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How is an audio system built from mixing, filtering, amplification and a power output stage?

Audio systems: the audio signal chain, the mixer based on a summing amplifier, tone control with filters, voltage and power amplification, gain in decibels, and driving a loudspeaker.

A focused answer to WJEC A-Level Electronics audio systems, covering the audio signal chain, the mixer built from a summing amplifier, tone control with filters, voltage and power amplification, expressing gain in decibels, and driving a loudspeaker.

Generated by Claude Opus 4.812 min answerUpdated 2026-06-16

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

Audio is a flagship application of analogue electronics, and Component 2 builds a complete audio system from sub-systems you already know. WJEC expects you to describe the audio signal chain, the mixer based on a summing amplifier, tone control with filters, voltage and power amplification, gain in decibels, and how a loudspeaker is driven. The mixer-gain calculation and the voltage-versus-power amplifier distinction are reliable, high-mark exam content.

The answer

The audio signal chain

The mixer

Tone control and gain in decibels

Tone control uses high-pass and low-pass filters to boost or cut treble and bass. Gain is conveniently expressed in decibels: a voltage gain of 10 is 20 dB, 100 is 40 dB, because the logarithmic scale matches how loudness is perceived.

Voltage versus power amplification

Setting channel levels in a mixer

A summing-amplifier mixer has two inputs. Input A should be at full level and input B at half level, using a feedback resistor of $10\,\text{k}\Omega$ and an input resistor of $10\,\text{k}\Omega$ for A. Find the input resistor for B.

step Find the gain for each input

Gain is $A_v = -\dfrac{R_f}{R_{in}}$ . For A: $-\dfrac{10}{10} = -1$ (full level).

step Set B to half the gain of A

Half level means half the gain magnitude, so B needs $|A_v| = 0.5$ . Then $R_{in,B} = \dfrac{R_f}{|A_v|} = \dfrac{10}{0.5} = 20\,\text{k}\Omega$ .

step State the design

Use a $20\,\text{k}\Omega$ input resistor for B. Because each input sees the virtual earth, B sits at half the level of A and the two do not interfere, which is exactly how a mixer balances channels.

Examples in context

Example 1. A band's stage mixer: Several microphones and instruments feed a summing-amplifier mixer, each through a fader (variable input resistor) that sets its level. The virtual earth keeps each channel independent, so the sound engineer can adjust one instrument without affecting the others, then send the mix to the power amplifier and speakers.
Example 2. Bass and treble controls: A tone-control stage uses adjustable high-pass and low-pass filters to cut or boost the high and low frequencies. Turning up the treble lifts the gain above a few kilohertz; turning up the bass lifts it below a few hundred hertz, shaping the sound to taste.
Example 3. Why the output stage gets hot: The power amplifier delivers watts into the loudspeaker, so it handles large currents and dissipates significant heat, which is why it needs a heat sink. The voltage stages before it handle tiny currents and stay cool, illustrating the division of labour in the chain.

Try this

Q1. An amplifier has a voltage gain of 50. Express this in decibels. [2 marks]

Cue. $G_{dB} = 20\log_{10}(50) = 20 \times 1.70 = 34\,\text{dB}$ .

Q2. State why a power amplifier, not a voltage amplifier, is used as the final stage to drive a loudspeaker. [2 marks]

Cue. The loudspeaker is a low-impedance load needing large current; a power amplifier supplies the current (and power) to move the cone, whereas a voltage amplifier supplies little current.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC Eduqas 20215 marksAn audio mixer combines two inputs using a summing amplifier with input resistors of

10\,\text{k}\Omega

each and a feedback resistor of

20\,\text{k}\Omega

. Calculate the gain applied to each input, and state why a summing amplifier is suitable for mixing audio channels.

Show worked answer →

In a summing amplifier each input is amplified by minus the feedback resistor over its own input resistor.

Gain per input: $A_v = -\dfrac{R_f}{R_{in}} = -\dfrac{20}{10} = -2$ .

So each input is amplified by 2 (inverted) and the two are added.

A summing amplifier is suitable because each input feeds the virtual earth, so the channels do not interact: adjusting or adding one input does not affect the others, and the output is the clean (inverted) sum of the inputs. Different input resistors would set different channel levels.

Markers reward the gain of $-2$ per input and the virtual-earth reason that the channels are independent.

WJEC Eduqas 20194 marksAn amplifier increases a signal's voltage by a factor of 100. Express this voltage gain in decibels, and explain the difference between a voltage amplifier and a power amplifier in an audio system.

Show worked answer →

Voltage gain in decibels is $G = 20\log_{10}(A_v) = 20\log_{10}(100) = 20 \times 2 = 40\,\text{dB}$ .

A voltage amplifier raises the signal voltage but supplies little current, so it cannot drive a low-impedance load like a loudspeaker. A power amplifier provides both voltage and the large current needed to deliver power into the speaker, so it is the final stage that actually moves the speaker cone.

In an audio system the voltage (pre-)amplifier brings the small signal up to the right level, and the power (output) amplifier then drives the loudspeaker.

Markers reward the $40\,\text{dB}$ gain and the distinction that a power amplifier supplies the current/power to drive the loudspeaker while a voltage amplifier only raises the voltage.

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Sources & how we know this

WJEC Eduqas GCE A-level Electronics specification — WJEC Eduqas (2017)