Notes · ElectronicsUK · A-Levels

Audio systems

An audio system is a complete signal chain that shows the systems approach at its clearest: input transducer, voltage amplification, tone shaping, power amplification and output transducer. This chapter develops each sub-system and, above all, how to combine amplifier and filter stages and manage the overall gain budget in decibels.

4 sections~15 min reading time3 competenciesLevel Standard 2 · Advanced 2

T·111111 / 16

Exam profile

AO1 · Describe the sub-systems of an audio chain: microphone, pre-amplifier, tone control, power amplifier and loudspeaker.AO2 · Calculate stage and overall voltage gain in decibels and the power delivered to a loudspeaker.AO3 · Design and evaluate a complete audio system by combining amplifier and filter sub-systems to a specification.

Operators:describecalculatedeterminedesignanalyseevaluate

basic level

At AS the focus is the block structure of an audio system and simple voltage-gain calculations for the amplification stages.

higher level

The full A-Level develops tone control with filters, power amplification and impedance matching, and the cascaded gain budget in decibels.

Depth

Text

Contents · 4 sections

Audio systems

The audio system block diagram#

StandardWJEC/Eduqas A Level Electronics — Unit 3 Analogue Design (audio systems)

Audio system sub-systems

Fig. 1The audio chain: microphone (input), pre-amplifier and tone control (process), power amplifier and loudspeaker (output).

An audio system is the clearest illustration in the whole course of the systems approach — the idea that a complex problem is solved by partitioning it into input, process and output sub-systems, each doing one job, connected in a chain. Sound enters at an input transducer (a microphone), the electrical signal is processed (amplified in voltage, shaped in tone, amplified in power), and it leaves at an output transducer (a loudspeaker) as sound again. Reasoning block by block is how the whole system is designed and understood.

The signal at each stage is at a very different level, and this dictates the order of the blocks. A microphone produces only a few millivolts, far too small to shape or to drive a loudspeaker, so it is first raised to a usable line level by a pre-amplifier — a high-gain, low-noise voltage amplifier. Because the pre-amplifier handles the tiniest signal, its noise performance sets the quality of the whole system: noise added here is amplified by everything that follows.

Once the signal is at line level, the tone-control stage shapes its frequency content — boosting or cutting bass and treble — using the filters of the AC-circuits chapter. Shaping is done at line level, before power amplification, because filters work best on a moderate signal and because it is wasteful to amplify to full power and then throw part of the signal away. The order of the blocks is thus not arbitrary; it follows from the signal levels and the job of each stage.

Finally the shaped line-level signal drives a power amplifier, whose job is not to increase the voltage much further but to supply the large current a loudspeaker needs — to deliver power. The power amplifier has a low output impedance so it can drive the low impedance of a loudspeaker (typically

4

8\,\Omega

) without its output voltage collapsing. The loudspeaker then converts the electrical power back into sound, completing the input-process-output chain.

Seeing the audio system as this chain of sub-systems lets each block be designed and tested independently and then combined, and it makes the overall behaviour predictable: the total gain is the sum of the stage gains, the frequency response is the combination of the stages' responses, and the output power follows from the final voltage and the loudspeaker impedance. This block-level, systems way of thinking is exactly what the design-and-realise task and the extended-response questions reward.

\text{input} \to \text{process} \to \text{output}

Systems approach

The audio chain: transducer, amplification and shaping, output transducer.

Worked example

Ordering the sub-systems

A student proposes: microphone, power amplifier, tone control, loudspeaker. Identify the flaw and give the correct order with a reason.

The flaw
Placing the power amplifier before the tone control amplifies the signal to full power and then filters part of it away, wasting power, and shapes the tone of a high-power signal, which is harder to do cleanly.
Correct order
Microphone, pre-amplifier, tone control, power amplifier, loudspeaker: amplify the tiny signal to line level, shape it, then raise it to power.
Reason
Shaping at line level is efficient and clean, and the pre-amplifier's low noise sets the quality before the signal is amplified further.

Result: The order must be microphone, pre-amplifier, tone control, power amplifier, loudspeaker — shape at line level, then amplify to power.

Typical mistakes

Putting the power amplifier before the tone control, wasting power on signal that is then filtered out.
Expecting the power amplifier to raise the voltage greatly; its job is to supply current (power) at low output impedance.

