How can I test the output power of an amplifier to determine if it's working properly?
NOTICE
The method described in this document is not designed to determine the actual "rated," "peak," or "dynamic" output power of an amplifier.
It is specifically written for the "do it yourself" car audio fanatic as a troubleshooting method designed to verify power output of a properly working amplifier. This document recommends using test equipment and measurement procedures that are fairly easy to work with, versus professionals who use oscilloscopes, specific voltage measurements, distortion limits and signal-to-noise parameters to accurately test and measure the actual output power of amplifiers per CTA-2006 guidelines.
WARNING
Please follow the steps outlined below to prevent possible damage to your audio system. You can measure the actual output power (in Watts) of your amplifier by using the following tools:
- Signal Generator -or- Test CD with test tones recorded at 0dB (all bits high.)
- A.C. Voltmeter.
| Using a Signal Generator to Measure Amplifier Output Voltage |
 - Disconnect the source inputs from the amplifier. These may either be Low-Level RCA's or Hi-Level input wires from the source unit.
- Disconnect all speaker wires from the output connectors of the amplifier.
- Disable any electronic crossovers built-in to the amplifier and/or disconnect any passive crossovers installed at the amplifier outputs.
- With the Signal Generator OFF, connect the output of the Signal Generator to the inputs of the amplifier using short RCA cables.
- With the A.C. Voltmeter set to ~ AC VOLTS, in a range of 10 to 100 Volts, connect the voltmeter (+) test lead to a single channel of the amplifier (+) output. Connect the voltmeter (-) test lead to the same amplifier channel's (-) terminal. If the amplifier is being operated in Bridged/Mono mode, connect the test leads to the (+) and (-) bridged output terminals of the amplifier.
- Turn the Signal Generator ON and adjust to a frequency used to normally power the speakers. For example, if measuring a subwoofer amplifier, tune the Signal Generator to 50Hz. If measuring a midrange/tweeter amplifier, tune the Signal Generator to 1kHz.
- Document the output voltage displayed on the voltmeter
|
| Using a Test CD to Measure Amplifier Output Voltage |
- Disconnect all speaker wires from the output connectors of the amplifier.
- Disable any electronic crossovers built-in to the amplifier and/or disconnect any passive crossovers installed at the amplifier outputs.
- Insert the Test CD into the source unit.
- With the A.C. Voltmeter set to ~ AC VOLTS, in a range of 10 to 100 Volts, connect the voltmeter (+) test lead to a single channel of the amplifier (+) output. Connect the voltmeter (-) test lead to the same amplifier channel's (-) terminal. If the amplifier is being operated in Bridged/Mono mode, connect the test leads to the (+) and (-) bridged output terminals of the amplifier.
- Select the appropriate track on the Test CD to a frequency used to normally power the speakers. For example, if measuring a subwoofer amplifier, select the 50Hz track recorded at "0dB". If measuring a midrange/tweeter amplifier, select the 1kHz track recorded at "0dB".
- Document the output voltage displayed on the voltmeter
|
| Formula for Calculating Amplifier Power (in Watts) |
Now that we have a measurement of output voltage from the amplifier, we can convert Volts into Watts. For this procedure we'll need the following:- The formula for converting Voltage to Watts (see above)
- The calculated impedance of the speaker system connected to the amplifier.
|
Example: We'll use the the above formula to calculate the actual output power of an a amplifier used to drive (2) 8-ohm subwoofers wired in parallel.
- Since our amplifier is driving subwoofers, we'll use the track on our Test-CD that generates 50Hz @ 0dB.
- We'll be operating our amplifier in Bridged/Mono mode. We measured the AC output voltage as 24.5V RMS using the L(+) and R(-) bridged amplifier output teminals.
- Our (2) 8-ohm subwoofers wired in parallel will present a 4-ohm amp load (see below.)
- Now we use our formula ... 24.5 (volts) x 24.5 (volts) = 600.25 (volts)
- 600.25 (volts) divided by 4 (ohms) = 150.0625 Watts
- Our amplifier will produce 150 Watts bridged.
- Each subwoofer will "see" 75 Watts.
