Simple Speaker Turn-on Delay Circuit

Today we are going to introduce you to a Simple Speaker Turn-on delay Circuit. Imagine after you finish building your own OCL power amplifier circuit.

But every time you turn it on, within the first 3 seconds, you hear a loud “tlub” sound from a speaker.

Wouldn’t you feel annoyed by that sound? Apart from the minor annoyance, this sound is caused by a voltage spike that can damage the speaker.

So let’s find a solution to this problem.

The main idea of the circuit

When we turn on a normal OCL power amplifier, at the first instance, the output voltage will swing between positive and negative, as high or as low as the power supply voltage (+24V and -24V). And then gradually decreases until it reaches 0V within 3 seconds.

This type of voltage spike has quite a high amplitude, although it only happens in a short time window. It is also considered a DC pulse signal, which can be harmful to the speaker.

Let’s sum up each point of the problem, find the simplest solution to it, and put them all into an easy-to-understand block diagram.

We can solve this problem by using a switch that will disconnect and connect the speaker from the output of the amplifier, as shown in the block diagram above.

In the first instance, after the amplifier turns on, the 3-second Turn-on Delay block will turn the switch off, disconnecting the amplifier and the speaker. 

And after three seconds have passed, the 3-second Turn-on Delay block will send out a command to turn on the switch, connecting the amplifier and the speaker as normal.

As a result, the voltage spike in the first few seconds of the amplifier turning on is prevented from reaching the speker.

Turning It Into Circuit Diagram

Now let’s pick the components we need according to the block diagram above, with the goal of making it as easy and efficient as possible. 

The Relay as a Switch

Start with the switch block. We are going to use a 2-contact 12V DC DPDT relay to manage the connection between the stereo output and two speakers. The relay here basically functions as a normal switch, where instead of a physical switch, it is controlled by the voltage applied to its coil.

We picked a 12V one because it is very easy to buy, given how widely used it is. 

Normally, the resistance of a relay coil sits in the 160Ω to 600Ω range, depending on the size of the contact pad. In this case, we use 180Ω, which requires a coil current of about 0.067A from I = 12V/180Ω. Hence, we are going to use a 12V 300mA power supply.

We also need a transistor to drive the relay; today we chose the S9013 NPN transistor. It has a maximum IC of 0.5A, which is definitely enough for the relay coil.

Now we put together those components into a simple sketch circuit, as shown below. This tansistor circuit is set up in a common emitter form, which has an exceptionally high gain and input impedance.

As a result, it can be operated with just a small amount of input current.

The circuit will only function when the input is supplied with 12V from the power supply. This base current flows through the input and R3 and goes directly to the B of Q1.

Then the transistor will start working after receiving this forward bias, letting the collector current flow freely through to the relay coil and triggering its contact.

Finding the Value of R2

We have explained the process of calculating the value of R2 (RB) many times already, but it is the basics that should be mastered, so let’s go over it again.

R2 = VR2/IR2 

VR2 = Vin-VBE

From Vin = 12V and VBE = 0.7V 

Substituting the VR2 = 12V – 0.7V 

VR2 = 11.3V

Then, Find IR2, which is the same as the IB of the transistor.

IB = IC/hFE

From IC = 0.067A and hFE = 200

Substituting the IB = 0.067A / 200

IB = 0.000335A  

R2 = VR2/IB 

Subtituting the R2 = 11.3V/0.000335A 

R2 = 33,731Ω 

We will use 33K instead.

Also, there should always be a diode (D1) across the relay coil to protect the transistor (Q1) from the high-voltage pulse generated by the relay coil switching off.

While the LED (with an R3 as its current limiter) indicates if the relay and the transistor function properly or not.

The 3-second Timer

Next, we need to find a way to make the transitor circuit remain on for 3 seconds after it turns on. The common way to make a timer circuit like this is to use a capacitor, as they are cheap and easy to put together. So in this case, we will be using them as well.

As you may already know about the basics of capacitors, their normal state is their discharge state. In this state, the resistance within them is quite low, and the voltage drop across them will be zero.

But when it receives an input voltage (Vin) of 12V, the capacitor will enter its charge state. The voltage inside it will gradually increase to match that of the input.

The RC time constant, which is the time that it takes for the voltage to charge a capacitor, can be an essential part of the timer circuit. We figure out the time through the use of this simple formula:

Time Constant (T) = R x C

So if R = 33K (33,000Ω), C=100µF(0.0001F)

Then T = 33,000 x 0.0001 = 3.3s, which is around the three seconds that we need.

It functioned fairly well when we tried to connect the output of this timer circuit to the input of the transistor circuit from earlier. Adding up to the finished circuit, as shown below.

In addition, we also add in C2 to increase the efficiency of the circuit by improving Q1 and D2 to help prevent the inputting of power supplies with the wrong polarity.

Testing circuit

We then test the circuit on the breadboard.

In the case that you cannot find a 2-contact relay, you might be able to substitute it with two 1-contact relays instead. Both relays will have to be connected together in parallel.

Which will lower their coil resistance from approximately 400 ohms each to around 200 ohms combined.

And if the circuit shows a sign of error—not working—it might be due to the deterioration of the transistor, which lowers its hFE value, causing it to not be able to amplify a low current from R2.

If changing the transistor is not plausible for you, you may try to increase the capacitance of C1 to 200uF and decrease the resistance of R2 to 22K. 

Or if the three seconds delay is not enough for you, then you can increase it to six seconds by changing C1 to 220uF.

Parts list

R1, R2: 33K 0.25W Resistors, tolerance: 5%

R3: 1.2K 0.25W Resistors, tolerance: 5%
RY1: Relay with DPDT 5A switch, Coil Voltage 12V, Coil resistance 150-600Ω
C1,C2: 100µF  25V  Electrolytic Capacitors
Q1: S9013 or equivalent, 40V 0.5A, TO-92 NPN Transistor
D1,D2: 1N4007, 1000V 1A, Silicon Diode
LED1: Red 3mm LED.

Conclusion

This circuit functions quite well and is suitable for a small OCL amplifier that lacks a speaker protection circuit. This function is normally built into a speaker protection circuit. So does the OTL amplifier circuit; it does not need one.

Anyhow, it is still a good way to learn about a turn-on delay time circuit. These types of circuits can have many other applications, such as an electric motor, a monitor, a heating coil, etc.

Because they can reduce the effect of a short voltage spike that will cause harm to that load.

Look: the loudspeaker protection circuit project

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