You need an audio wattmeter to measure the power output of your audio amplifier?
You need one LM3915 IC and add a few passive components and you got a simple but effective audio wattmeter.
This sound wattmeter uses a row of colored LEDs as a scale to show the relative power output of your amplifier in watts. It is easy to be inserted into the speaker box. All you need is to hook a supply voltage to it. The value of R1 depends upon the impedance of the speaker being used. The table near the sound wattmeter diagram shows the necessary values of R1.
LM3915 audio wattmeter circuit diagram
If you want to apply the circuit to stereo systems you must build two identical circuits.
The supply voltage of the circuit is 12…20 volts/50 mA DC adapter. Take note that the LM3915 indicates only the positive swing of the signal. Anyway, by testing amplifiers it does not matter anyhow since a sine wave is normally used as a test signal. To test the circuit without using an actual speaker, you can connect a dummy resistor with a value of either 8 or 4 ohms.