Circuit Theory in Loudspeakers and Amplifiers

Circuit Theory in Loudspeakers and Amplifiers
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In a general way we all know what a loudspeaker or simply a speaker is. We have seen them in different sizes and shapes. Each one of us has grown up with these outstanding music and speech reproducing devices which today has become an indispensable part of our life as far as fun and amusement is concerned.

Similarly we are also quite familiar with audio amplifiers which are definitely the integral part of loudspeakers and perhaps the two are incomplete without each other. But do you know how these amazing systems actually work?

In this article we’ll go through a thorough discussion regarding loudspeaker and amplifier circuit theory separately.

What is a Loudspeaker?

Loudspeaker Component, Diagram Image

A Loudspeaker basically is a device able to convert or reproduce electrical signals or frequencies into corresponding music and/or speech vibrations in the air. These electrical signals, which are used to drive a loudspeaker, are always received from audio amplifiers. We’ll study regarding amplifiers later on in the article.

The functioning of a loudspeaker may be easily understood through the following points:

  • Normally a loudspeaker is made up of a diaphragm or a cone, a magnet, and a coil.

  • The coil is generally wound over a paper or some light synthetic bobbin and is arranged or mounted over the magnet in such a way that it is able to slide over the entire length of the magnet.

  • The input signal is applied to the terminals of the speaker assembly where the two ends of the coil are connected.

  • But before the ends of the coil can reach the output terminals, they are made to run across a certain length of the diaphragm and the portion is firmly glued with epoxy adhesive.

  • When an input electrical music or speech signal is applied to the coil through its terminals, a varying magnetic field corresponding to the received electrical signals is created around the coil.

  • Since the coil is mounted over a permanent magnet, the magnetic field created around it interacts with the magnetic field of the permanent magnet.

  • This interaction between the two magnetic fields creates a pressure over the coil assembly and it is forced to move or slide to and fro over the magnet. The to and fro displacements of the coil is exactly equivalent to the varying pitches of the input electrical signals.

  • Now since the coil terminals are attached to the cone or diaphragm, its to and fro motion or the pressure is directly applied to the diaphragm which also starts vibrating correspondingly. The air volume surrounding the cone responds and starts vibrating accordingly and audible sound waves are thus created.

What is an Amplifier (Audio)?

The general circuit operation of amplifiers may be understood from the following given points:

  • In simple words an audio amplifier may be explained as a device which is able to increase or “magnify” input audio electrical signals to a magnitude which may be relatively much higher than the applied input. This conversion of a low power input signal into a high power output signal is termed as amplification and is measured in watts.

  • This amplified signal is fed into a speaker so that the received input information may be reproduced or transformed into the corresponding vibrations audible to the human ear. The input signal to the amplifiers is normally derived from a source that may be in the form of a sensor able to decode the audio information from storage devices like DVDs, CDs, pen drives, etc. into corresponding electrical pulses or it can be simply a mic that can directly convert any audio vibrations in the air into minute electrical frequencies.

  • Amplifiers are generally classified as low signal amplifiers and power amplifiers. The low signal amplifiers are also known as preamplifiers.

  • Power amplifiers require higher input levels, generally around 100 to 200 mV to be able to sense them and amplify to the desired level. Since the audio signals received from sensors like the lens in DVD players or microphones are too low, they cannot be directly configured with the power amplifiers.

  • It becomes very essential to lift these minute signals to a level suitable for the power amplifier. Therefore a preamplifier is incorporated in between the small source signal and power amplifier so that it becomes feasible to amplify the minute signals into large output signals powerful enough for driving the loudspeakers.

The technical specifications generally involved with amplifiers are briefly explained as under:

  • Input and Output Impedance: It is the resistance offered by the inputs and the output terminals to varying voltages. The quantity is expressed in Ohms.

  • Total Harmonic Distortion (THD): It can be calculated by dividing the total power of the harmonics by the total power of the fundamental frequencies present in the music content.

  • Quiescent Current: It is the current consumed by an amplifier in the idle state.

  • Bandwidth: It is the maximum range of frequencies for which the amplifier is able to provide an optimum performance.

  • Gain: It can be found by dividing the average output power by average power of the applied input. The unit of measurement is in decibels (db).

From the above explanations, you hopefully now clearly understand the basic general loudspeaker and amplifier circuit theory and how they are correlated. If you have any further queries about them, feel free to post your comments. (Comments need moderation and may take time to appear.)


  • Author’s own experience.

    Images - Drawn by the author.