How to design audio circuits and analyze speaker principles

During my electrician internship, I made a very interesting small speaker. Although the sound quality is not very good, the circuit principle behind it made me very interested. So I am doing an introductory research on it.

Start with the speaker principle (magnetic speaker)

To understand speakers, let’s start with the eardrums we are most familiar with:

There is a thin layer of skin on the inside of the ear called the eardrum. When the eardrum vibrates, the brain interprets the vibrations as sound, which is the sense of hearing. Rapid changes in air pressure are the most common cause of eardrum vibrations. When objects vibrate in the air, they make sound (sound can also travel through liquids and solids, but air is the medium through which we hear the sound from a speaker). When an object vibrates, it moves the air molecules around it. These air molecules, in turn, push against the air molecules around them, spreading the vibrations through the air as a propagating disturbance.

The microphone works similarly to the human ear. It has a diaphragm that is vibrated by surrounding sound waves. The signal from the microphone is encoded as an electronic signal and stored on a tape or CD. When this signal is played back in a stereo system, the amplifier sends it to the speakers, which redefine it as mechanical vibrations. The air pressure waves produced by a good speaker are exactly the same as the waves originally picked up by the microphone.

So how does the speaker complete the process of [electrical signal ~ mechanical vibration ~ sound]?

Let's take a backward approach.

The audio speakers we usually see usually look like this [Figure 1]

The shiny part with a white circle around it is called a flexible cone or diaphragm.

1. Flexible cone or diaphragm

what is diaphragm?

The flexible cone is usually made of paper, plastic or metal and is attached to the wide end of the suspension frame.

The suspension or surround is the rim of flexible material that allows the cone to move and is connected to the metal frame of the driver called the basket. (That's the outermost gray material that is very soft.)

The narrow end of the cone is connected to the voice coil. (What is a voice coil? See below)

The voice coil is attached to the basket by a tripod (a ring of flexible material). The tripod holds the voice coil in place but allows it to swing back and forth freely.

[Figure 2] Violent disassembly of the first layer: It is indeed a diaphragm made of paper and plastic (the outermost boundary metal bracket is the wide end of the suspension frame)

[Figure 3] After violently disassembling the second layer, we can see the structure underneath the diaphragm

[Figure 4] After violently disassembling the third layer, we can see the magnet and voice coil, where the voice coil is a wound wire. (The voice coil and the cone are connected in the middle)

[Figure 5] Close-up of the voice coil (in the figure, you can see two thin wires being led out, which are connected to the two wires on the diaphragm and then to the external wires (Figure 4))

2. Voice coil and magnet

(1) Voice coil:

The voice coil is a basic electromagnet. An electromagnet is a coil of wire, usually wrapped around a magnetic metal object, such as iron. Electric current flows through the coil, creating a magnetic field around the coil, which magnetizes the metal object the coil is wrapped around. This area is equivalent to the magnetic field around a permanent magnet: it has two polarities, one "north" and the other "south", and attracts ferrous objects. Unlike a permanent magnet, it can change the polarity of the electromagnetic field. Change the direction of the current, and the north and south poles of the electromagnetic field will change accordingly. This is what a stereo signal does - it constantly changes the direction of the current. If you have ever built a sound system, you know that each speaker has two output wires, usually one black and the other red. In fact, the amplifier is constantly switching the electronic signal, and the red wire fluctuates between positive and negative charges. Because electrons always flow in the same direction between positive and negative particles, the current comes out of the speaker, flows in one direction and then turns around and flows out the other way. This alternating current causes the polarity of the electromagnetic field to change many times a second.

(2) Magnet:

So how does this fluctuation cause the speaker's voice coil to move back and forth?