Active revision

Draw a labelled block diagram of an audio system from microphone to loudspeaker, stating the approximate signal level at each stage and the main function of each block.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

Input and voltage amplification#

StandardWJEC/Eduqas A Level Electronics — Unit 3 Analogue Design (pre-amplifier)

Microphone input stage

Fig. 2The microphone is AC-coupled into a non-inverting pre-amplifier, whose high input impedance does not load the microphone and whose gain raises millivolts to line level.

The input transducer of an audio system is usually a microphone, which converts the pressure variations of sound into a small alternating voltage — typically only a few millivolts. Because the signal is so small and rides on no useful DC, it is coupled into the first amplifier through a capacitor that blocks any DC offset while passing the audio (the high-pass coupling of the AC chapter). The choice of coupling capacitor sets the low-frequency limit, which must be below the lowest wanted note.

The pre-amplifier raises this millivolt signal to line level, around a volt, ready for tone shaping and power amplification. It is built from an op-amp — commonly a non-inverting amplifier, whose very high input impedance does not load the microphone, with a gain of

1 + R_f/R_{in}

set by two resistors. The pre-amplifier must combine high gain with low noise, because any noise it adds is amplified by every stage that follows, so it dominates the signal-to-noise ratio of the whole system.

The gain of the voltage stage is chosen from the ratio of the wanted line-level output to the microphone's output. If a

5\,\text{mV}

microphone signal must reach

0.5\,\text{V}

, the stage needs a voltage gain of

100

, which is

40\,\text{dB}

. Expressing gains in decibels,

G = 20\log_{10}(V_{out}/V_{in})

, makes the later cascade arithmetic simple, because the stage gains in decibels simply add along the chain.

The pre-amplifier's high input impedance matters as much as its gain. A microphone is a relatively high-impedance source, so an amplifier that drew appreciable current would load it and lose signal — the loading effect again. The non-inverting op-amp configuration, with its near-infinite input impedance, is ideal because it takes almost no current from the microphone, capturing the full signal. This is the same reasoning that made the voltage follower useful as a buffer.

The voltage-amplification stage is where the systems ideas of the course become concrete: it applies op-amp gain (from the op-amp chapter), high-pass coupling (from the AC chapter), impedance considerations (from instrumentation) and the decibel (from filters), all in one block. Designing it — choosing the gain for the required output, the configuration for high input impedance, and the coupling capacitor for the low-frequency limit — draws together several strands of the specification into a single practical task.

A_V = 1 + \dfrac{R_f}{R_{in}}, \quad G = 20\log_{10}\!\left(\dfrac{V_{out}}{V_{in}}\right)

Pre-amplifier gain

Non-inverting gain, expressed as a ratio and in decibels.

Worked example

Designing the pre-amplifier gain

A microphone produces $5\,\text{mV}$ and must reach a $0.5\,\text{V}$ line level. Find the voltage gain as a ratio and in decibels, and choose resistors for a non-inverting op-amp stage.

Gain ratio
$A_V = V_{out}/V_{in} = 0.5/0.005 = 100$ .
In decibels
$G = 20\log_{10}(100) = 40\,\text{dB}$ .
$G = 20\log_{10}(100) = 40\,\text{dB}$
Resistors
For a non-inverting stage, $A_V = 1 + R_f/R_{in} = 100$ , so $R_f/R_{in} = 99$ . Choosing $R_{in} = 1.0\,\text{k}\Omega$ gives $R_f = 99\,\text{k}\Omega$ (use $100\,\text{k}\Omega$ for a gain of $101$ ).

Result: A gain of $100$ ( $40\,\text{dB}$ ) is needed; $R_{in}=1.0\,\text{k}\Omega$ with $R_f\approx 100\,\text{k}\Omega$ in a non-inverting stage achieves it with high input impedance.

Typical mistakes

Choosing an inverting amplifier with a low input impedance that loads the microphone.
Forgetting the coupling capacitor, so a DC offset is amplified and can push the stage toward a rail.

Active revision

A microphone produces $8\,\text{mV}$ and must be raised to a $0.8\,\text{V}$ line level. Find the required voltage gain as a ratio and in decibels, and give suitable resistor values for a non-inverting op-amp stage.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

Tone control and filtering#

AdvancedWJEC/Eduqas A Level Electronics — Unit 3 Analogue Design (tone control)

Bass and treble shelving responses

Fig. 3Shelving tone-control responses: the bass control boosts low frequencies (falling shelf) and the treble control boosts high frequencies (rising shelf), each about

+6\,\text{dB}

at the extremes.

