Audio Pro B2-50 User 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 de-

energized.

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 capaci-

tances 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.

1.5 Ace-Bass Circuits

The Power Amplifier together with Z401 (1, 2, 3) and Z401 (12,

13, 14) form the Ace-Bass amplifier, which can be represented

by the diagram below.

R418 and Z401 (12, 13, 14) sense the current through the drivers.

The voltage in test point (J) equals the driver current (1V/1A).

Z401 (2, 3, 1) works as an active bandpass filter, and also sums the

signals in test points (E) and (J).

The Power Amplifier is of inverting type , and also acts as

a summing amplifier, adding signals (F) and (J).

Part of the driver current signal is fed back positively through

the Power Amplifier, which gives the Power Amplifier a negative

output resistance Rs. The amount of positive feedback, and thereby,

the value of Rs, can be adjusted by trimpot Rrs.

The parallel circuit Lp, Rp, Cp in the Ace-Bass amplifier is

accomplished through negative feedback of the driver current

signal via the bandpass filter, (Z401 (2, 3, 1) ).

1.6 Power Amplifier

The input signal to the Power Amplifier is DC decoupled by

capacitor C414, and is amplified linearly to the output (test

point G).

The output signal is fed back negatively to the input via R511,

C503.

When the output signal is large, negative feedback is also

obtained via a diode - resistor network. The amplitude of the

output signal is compared to the supply voltages (+40 V) in this

network, and about 2 volts before clipping, the output signal is

rounded off by the diode network so clipping will be soft.

The differential input stage (V503, V506) is supplied via a

current generator (V504). Transistors V501, V502, V505 form

a current mirror, so the output stages from V507, V509 are

symmetrically driven.

The output stage is of triple darlington type with transistors

V512-V517. AC gain is 1. V508 senses the temperature of the

output transistors and assures constant bias current.

V510 and V511 filter the supply voltages to the input stages,

removing hum and transients.

NOTE: If there is a"click" in B2-50 when it is switched on,

the cause could be a short circuit in one of the tran-

sistors V510, V511.

2. TROUBLE SHOOTING TIPS

Use + pole of C104 or - pole of C103 as common point for

measurements of DC or AC voltages.

2.1 B2-50 Does Not Work

Set toggle switch in ON position.

2.1-1 Check that LEDs AUTO and ON are lit.

2.1-2 Check power fuse and fuses F501, F502.

2.1-3 Check that DC voltages are OK.

+40 V : 40+-2 V

-40 V : 40+-2 V

+15 V : 15+-2 V

-15 V : 15+-2 V

2.1-4 To localize the fault, connect a signal generator to

DIN plug J303, pins 1 and 4 in parallel. Set generator

to 5mV, 100Hz sine wave. Turn "sensitivity" (R322) and

"crossover frequency - subwoofer" (R402) to max.clock-

wise position. You should now measure as follows:

Test Point AC Volts

A approx. 5mV

B approx. 13mV

C approx. 95mV

D approx.90mV

E approx. 90mV

F approx.40mV

G approx.0,85V

with drivers connected

}

2.2 Fuses F501, F502 Blow

2.2-1 Disconnect drivers, and repeat test per 2.1-4. Signal

at test point (G) now should be around .5V.

2.2-2 Measure resistance of voice coils in drivers. Should

be 6.6 ohm each, 3.3 ohm in parallel.

2.3 Automatic ON/OFF

This can also be checked with the same setup as in 2.1-4.

2.3-1 Set toggle switch in position "AUTO".

2.3-2 Signal at test point (H) should be clipped and measure

7 V peak to peak. (Oscilloscope) Signal at test point

(I) should be about 20 V DC.

2.3-3 Shunt R210 by 100k ohm to shorten off - delay.

Check that ON is actuated when signal from generator

exceeds .5mV at 500Hz.

2.4 B2-50 Oscillates Between ON and OFF with no audio signal

on input.

2.4-1 Check DC offset at test point (G). Should be between

+10mV and +100mV.

If DC offset is outside these limits, fault can be in

C414 or in the Power Amplifier. (See also section 1.1).

NOTE: If a large DC voltage has appeared at the output of the

Power Amplifier (caused by a fault), C414 will probably

be damaged, and it should be replaced.

3. ADJUSTMENTS

After certain repairs have been made in B2-50, the following

adjustments must be performed.

3.1 Bias Adjustments in Power Amplifier

After repairs have been made in the Power Amplifier, the bias

current must be readjusted.

Measure the voltage between emitters on V516 - V517 (no

signal on input terminals of B2-50). Adjust trimpot Rqc (R525)

till voltage reads 22mV. (This corresponds to a bias current

of 50mA).

Make final adjustment after B2-50 has been ON for a few minutes,

so the bias current has reached steady state value.

NOTE: When replacing power transistors, temperature sensor

on V508, heatsink compound (and isolating shims) must

be used.

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