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Audio Power Amplifier Calculator

Calculate amplifier output power from supply voltage and speaker impedance. Get max power, RMS power, THD estimate by class (A/AB/D), SNR, and input sensitivity for speaker matching.

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Formula

Pout=Vout,peak22RL,Vout,peak=Vcc×0.92P_{out} = \frac{V_{out,peak}^2}{2 R_L},\quad V_{out,peak} = \frac{V_{cc} \times 0.9}{2}

Reference: Cordell, "Designing Audio Power Amplifiers" 2nd ed.

Vcc— Supply voltage (V)
RL— Speaker impedance (Ω)
Vout_peak— Peak output voltage swing (V)
η— Amplifier efficiency (%)

How It Works

This calculator computes maximum output power, voltage swing, and efficiency for audio power amplifiers based on supply voltage, load impedance, and amplifier class. Audio engineers, electronics designers, and DIY builders use it to size amplifiers for speaker loads and estimate thermal dissipation. Maximum output power follows P_max = (V_peak)^2/(2*Z_L), where V_peak is approximately 0.9*V_supply for Class AB (accounting for output stage saturation voltage). According to AES measurements, Class A achieves 25% efficiency (75% heat), Class AB achieves 50-65% efficiency, and Class D achieves 85-95% efficiency. A 12 V single-supply Class AB amplifier delivers 1.8 W into 8 ohms with 50% efficiency (1.8 W dissipated as heat). The IEC 60268-3 standard specifies power measurement using 1 kHz sine wave at 1% THD threshold. Understanding amplifier classes per IEEE 1789-2015 is critical for thermal design - a 100 W Class AB amplifier at 50% efficiency dissipates 100 W as heat requiring substantial heatsinking.

Worked Example

Problem

Design a Class AB amplifier for automotive 12 V system driving 4-ohm speakers at maximum clean output per IEC standards.

Solution
  1. Supply voltage: Vcc = 12 V (single supply with virtual ground)
  2. Effective swing: V_peak = 0.9 * (12/2) = 5.4 V (half-supply reference)
  3. Maximum RMS voltage: V_rms = 5.4/sqrt(2) = 3.82 V
  4. Maximum power: P = (5.4)^2/(2*4) = 29.16/8 = 3.65 W per channel
  5. For bridge-tied-load (BTL): V_peak doubles to 10.8 V
  6. BTL power: P = (10.8)^2/(2*4) = 14.6 W per channel
Efficiency calculation at 14.6 W output (Class AB):
  • Output power: 14.6 W
  • Theoretical Class AB efficiency: eta = (pi/4)*(V_out/V_supply) = 78.5% at max output
  • Actual efficiency with losses: ~65% (typical)
  • Power dissipation: 14.6/0.65 - 14.6 = 8.0 W
  • Heatsink requirement: 8 W at 40C ambient, 85C junction -> Rth_ja < 5.6 C/W
For higher power, use TDA7293/TDA7294 with +-25 V supplies: P = (22.5)^2/(2*8) = 31.6 W into 8 ohms

Practical Tips

  • ✓For Class AB, thermal design is critical: at 50% efficiency, half of input power becomes heat. A 100 W amplifier dissipates up to 100 W requiring a heatsink with Rth < 0.5 C/W (large finned heatsink or forced air). Class D at 90% efficiency dissipates only 11 W for same output - often heatsink-free per typical reference designs.
  • ✓Match amplifier power to speaker sensitivity and room size per this guideline: for 90 dB/W/m speaker in 30 m2 room at 3 m distance, 50 W provides 100 dB peaks (adequate for most music). For 85 dB/W/m speaker, same scenario requires 160 W. Use the speaker sensitivity calculator to determine requirements.
  • ✓Class D amplifiers (TPA3116, TPA3255, ICEpower) achieve 90-95% efficiency but require LC output filters that can ring with capacitive speaker cables. Keep speaker cables under 3 m or add Zobel networks (10 ohm + 100 nF series to ground) per TI application notes.
  • ✓For battery-powered applications, Class D is mandatory: a Class AB amplifier delivering 10 W at 50% efficiency draws 20 W from battery; Class D at 90% efficiency draws only 11.1 W - nearly 2x battery life. Modern Class D THD+N reaches 0.01-0.05%, competitive with Class AB per Audio Precision measurements.

Common Mistakes

  • ✗Using theoretical Class AB efficiency (78.5%) instead of real-world values (50-65%) - output stage quiescent current, driver stage losses, and power supply resistance reduce efficiency by 15-25 percentage points. Budget for worst-case 50% efficiency in thermal calculations per AES guidelines.
  • ✗Confusing peak power with continuous (RMS) power - marketing specs often quote peak or PMPO ratings that are 4-8x higher than continuous ratings. IEC 60268-3 specifies RMS power at 1% THD with 1 kHz sine wave. A '200 W PMPO' amplifier typically delivers only 25-50 W RMS continuous.
  • ✗Ignoring speaker impedance variations - a nominal 8-ohm speaker may drop to 3-4 ohms at certain frequencies, demanding 2-3x the current. This causes current limiting in amplifiers designed for 8-ohm minimum, resulting in clipping and distortion at those frequencies. Check minimum impedance on speaker datasheet.
  • ✗Forgetting voltage headroom for op-amp/driver stages - rail-to-rail output ICs lose 0.5-1.0 V per rail; conventional op-amps lose 2-3 V per rail. A 'rail-to-rail' TPA3116 with 24 V supply actually swings to 22-23 V peak, reducing calculated power by 10-15%.

Frequently Asked Questions

Per blind listening tests (Harman, AES conventions), modern Class D amplifiers are indistinguishable from Class AB when both are operating within linear range. Class A provides lowest crossover distortion (no zero-crossing artifacts) with THD < 0.001%, but at 25% efficiency is impractical above 20-50 W. Class AB is the traditional choice with THD 0.01-0.1%. Modern Class D (Purifi, Hypex, Pascal) achieves THD < 0.001% and SNR > 120 dB - exceeding most Class AB designs.
Class does not directly affect maximum power (that depends on supply voltage and load), but efficiency determines sustainable power. A Class AB amplifier with 50 W output at 50% efficiency dissipates 50 W as heat; the same supply/load with Class D at 90% efficiency dissipates only 5.5 W heat. This allows Class D amplifiers to deliver rated power continuously without thermal limiting, while Class AB may thermally derate 30-50% under sustained full-power operation per AES2-1984 power compression testing.

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