Tone control shapes the frequency content of the audio signal — adjusting how much bass and treble the system delivers — using the passive and active filters of the AC-circuits chapter. A treble control is built around a high-frequency shelving response: turning it up boosts frequencies above a chosen point, turning it down cuts them. A bass control does the same at low frequencies. Together they let the listener tailor the sound, or the designer correct the response of a loudspeaker.

The building block is the filter. A low-pass section attenuates the treble (leaving the bass); a high-pass section attenuates the bass (leaving the treble); combining them, often around an op-amp so the boost as well as cut is possible, gives a tone-control stage. The cut-off frequencies, set by the RC values through

f_c = 1/(2\pi R C)

, decide where the bass and treble regions begin, and the amount of boost or cut is set by the surrounding gain.

The response is best understood on a gain-versus-frequency graph in decibels. A flat setting gives

0\,\text{dB}

across the audio band; boosting the bass raises the low-frequency end while leaving the midrange unchanged; boosting the treble raises the high-frequency end. Reading such a response — identifying the flat midband, the bass and treble regions, and the amount of boost in decibels — is the key skill, and it ties directly to the decibel and roll-off work from the filter chapter.

Because the decibel is logarithmic, the amount of boost translates simply into a voltage ratio: a

+6\,\text{dB}

treble boost doubles the amplitude of the high frequencies (

10^{6/20} = 2.0

), while a

-6\,\text{dB}

cut halves them. This lets a design specify a tone control quantitatively — so many decibels of boost at so many hertz — and lets the response of the finished stage be checked against the specification. Being able to convert between decibels of boost and voltage ratios is essential here.

Tone control shows the filter theory of the course put to a real, everyday purpose, and it must be placed correctly in the chain — at line level, between the pre-amplifier and the power amplifier — so that it shapes a moderate signal cleanly and efficiently. Designing a tone-control stage, choosing the cut-off frequencies for the bass and treble regions and the boost or cut in decibels, and evaluating its response, is a rich application of the AC-circuits and decibel work within the audio system.

\text{ratio} = 10^{G/20}, \quad f_c = \dfrac{1}{2\pi R C}

Boost ratio and cut-off

Decibels of boost convert to a voltage ratio; the RC sets where the region begins.

Worked example

Decibels of boost to voltage ratio

A tone control provides $+6\,\text{dB}$ of treble boost and $-6\,\text{dB}$ of bass cut. Find the voltage ratio for each, and describe the effect on the sound.

Treble boost
$\text{ratio} = 10^{G/20} = 10^{6/20} = 10^{0.30} = 2.0$ — the high frequencies are doubled in amplitude.
Bass cut
$\text{ratio} = 10^{-6/20} = 10^{-0.30} = 0.50$ — the low frequencies are halved in amplitude.
$10^{6/20} = 2.0, \quad 10^{-6/20} = 0.50$
Effect
The treble is twice as loud relative to the midrange and the bass is half as loud, giving a brighter, thinner sound.

Result: $+6\,\text{dB}$ doubles the treble amplitude and $-6\,\text{dB}$ halves the bass — a brighter overall balance.

Typical mistakes

Confusing which filter affects bass and which affects treble (low-pass keeps bass, high-pass keeps treble).
Muddling the decibel conversion — $+6\,\text{dB}$ is a voltage ratio of $2$ , not $4$ .

Active revision

A treble control gives a boost of $+6\,\text{dB}$ above $3\,\text{kHz}$ . State the voltage ratio this represents, and describe how the response graph changes as the control is turned from flat to full boost.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

Power amplification and the overall gain budget#

AdvancedWJEC/Eduqas A Level Electronics — Unit 3 Analogue Design (power amplifier)

Cascaded gain budget

Fig. 4The gain budget: decibel gains add along the chain, taking a

5\,\text{mV}

microphone to

5\,\text{V}

across an

8\,\Omega

loudspeaker (about

3.1\,\text{W}

The power amplifier is the output stage that delivers the current a loudspeaker needs. A loudspeaker has a low impedance — typically

4

8\,\Omega

— so producing useful sound power means driving amps of current through it, which the earlier voltage stages cannot supply. The power amplifier therefore has a modest voltage gain but a very low output impedance, so it can drive the low load impedance without its output voltage collapsing. The power delivered is

P = V_{rms}^2/R

for a resistive loudspeaker.