The voice coil can be inserted
(it can vibrate slightly)

The electromagnetic field is located in the constant magnetic field created by the permanent magnet. The two magnets (electromagnet and permanent magnet) interact like any two magnets. The positive pole of the electromagnet attracts the negative pole of the permanent magnet, and the negative pole of the electromagnet repels the negative pole of the permanent magnet. When the polarity of the electromagnet switches, the polarity of its repulsion and attraction also changes. Thus, the alternating current constantly changes the magnetic force between the voice coil and the permanent magnet. This pushes the voice coil back and forth rapidly like a piston. When the current flowing through the voice coil changes direction, the polarity of the voice coil also changes. This changes the magnetic force between the voice coil and the permanent magnet, causing the voice coil and the diaphragm to which it is connected to move back and forth. As the voice coil moves, it pushes and pulls on the speaker cone. This causes the air in front of the speaker to vibrate, creating sound waves. The electronic audio signal can also be thought of as a wave. The frequency and amplitude of the wave, which represents the original sound wave, reflects the speed and distance that the voice coil moved. In turn, this speed and distance determine the frequency and amplitude of the sound waves produced by the movement of the diaphragm. Different driver sizes are better suited to certain frequency ranges. Therefore, loudspeakers usually divide a large frequency range among multiple drivers.

3. Summary

So the sound-generating process is actually this: when the speaker receives the electrical signal output by the sound source device, the current will pass through the coil (voice coil) on the speaker and generate a magnetic field reaction. The current passing through the coil is an alternating current, and its positive and negative poles are constantly changing; the positive and negative poles will attract each other when they meet, and the coil will move backward (inside the box) due to the attraction of the magnet on the speaker; the positive and positive poles will repel each other when they meet, and the coil will move outward (outside the box). This rhythm of contraction and expansion will generate sound waves and airflow, and produce sound, which has the same effect as the vibration of our throat when we speak. [Voice coil drives the diaphragm]

So, what form of electrical signal is needed to drive the speaker?

We need an audio signal, but the audio signal is generally weak. If we use the supplied audio signal directly, it is definitely not enough to drive the speaker to make a sound (or it is very weak). At this time, we need a power amplifier circuit.

The power amplifier circuit built with discrete components is generally not as stable as the chips made directly by the manufacturer, and considering the cost, we can directly use the driver chip. Here we use the TDA2822M from ST.

1. TDA2822M chip

1. Chip Introduction

TDA2822M is a dual-channel monolithic power amplifier integrated circuit developed by STMicroelectronics (ST). It is usually used as an audio amplifier in pocket cassette players, recorders and multimedia active speakers. At the same time, TDA2822M has low quiescent current and crossover distortion. The power supply voltage range is 1.8~15V, so it can work within this range. The output power can be made relatively high. TDA2822M has the characteristics of simple circuit, good sound quality and wide voltage range.

2. Chip physical picture

3. Chip pin distribution diagram

4. Chip internal circuit

5. Manufacturers provide reference typical applications

From this figure we can see that this chip is simply a circuit that can amplify two weak signals (so it can be used for left and right channel amplification)

2. Now we can move on to the next circuit. The following circuit forms a relatively complete basic audio circuit.

1. Circuit Diagram

2. Circuit diagram analysis:

(1) The left L-IN and R-IN are audio signal inputs. The input lines are as shown below: - Signal input

(divided into three wires: one GND wire, one left channel L-IN, and one right channel R-IN)

(2) The audio signal is input to the input terminal (VOL) of the stereo disc potentiometer - volume control circuit

This knob is actually two 50KΩ potentiometers

(3) The two audio signals are then coupled to the input terminals 6 and 7 of the power amplifier integrated circuit D2822 through R1, C1, R4, and C4 respectively.

(4) After being amplified by the internal power of IC1 (D2822), the amplified audio signal is output from its 1st and 3rd pins to drive the left and right speakers.

(5) The LED in the circuit serves as a power-on indicator. The toggle switch K1 can control the power on or off. The DC power socket DC serves as an external power source for the circuit.

Summary TIPS:

The power amplifier circuit TDA2822 is an integrated circuit with a dual-channel audio power amplifier. Its operating voltage range is 1.8~15V. Pin 2 is the power input terminal, pin 4 is the common ground terminal, pins 7, 8, and 1 are the forward input, reverse input, and output terminals of one of the amplifiers, and pins 6, 5, and 3 are the forward input, reverse input, and output terminals of the other amplifier. When the chip is working, the voltage of the two output terminals is approximately equal to 1/2 of the power supply voltage.