Measure Output Power of Audio Amplifier Using Power Resistor

Are you worrying about the audio amplifier wattage calculation? Most people don’t know how to find the real output power of an amplifier. So in this article I will share the information on how to Measure output power of an audio amplifier using a power resistor. To measure audio amplifier power using a resistor you need to connect a known resistance (matching the speaker impedance) in series with the speaker output then measure the voltage across the resistor. After measuring the voltage across the resistor calculate power using the formula of

• P = Power in RMS
• V = Voltage measured across the resistor.
• R = Known resistance of the connected power resistor.

Amplifier Output Power Calculator

What is Power Resistor

The power resistor is a resistor specially designed to handle higher power levels compared to standard resistors. When testing an amplifier's output watt a power resistor can be used as a load to simulate the speaker impedance. It will be 4 ohms or 8 ohms with high wattage resistors. After connecting the resistor we will measuring the voltage and current across the resistor the amplifier's output power can be calculated.

How to Measure Output Power of Audio Amplifier Using Power Resistor

Choose a resistor with a resistance value equal to the speaker's impedance (e.g., 8 ohms or 4 ohms). Ensure the resistor has a power rating that is significantly higher than the expected power output of the amplifier. Power rating is indicated in watts if you are expecting 20 watt of amplifier output please choose 30 watt resistor for testing. 

Connect the resistor in series with the speaker output and effectively replace the speaker as the load. Feed a test signal (e.g., a sine wave) into the amplifier's input. The perfect signal frequency for testing is 1KHz sine wave. You can use a Function generator circuit to get the appropriate sound signals for testing. Use a multimeter or oscilloscope to measure the voltage across the resistor.

Before measuring the voltage increase the input signal level until you observe clipping on the oscilloscope waveform, then slightly reduce the level to avoid distortion. Be cautious when testing high-power amplifiers, as prolonged clipping or high power test tones can damage speakers diaphragms.

Calculation

If using an oscilloscope, measure the peak-to-peak voltage and calculate the RMS voltage. Use the formula P = V² / R, where P is power (in watts), V is the RMS voltage, and R is the resistance value. 

Example: If you measure 20 volts RMS across an 8-ohm resistor, the power is (20²) / 8 = 50 watts.