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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