Using this circuit, you can measure the actual output power of your amplifier. You can put this circuit in a box as a measurement instrument, or you can integrate it inside your power amplifier to get real power display. The circuit’s schematic diagram is shown in the figure below.
This circuit would be a handy instrument for audio engineer, for field testing and checking of sound system installations. Because the scale is logarithmic, you can measure wide range of audio output with only ten scales. Look at the pin 5 of the LM3915, you can see that the input is not rectified. The negative swing will present at this input pin, but it harmless because the current is limited by R1, and the LM3915 will respond to positive cycle only.
The absence of peak detector or average detector will give the circuit a fast reading of instantenous power, and this gives us insight of both average and peak condition. For more readable peak or average mesurement, you can use peak or average detector circuit. [Circuit’s schematic diagram source: National Semiconductor Applictaion Notes]