish
PERFORMANCE SPECIFICATIONS
Amplifier Topology Class G (High efficiency)
Amplifier Power Woofer channel (single): 100 watts min. into 4Ω@100Hz
Satellite channels (each): 50 watts min. into 8Ω@ 1 kHz
Output Short Circuit Current Threshold: 5 ±0.2 amperes
Protection Duration: Infinite
Input Impedance System: Balanced: 13.0 kΩ
Unbalanced: 26.5 kΩ
Satellites: 4Ω
Woofer: 4Ω
Distortion Bass Channel @ 100 W,100 Hz <0.1% THD typical
L/R Channels @ 50 W,1 kHz <0.1% THD typical
Output Noise Bass Channel (80 kHz LPF,inputs shorted) <1 mV
L/R Channels (A weighted,inputs shorted) <300µVrms
Output Hum Bass Channel <500µVrms (unweighted)
Compressor Attack time: 10 ±5 ms
Release time: 80 ±20 ms
Attenuation Range: 12 dB
Channel @1kHz >40 dB
Separation @10 kHz >30 dB
3

4
PERFORMANCE SPECIFICATIONS (Continued)
Dynamic 0-16 dB variable boost @ 55 Hz (level dependent)
Equalization
Crossover Transition 180 Hz
Frequency
Input Sensitivity 0.4 Vrms @1kHz produces rated
power in L/R channel
outputs (at full volume)
Turn-On Delay 80ms ±40 ms
(Auto)
Turn-on 300 µVrms ±100 µVrms
Threshold
Turn-Off Delay 90 sec. ±30 sec.
(Auto)
Acoustic Output 95 dB SPL in IEC standard room @ 50W
Power Consump- 350 W max.
tion
Standby Power 8 W max.
Consumption
DC Offset Bass Channel,L/R Channels <70mVDC
(with audio inputs shorted)

5
Figure 1. Bass Channel Frequency Response Curve
Figure 2. Left/Right Channel Frequency Response Curve
Frequency Response Curves
Bass channel frequency response curve with tone controls at min.,
max., and flat positions (for 30 and 200 mVrms input level)
Left/right channel frequency response with tone controls at min.,
max.,and flat positions.

6
BLOCK DIAGRAM
Figure 3. Main PCB (Amplifier) Block Diagram
MAIN

7
THEORY OF OPERATION
The following discussion references the Main PCB Schematic and Block Diagram. The
schematic is located in the back of this manual and the block diagram is shown in Figure 3.
GENERAL
The FreeSpace™business music system electronics module powers the FreeSpace loud-
speakersystem,whichisaderivativeoftheAM-5®poweredloudspeakersystem. Asapowered
system, FreeSpace is meant to be used in conjunction with the customer's audio source.
The FreeSpace system offers the following features:
- Automatic turn-on/ turn-off (audio sense or signal from the customer's audio source)
- Automatic (Bose®patented) dynamic equalization
- Bi-amplification for better power distribution to speakers
- High Efficiency Class G Power Amp stages (100W + 50W +50W)
- Active equalization for smoother frequency response
- Amplifier short-circuit protection
- Bass/treble room compensation controls
- Differential input stage (to reject hum)
- Dynamic compressor to prevent amplifier output overload distortion
- Electronic remote volume
BLOCK DIAGRAM DESCRIPTION
Refer to the block diagram and schematic mentioned above. For discussion of L/R channels,
only the right channel is discussed. Left channel operation is identical.
1. Power Supply
TwopowersuppliesareusedintheFreeSpacesystem. Oneisalowpowerregulated±15VDC
supply used to power the signal processing circuitry, the other is the high voltage main power
supplyusedtopowerthethree Class G power amplifier stages. Two voltagesareprovidedfor
class G operation, ±20VDC and ±40VDC. The regulated ±15VDC supply always remains
energized as long as the unit is wired to an active mains voltage source. The main power
amplifier supply is turned on and off by the audio sensing circuit (discussed elsewhere).

