BOSE OC-1, ACOUSTIC Service Manual

OC-1 AWCS-II Option Card

®

©

1994 Bose Corporation Service Manual

TABLE OF CONTENTS

1. SPECIFICATIONS.................................................................................................... 2

2. THEORY OF OPERATION....................................................................................... 3

3. EQ CURVES............................................................................................................. 5

4. BLOCK DIAGRAM.................................................................................................... 8

5. INSTALLATION INSTRUCTIONS........................................................................... 9

6. TEST PROCEDURES.............................................................................................. 12

7. PARTS LIST ..........................................................................................................

CAUTION: THE OC-1 OPTION CARD CONTAINS NO USER SER­VICEABLE PARTS. TO PREVENT WARRANTY INFRACTIONS, REFER
SERVICING TO WARRANTY SERVICE STATIONS OR FACTORY SERVICE.

PROPRIETARY INFORMATION

16

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

OC-1 SPECIFICATIONS

Dimensions: .63" Hx1.29"Wx8.98"D(1.6x3.3x22.8 cm)
Weight: .09 lbs. (.042 kg.)

Input/Output: 2 audio inputs,Channel 1 and 2 high

frequency outputs, bass channnel output

Input Impedance: 482 kΩ (nominal)
(feedback input)

Output Level: 5.0 Vrms into 10 kΩ
Crossover Frequency: 125 Hz,Roll-off slope:

Output Noise: ≤ 40µV (A-weighted)

Channel Separation: 50 dB (min.) @ 5 kHz

24 dB/oct.

2

THEORY OF OPERATION

Overview

The OC-1 is a small plug-in module for use with Bose 502™, 402™, and 802® II controllers. The
OC-1 allows the use of the Cannon™ bass loudspeaker with these controllers and their
loudspeakers. Refer to the block diagram, equalization curves and OC-1 schematic for further
information.

The OC-1 provides three major functions:

A. Low frequency equalization.
B. Crossover filtering of the high frequency signals.
C. Protection of the Cannon against excessive power.

1. Low Frequency Equalization Circuit
U5's four op-amp stages provide low-pass crossover filtering, high-pass filtering to eliminate

subsonic material, and frequency contouring to optimize the overall response.

2. High Frequency Equalization Circuit
U3, section 1 and U5, section 2 provide two channels of high-pass filtering to remove bass from

the main loudspeakers.

3. Protection Circuit

The protection circuit consists of a compressor and a mute circuit. The protection circuit is
connected so that it can monitor the low frequency voltage applied to the Cannon driver. When this
voltage exceeds the power limit defined for the driver, a compressor begins to reduce the gain in
the low frequency path so that the power does not continue to rise.

4. Compressor Circuit

The compressor consists of the following functional blocks:

A. Differential input buffer
B. Full-wave peak detector/comparator
C. Averaging circuit
D. Voltage-controlled amplifier (VCA)

The differential input buffer (U1 section 1) features protection against Radio Frequency
Interference (RFI), Electrostatic Discharge (ESD), and overvoltage. It has a gain which is much
less than 1 so that it can attenuate the high level signals coming from the Cannon (over 40 volts at
full power).

3

U4, sections 1 and 2 form a full-wave peak detector. When the input to these stages exceeds
+/-2V peak, the outputs go high and a control voltage is created in the averaging circuit (C27, C28,
etc.). The averaging circuit captures peaks with a short attack time and holds them with a longer
release time. U2, section 2 inverts and scales the averaged control voltage and applies it to the
VCA control input.

U7 is a current-in/current-out VCA which is controlled by the voltage on pin 2.

5. Mute Circuit

The mute circuit acts as a watchdog, and prevents the Cannon™ from operating unless the
feedback signal from the Cannon is connected properly to the controller/OC-1 protection input.
When the mute circuit detects that no audio has occurred for about two minutes, it reduces the gain
in the low frequency path.

The mute circuit consists of a gain stage and a comparator. U1, section 2 amplifies the signal from
the protection circuit differential input, and U2, section 1 compares it to a reference. When the
sensed level exceeds the threshold, a control voltage (generated in U2, section 2) causes U2,
section 1 to go from high to low and reduces the compressor gain by about 40 dB.

4

Output is measured from U3 pin 7 or U6 pin 1. Controls are set at:: Mode 4 (Option),
Output Mode: Normal, Low Frequency Level: 0 dB, and Input Level: +4 dB.

Figure 1. High Frequency EQ

Output is measured from U5 pin 14. Controls are set as in Figure 1.

Figure 2. EQA + EQB

5

Output is measured from U5 pin 1. Controls are set as in Figure 1.

Figure 3. EQA + EQB + EQC

Output is measured from U5 pin 7. Controls are set as in Figure 1.

Figure 4. EQA + EQB + EQC + EQD

6