AudioFebruary 27, 202610 min read

Audio Amplifier Design: Power, Impedance, and Noise

A practical guide to designing audio amplifier stages: calculating power output, matching speaker impedance, managing noise floor, and choosing between Class AB and Class D.

Power Amplifier Fundamentals

An audio power amplifier takes a low-level line signal (typically 1 Vrms, 0 dBV) and drives a speaker (4–8 Ω) to produce acoustic output. The key challenge is delivering tens to hundreds of watts while maintaining low distortion and high efficiency.

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Power Output Calculations

Maximum output power for a class AB amplifier:

P_{max} = \frac{V_{peak}^2}{2 R_L}

where $V_{peak} \approx 0.9 V_{supply}$ (Class AB typically swings within 10% of the rails).

For a ±18V (36V dual) supply into 8Ω:

P_{max} = \frac{(0.9 \times 18)^2}{2 \times 8} = \frac{262}{16} = 16.4\,\text{W per channel}

Use the Amplifier Clipping calculator to find peak voltage and clipping power.

Use the Power Amplifier Gain calculator to verify voltage gain (typically 26–34 dB for power amps).

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Speaker Impedance Matching

Amplifiers are rated into specific loads. Operating into a lower impedance draws more current:

I_{peak} = \frac{V_{peak}}{Z_{speaker}}

A 100W/8Ω amplifier produces $V_{peak} = \sqrt{2 \times 100 \times 8} = 40V$ peak. Into 4Ω at the same voltage, power doubles to 200W — but current also doubles to 10A peak. The output transistors must handle this.

Speaker sensitivity determines how loud it will play for a given power:

\text{SPL} = S + 10 \log_{10}(P) - 20 \log_{10}(d)

where $S$ is sensitivity in dB/W/m. A 90 dB/W/m speaker at 100W produces 110 dB SPL at 1m.

Use the Speaker Sensitivity calculator to predict SPL at listening distance.

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Amplifier Classes Compared

Class	Quiescent Current	Efficiency	Distortion	Best For
A	High (equal to peak)	25–50%	Very low	Hi-fi headphones
AB	Low	50–70%	Low	Home audio
D	~0	85–98%	Low (with feedback)	Portable, automotive
G/H	Low	60–80%	Low	High-power home audio

Class AB: The Standard

Class AB operates with small quiescent current to eliminate crossover distortion, while delivering efficiency better than Class A. Output transistors idle at 10–50 mA each.

Power dissipation at maximum output is actually lower than at half power (counter-intuitive). Worst case dissipation occurs at $V_{out} = 0.64 V_{supply}$ .

Class D: The Modern Choice

Class D uses PWM to switch output transistors fully on or off. Typical efficiency: 85–95%.

Use the Class D Efficiency calculator to estimate efficiency from MOSFET RDS(on) and quiescent current.

Trade-offs: Requires output LC filter (adds cost and size), EMI from switching frequency, may require careful layout. Integrated Class D ICs (TPA3116, MAX9744) include the filter and handle most of the complexity.

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Headphone Amplifiers

Headphone amplifiers face a different design problem: driving high-impedance loads (32–600Ω) from a low supply voltage.

Required power for 110 dB SPL from a 300Ω/100 dB/mW headphone:

P_{req} = 10^{(110-100)/10} = 10\,\text{mW}

V_{rms} = \sqrt{P \times Z} = \sqrt{0.01 \times 300} = 1.73\,\text{V}

Use the Headphone Power calculator to calculate required voltage and current from your headphone specs.

Output impedance matters: for minimum frequency response deviation, the amplifier output impedance should be < 1/8 of the headphone impedance. For 32Ω cans, keep

Z_{out} < 4\Omega

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Noise Floor and SNR

The noise floor determines the dynamic range of the amplifier. For an audio system:

\text{SNR} = \frac{V_{signal,max}}{V_{noise,RMS}}

A 120 dB SNR (state of the art) means the noise is 1 million times smaller than full-scale signal.

Sources of Noise

1. Johnson noise in resistors: $V_n = \sqrt{4kTRB}$ 2. Op-amp input noise: specified as nV/√Hz voltage noise + pA/√Hz current noise 3. Power supply noise: must be well-filtered; use LC filter + local capacitor

Use the Audio SNR calculator to calculate SNR from signal and noise levels.

Op-Amp Selection for Audio

For audio preamplifiers:

NE5532: classic, low noise (5 nV/√Hz), inexpensive
OPA2134: JFET input, very low distortion, 8 nV/√Hz
LM4562: 2.7 nV/√Hz, excellent for precision stages

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Protection Circuits

Every power amplifier needs:

1. DC offset protection: A relay that disconnects the speaker if DC offset exceeds ~50–100 mV. Protects the speaker from DC current.

2. Thermal protection: Thermistor on the heatsink that reduces gain or disconnects if temperature exceeds 80°C.

3. Short-circuit protection: Current limiting (reduce drive if $I > I_{max}$ ) or fuses at the output.

4. Tweeter protection capacitor: First-order high-pass filter to block low frequencies from the tweeter.

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Practical Design Checklist

[ ] Calculate max output power from supply voltage and load impedance
[ ] Verify transistor/IC current rating (1.5× peak)
[ ] Calculate gain (typically 26–34 dB, set by resistor ratio)
[ ] Check slew rate for full-power bandwidth ≥ 20 kHz
[ ] Size heatsink: P_dissipation at 1/3 full power (worst case for AB)
[ ] Verify SNR > 90 dB (noise floor < −90 dBV)
[ ] Add DC protection relay
[ ] Decouple supply rails locally (10 μF + 100 nF ceramic)

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