Index Signal & AmplificationAmplifier Voltage & Current

Amplifier Voltage & Current

What a power rating means in volts and amps at the speaker terminals.

Voltage, rms28.3V

Voltage, peak40V

Current, rms3.54A

Current, peak5A

Peak to peak80V

Power level20.0dBW

Voltage at the speaker terminals

The supply rails must sit a few volts beyond ±40 V to deliver this without clipping.

Same voltage into other loads

16 Ω, 1.77 A rms50 W

8 Ω, 3.54 A rms100 W

4 Ω, 7.07 A rms200 W

2 Ω, 14.1 A rms400 W

An ideal voltage source doubles its power each time the impedance halves. The current doubles too, and that is what real amplifiers run out of.

Input

Continuous power

P

Load impedance (Ω)

How it works

Power, voltage and current

A power rating is only meaningful together with the load it was measured into. For a resistive load $R$ :

V_{rms} = \sqrt{P\,R}

I_{rms} = \sqrt{\frac{P}{R}}

P = \frac{V_{rms}^{2}}{R}

Peaks

A sine wave peaks at $\sqrt{2}$ times its rms value, so the output stage and its supply must handle

V_{pk} = \sqrt{2\,P\,R}

I_{pk} = \sqrt{\frac{2P}{R}}

100 W into 8 Ω needs ±40 V and 5 A at the peaks. The same 100 W into 4 Ω needs only ±28 V but 7 A.

Decibel watts

L_{dBW} = 10\log_{10} P

Loudness follows this scale, not the watt figure. Going from 50 W to 100 W adds 3 dB, a small step. Ten times the power is needed for music to sound roughly twice as loud.