Audio Pro B2-50 Service Manual

SERVICE MANUAL

FOR

SUBWOOFER B2-50

Audio Pro AB

LIST OF CONTENTS

1. TECHNICAL DESCRIPTION

1.1 Power Supply

1.2 Automatic ON/OFF

1.3 Input Stage

1.4 Filters

1.5 Ace-Bass Amplifier

1.6 Power Amplifier

2. TROUBLE SHOOTING TIPS

2.1 B2-50 Does Not Work

2.2 Fuses F501, F502 Blow

2.3 Automatic ON/OFF Faulty

2.4 B2-50 Goes On and Off Repeatedly with no signal in.

3. ADJUSTMENTS

3.1 Bias Adjustment in Power Amplifier

3.2 Negative Output Resistance (Rs) in Ace-Bass Amplifier

4. SPARE PARTS

5. ENCLOSURES

5.1 Circuit Diagram 2-125010

5.2 Component Layout 3-125004

1. TECHNICAL DESCRIPTION

The theoretical Background for the Ace-Bass principle used in
Subwoofer B2-50 is described in AES Preprint #1381.

The functions described below are separated by dotted lines
on the Circuit Diagram.

1.1 Power Supply

The primary of the power transformer is always connected
to the power cord via a voltage selector and a power fuse (5.0A
slow at 117 V, 2.5A slow at 220-250 V). The idle current in this
transformer is very low (approx. 25mA at 117 V supply).

The center tapped secondary winding supplies the Automatic
ON/OFF via two diodes D201, D202, and a double rectifier D101
via a DPST switch K101. This switch is operated by a relay in
the Automatic ON/OFF circuits.

Rectifier D101 supplies +- 40 V to the Power Amplifier, and also
stabilized +- 15 V via voItage regulators, V101, V103.

It is very important that the -15 V tracks the +15 V accurately
when power is switched on or off. Mistracking can cause transients
or oscillations in the electronic circuits.

To maintain tracking of the +15 V and -15 V when power is
disconnected, the +40 V must decay faster than the -40 V.
For this reason the Power Amplifier is built to always maintain
a small positive offset voltage (10-100mV) on its output terminals
under operating conditions.

If the B2-50 is tested without any load on the Power Amplifier,
or with fuses F501, F502 removed, the balance between +15 V and

-15 V may be upset during on/off switching. In such cases the
operational amplifiers may start oscillating when power is dis-
connected, so that Automatic ON is actuated. This is normal under
these conditions, and the electronics will not be damaged.

1.2 Automatic ON/OFF

This circuit is powered via diodes D201, D202 and capacitor C201.

The power transformer and the Automatic ON/OFF circuit are powered
at all times when B2-50 is plugged in to a live outlet.
The other circuits in B2-50 are powered only when relay K101 is
actuated.

The audio signal on left channel is sensed by operational amplifier
Z201 (3, 2, 1) which acts as an amplifier with gain 5,000 at 500Hz.
The amplified voltage (point H) is limited to +7.0 V and -0,7 V
by zener diode D208.
Operational amplifier Z201 (6, 5, 7) and transistor V201 work
as a Schmitt trigger with ON level 7 V and OFF level 5 V. The
input (pin 5) senses the voltage from Z201 (3, 2, 1) after
rectification in D209 and capacitor C206.

Transistor V201 drives relay K101 and a red LED indicating "ON".
When toggle switch 5201 is in position "AUTO", relay K101 is
de-energized about 5 minutes after the audio signal on left
channel input disappears. The time delay is determined by C206
and R210. (During testing R210 may be shunted by 100 kohm, so
delay time is reduced).

When toggle switch 5201 is in position "ON" the sensing circuits
are bypassed, and relay K101 is energized as soon as B2-50 is
plugged in to a live outlet.

Temperature sensor TS3-85 senses the temperature of the Power
Amplifier heatsink. If this temperature becomes too high, the
resistance in TS3-85 drops to a low value, and relay K101 is de­energized.

1.3 Input Stage

The audio signals on DIN plug J303 pin 1 (left channel) and
pin 4 (right channel) pass through variable high pass filters
formed by C303, R315, R317 and C304 R316, R317 and buffer
amplifiers Z301 (5, 6, 7) and Z301 (3 ,2, 1) to DIN plug J303
pins 3 and 5. From pins 3 and 5, they can be returned to an
external power amplifier driving the satellite speakers. The
buffer amplifiers provide low output impedance, so cable capaci­tances will not affect the treble response.

The audio signals on DIN plug J303 pins 1 and 4, or alternatively
on speaker plugs J301, J304, are summed by resistors R318, R319
into a first active low pass filter with kneepoint 138Hz and
8dB gain at 100Hz (when channels are in parallel).

The signal from the first low pass filter is passed via a
volume control R322 to a second amplifier stage Z301 (10, 9 ,8)
with 17dB gain.

1.4 Filter

The signal from Z301 pin 8 (test point C) passes a variable low
pass filter including R401, R402, R403, C401, C402 and Z401 (10,
9, 8), and then on to a fixed high pass filter with kneepoint

22.8Hz. The output terminal is test point (E).

Remaining components in the filter circuits are part of the
Ace-Bass amplifier, and is described in section 1.5.