AVT 2854 Quick Start Manual

Loudspeaker Connection Delayer
kits
A simple, useful circuit for audio power amplifiers.
Prevents knocks in the speakers. It realizes dual
function: connects speakers to the amplifier output
with a delay of a few seconds, but disconnects
them immediately upon switching off supply
voltage.

Circuit description

Schematic diagram of the device is shown in Figure 1.
The circuit uses an AC voltage directly from the mains
transformer used in the power amplifier. Voltage
from secondary winding is pre-straightened with an
M1 bridge. Filter capacitor C2 has a very small
capacitance and is loaded with a resistor divider R1,
R6. Because of this, there are significant ripples on C2
and what important, the voltage drops quickly when
disconnected from mains voltage. This fact is used to
rapid disconnection of speakers. However, the main
filter capacitor and energy storage is "electrolyte" C1,
connected via diode D1. Voltage on it contains only
slight ripples. After the mains voltage is switched on,
it quickly charges capacitor C1 and C2. Transistor T1
is opened and shorts the base of transistor T2 to
ground. This allows charging the capacitor C3. C3 is

Characteristics

• switch-on delay approx. 2 s
• switch-off delay of less than 0.5 s
• executive element - relay
• two pairs of contacts 2×8 A
• can be used with amplifiers up to 500W/channel
• circuit connected only to power supply and loudspeaker
circuits
• power supply: 20-30 VAC
charged by R3. The resistance of R7 has no effect on
charging, because the two connectors connected in
parallel base - emitter T4 and T5 hold on voltage R7
during charging of about 1.2V. Transistors T4 and T5
are open during charging of C3. Open transistor T5
short-circuits the T3 base to ground. T5 collector
voltage is practically equal to the ground potential.
Growing voltage on capacitor C3 and base T6 causes
also increase of the voltage at the emitter of T6, and
the voltage occurring at the T6 emitter is divided: the
major part occurs on the relay coil, the smaller on the
resistor R5. A few seconds after switching on power
supply, the voltage on the relay will increase enough
to short contacts and connect the speakers to the
amplifier output. The voltage on C3 continues to rise,
and what important, as charging decreases the
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ASSEMBLY DIFFICULTY
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