LIST OF CONTENTS
1. TECHNICAL DESCRIPTION
1.1 Power Supply Circuits
1.2 Automatic ON/OFF
1.3 Overload protection for Midrange & Tweeter
1.4 Signal Amplifier Circuits
1.5 Bass Circuits
1.6 Ace-Bass Circuit
1.7 Midrange & Treble Circuits
1.8 Power Stage for Woofers
1.9 Power Amplifier for Midrange & Tweeter
2. FAULT FINDING
3. ADJUSTMENTS
4. SPARE PARTS
5. ENCLOSURES
5.1 Circuit diagram 2-141502 (A4-14, S/N through 12399)
5.2 Component layout 3-141504 (A4-14, S/N through 12399)
5.3 Circuit diagram 5520991-01 (A4-14 Mk2, S/N from 13400)
5.4 Component layout 4-80150 (A4-14 Mk2, S/N from 13400)
1. TECHNICAL DESCRIPTION
1.1 Power Supply Circuits
Unregulated voltages +30V & -30V supply the Power
Amplifier & also a regulator for ±15V
-15V supply tracks +15V in order to avoid transients
& oscillations when power is switched on or off. The
positive supply must decay faster than the negative
supply when power is disconnected, which is assured
by a 2k2Ω dummy load (R101) on the +30V capacitor.
1.2 Automatic On/Off
The Auto ON/OFF circuits are powered via D201,
D202 & C201. The power transformers & the auto
ON/OFF circuits are powered as long as the speaker
A4-14 is plugged in to a live outlet. The remaining
circuits are powered only in "ON" state when relay
K101 is closed.
The audio signals enter via pins 3 & 5 in DIN
connector J301. The other A4-14 in a stereo pair
receives signals via pins 3 & 5 in DIN connector
J302. The signals enter the amplifier circuits via
push-button switches S301-S302 which select Left
or Right Channel.
The input stage of auto ON/OFF, Z202 (1,2,3) works
as amplifier with gain -7 determined by feedback
network R204, R205. D208 & D209 limit the amplifier
signal to ±7V. The second stage, Z201 (1,2,3) works as
an amplifier with gain of ~2,000 at 500Hz.
The signal at pin 1 is limited to +7.0V/-7.0V. Z201
(5, 6, 7) plus transistor V201 works as a Schmitt
trigger with ON level approx. 7V & OFF level
approx. 5V. The input, pin 5, senses a DC voltage
which is obtained via diode D211 & capacitor C206.
Transistor V201 drives relay K101, which connects
power to the audio circuits in A4-14. When switch
S201 is in "AUTO" position, relay K101 will open
about 5 minutes after the signal at the input
disappears. The time relay is determined by C206 &
R210. During testing you may shunt R210 by 100kΩ
to obtain shorter OFF-delay.
When switch S201 is in "ON" position, the auto
ON/OFF circuits are bypassed & A4-14 remains
permanently ON.
A thermistor TS3-75 senses the temperature of the
heatsink. When this temperature is too high, the
thermistor resistance becomes so low, that relay
K101 opens.
1.3 Overload Protection For Midrange And Tweeter
Z202 (4-13) acts as an overload protection device
which is actuated if there is risk for damage to the
midrange driver or the tweeter.
The voltage across the voice coils are rectified in
diodes D220, D219, & the rectified voltages charge
capacitors C212, C211. The time constants for
discharge are longer than the charging time
constants. If the voltage at Z202 pins 5 or 6 exceeds
a threshold value (approx. 3.4V), the mono-stable
flip-flop Z202 (8-13) is triggered for 10 seconds.
This causes V201 to be shut off, so relay K101
opens & A4-14 is de-energised for 10 seconds.
1.4 Input Stage
When the Bass-Blend switch is off, the signals enter
via R302, C302, R303.
When the Bass-Blend switch is ON, the bass
frequency signals in left & right channels are added
via filters C301, R301 & R304, C303, R305.
C301, R301 is a high-pass filter with cut-off
frequency 100Hz. Lower frequencies are attenuated
by a factor of 2.
R304, C303, R305 is a Low-pass filter with cut-off
frequency 100Hz, & attenuation 1:2 relative filter
R302, C302, R303.
The signals below 100Hz are summed at Z301, pin
9. Because of the attenuation introduced in each
channel, the bass level is the same as when Bass-
Blend is not used.
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