Avt 2854 User manual

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.

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

Circuit description

Loudspeaker Connection Delayer

AVT 2854

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

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

kits

ASSEMBLY DIFFICULTY

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Fig. 1 Schematic diagram

Fig. 2 Arrangement of components on the PCB.

BC548

2N5551

BC548

2N5551

470uF/63V

220uF/63V

1uF

22k

22k 22k

22k 330

1k 22k 68

1N4007

1N4148

ZAS

R2 R3

R4 R5

R6 R7 R8

PK1

SPK1

SPK2

current flowing through the R3. The current flowing

in the base circuit of T4 and T5 also decreases.

When capacitor C3 is almost completely charged, the

charging current is already so low that the current in

T4 base decreases to zero, resulting in plugging of T4

and T5. Previously, transistor T5 shorted the base of

T3 to ground, and a current flows through the coil of

the relay with a value of close to its nominal current.

After plugging T5 a current-saving circuit is activated.

This takes advantage of the fact that in a relay the

hold current is several times less than the rated

current. Therefore, after the relay has operated you

can confidently lower the current and voltage on the

relay. This is realised by the T3 and R8, which,

together with the T6 form a typical current source

system. A voltage drop on R8 causes a partial

opening of the T3, and the same, such a reduction in

voltage on C3 and on the relay to maintain a voltage

on R8 of approx. 0.7V. The value of R8 is chosen so

that when T5 is plugged the relay current will be

limited to approximately 10 mA. This is advantageous

because of less heating relay and R5. Importantly,

resistors R3, R7 are so selected that first reliably

switched on is PK1 relay and only then T4, T5 will

plug and the relay current is limited. The circuit is

designed for amplifiers of any power rating. Typically,

higher power amplifiers are powered by symmetrical

voltage and the transformers used in them have two

identical windings. Type of the transformer used is

irrelevant.

Mount the circuit on a board shown in Figure 2. The

mounting is classic.

First, solder the smallest components, then the

bigger ones. The circuit, assembled from efficient

components, will not require any start up procedure

or calibration and, when connected to a transformer

with a voltage of 20...30 VAC, should work properly

straight away - see Figure 3.

Mounting and start-up

Resistors:

R1-R4, R7:.........................................................22 kΩ

R5: .......................................................................330 Ω

R6:..........................................................................1 kΩ

R8:..........................................................................68 Ω

Capacitors:

C1:.........................................................470 μF / 63 V

C2:...................................................1 μF / 63 V solid

C3:.........................................................220 μF / 63 V

Semiconductors:

D1: ...................................................................1N4007

D2, D3: ...........................................................1N4148

T1, T5:............................................BC548 or similar

T2-T4, T6: ......................................................2N5551

M1: ...................................................Rectifier bridge

Other:

PK1:..........................................RM84P24 or similar

ZAS, SPK1, SPK2: ..............................DG301-5.0/2

Power amplifier

Power

supply

Fig. 3.

List of components

Notes

AVT SPV reserves the right to make changes without prior notice.Installation and connection of the appliance not in accordance with the instructions, unauthorised modification of

components and any structural alterations may cause damage to the appliance and endanger persons using it. In such a case, the manufacturer and its authorised representatives shall

not be liable for any damage arising directly or indirectly from the use or malfunction of the product.

The self-assembly kits are intended for educational and demonstration purposes only. They are not intended for use in commercial applications. If they are used in such applications, the

purchaser assumes all responsibility for ensuring compliance with all regulations

This symbol means do not dispose of your

product with your other household waste.

Instead, you should protect human health

and the environment by handing over your

waste equipment to a designated collection

point for the recycling of waste electrical

and electronic equipment.

Leszczynowa 11 Street,

03-197 Warsaw, Poland

https://sklep.avt.pl/

AVT SPV Sp. z o.o.