8
2. Differential Amplifier
AudioinputisatbarrierstripP5,whichislocatedontheI/OPCBAssembly(seetheschematic
located in the back of this manual). This connects directly to the left and right differential
amplifierbuffersontheMainPCB. Thisstageneatlyaccomplishesseveralfunctions: 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
overloadprotectionthroughdiodes(D101-104); stripsoffradiofrequencyinterferencethrough
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(pins5,6,and7),undernormalaudioconditions,operatesasaconventionalinvertingop-
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
stageU203(pins1,2,and3),andispresentedatthepin1inputofturn-onchipU204(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.
Withoutaudioinput,U204pin7willgolowafterapproximately90seconds,andwillde-energize
relay K1 and the main supply. This turn-off time is set by capacitor C217 .
5. Tone /Electronic Volume Control
AseparatetonecontrolPCBattachestotheMainPCBthroughasevenconductorribboncable
connector. Power is derived from the main board. The tone control is a standard Baxandall
circuitwith+/-6dBbass and treble controls. Volume control function isaccomplishedthrough
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
potentiometerthatindirectly attachesacrossconnectorJ502 throughtheI/OPCBbarrier strip
terminal P5,pins 7 and 8. At minimum volume (0Ωbetween J502,pins 1 and 2),the voltage at
the base of Q502 should be .36 ±.1 VDC. At maximum volume (10 KΩbetween 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.

9
6. High Frequency Equalization
HighfrequencyequalizationisachievedthroughfourcascadedactiveEQstagesconsisting of
U101,U102,U103andU104.Thepurposeofequalizationistwo-fold. (1)Itprovidesverysharp
low frequency roll-off below 180 Hz (the crossover frequency of the acoustics) to prevent
overload of the L/R speakers; and (2) It provides finely tailored correction in the L/R speaker
pass band so that overall frequency response of the system is correct.
7. Automatic Dynamic Loudness
In order to compensate for the ear’s loss of bass response at low listening levels, a Bose®
patentedautomaticloudnesscircuitisemployed. Thiscircuitautomaticallysensesthevolume
leveloftheincomingaudiosignal,andproperlyadjuststheamountoflowfrequencybassboost.
When the volume level is high, the frequency response of the circuit is flat. When the volume
level is decreased (at the customer's audio source or through the electronic volume control),
the low frequency gain is increased in the region between 50 and 150 Hz.
Circuitoperationisasfollows: AfterpassingthroughtheToneControlPCB,audioatU201and
U202(pin 7) is summed byresistors R101and R131. The AC signalat pin5 of level detect op-
ampU203isnegative-peakdetected,andispresentedacrossemitterresistorR236. Thepeak
detected signal has a 4 second hold time, and a 4 sec / 10 dB release time constant which is
developed within the feedback loop of U203 (pins 5,6 and 7). The voltage across R236 is
convertedtoacurrentsourcethroughtransistorQ203,andflowsintopin5oftransconductance
amp U206. U206 and U207 (pins 1,2 and 3) form a variable gain stage (delta-G). Action is
such that increased current into pin 5 of U206 causes gain of the stage to decrease.
The variable gain stage is followed by a 55 Hz, Q=2.3 band pass filter (U207 pins 5,6 and 7).
TheoutputofthisstageissummedthroughresistorR247withtheflatresponse signal present
atresistorR248.Thiscreates acompositefrequencyresponsecurvewithvaryinglevelsof55
Hz boost. The overall desired loudness contour response is visible at U208 pin 1.
8. Low Frequency Equalization
A single stage Sallen-Key 2nd order high-pass / low pass network (U208 pins 5,6 and 7)
provides bass channel equalization and low frequency roll-off to prevent subsonic low
frequency overload.
9. Power Amplifier
Allthree poweramplifiersareofidentical topologyandusehighefficiency ClassG poweramp
stages. The L/R amplifier channels are both rated at 50W. The bass channel is 100W, and
because of its higher power, uses higher current output transistors and slightly different
component values. Otherwise,operation is the same.