Matching the amplifier to the loudspeaker is an impedance question. For maximum power transfer the source and load impedances would be equal, but audio power amplifiers instead use a very low output impedance so that the output voltage is nearly independent of the load (voltage bridging), which gives better control of the loudspeaker cone and lower distortion. The amplifier must, of course, be rated to supply the peak current and to dissipate the heat it generates, which is why power amplifiers use large transistors on heat sinks.

The overall performance of the system is captured by its gain budget — the sum of the stage gains in decibels. Because decibels add, the total gain from microphone to loudspeaker is simply the pre-amplifier gain plus the tone-control gain plus the power-amplifier gain, all in decibels. This makes the whole chain easy to design: choose each stage's gain so that the tiny microphone signal ends up at the voltage the loudspeaker needs, and the cumulative decibel figure confirms the design at a glance.

From the final output voltage and the loudspeaker impedance the sound power follows directly. If the chain delivers a certain rms voltage across an

8\,\Omega

loudspeaker, the power is that voltage squared divided by

8

. Working backwards, a required output power fixes the output voltage, which fixes the total gain needed from the millivolt input, which the gain budget then distributes among the stages. This end-to-end calculation is the capstone design task of the audio topic.

The complete audio system is the course's fullest example of combining sub-systems: op-amp voltage gain, RC filters for tone, and a power output stage, connected in a chain whose overall gain, frequency response and output power are all predictable from the individual blocks. Designing such a system to a specification — setting each stage's gain and response, managing the gain budget in decibels, and delivering the required power to the loudspeaker — brings together nearly every analogue idea in the specification, which is exactly why it is a favourite extended-design question.

G_{total} = G_1 + G_2 + G_3 \ (\text{dB}), \quad P = \dfrac{V_{rms}^2}{R}

Gain budget and power

Decibel gains add along the chain; power follows from the output voltage and loudspeaker impedance.

Worked example

Gain budget and output power

An audio chain has a $+40\,\text{dB}$ pre-amplifier, a flat tone control and a $+20\,\text{dB}$ power amplifier, fed by a $5\,\text{mV}$ rms microphone. Find the overall gain, output voltage and power into an $8\,\Omega$ loudspeaker.

Overall gain
$G_{total} = 40 + 0 + 20 = 60\,\text{dB}$ , which is a voltage ratio of $10^{60/20} = 10^3 = 1000$ .
Output voltage
$V_{out} = 1000 \times 5\,\text{mV} = 5.0\,\text{V}$ rms.
Loudspeaker power
$P = V_{rms}^2/R = 5.0^2/8 = 25/8 = 3.1\,\text{W}$ .
$P = \dfrac{5.0^2}{8} = 3.1\,\text{W}$

Result: The overall gain is $60\,\text{dB}$ ( $\times 1000$ ), giving $5.0\,\text{V}$ rms and about $3.1\,\text{W}$ into the $8\,\Omega$ loudspeaker.

Typical mistakes

Multiplying the stage gains in decibels instead of adding them.
Using peak instead of rms voltage when calculating the average power into the loudspeaker.

Active revision

An audio chain has a $+40\,\text{dB}$ pre-amplifier, a flat ( $0\,\text{dB}$ ) tone control and a $+20\,\text{dB}$ power amplifier, driven by a $5\,\text{mV}$ microphone. Find the overall gain, the output voltage and the power delivered to an $8\,\Omega$ loudspeaker.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas GCE Electronics specification (WJEC / Eduqas)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas GCE Electronics specification

Audio systems

4 sections~15 min reading time3 competenciesLevel Standard 2 · Advanced 2

Audio systems

Key points

Ordering the sub-systems

Key points

Designing the pre-amplifier gain

Key points

Decibels of boost to voltage ratio

Key points

Gain budget and output power

Sources

Audio systems

Key points

Ordering the sub-systems

Key points

Designing the pre-amplifier gain

Key points

Decibels of boost to voltage ratio

Key points

Gain budget and output power

Sources