I have a Sony STR-DE845 home theater amplifier with 8 Ohm speakers. I want to measure the voltage sent to the speakers to infer their volume using an Arduino. The AC voltage measured across a speaker ranges from 20mV to 180mV using a true RMS voltmeter when playing music or various test tones over the audible volume range (are those readings too low?).
To do this I want to:
-amplify the AC signal with a gain of ~100 (output is 2VRMS to 18VRMS)
-put the amplifier signal through a full bridge rectifier made of 1N4148's (output is ~0V to 16VDC)
-Use a voltage divider and zener diode to give ~0 to 5V DC for the arduino's analog pin
If the above makes sense, what kind of circuit should I use to amplify the AC signal by 100? I have some AD623 instrumentation amplifiers, 2N2222 transistors, 1N4148 diodes, etc.
Best Answer
It might make more sense to use a precision rectifier and avoid the bridge. Below is an example that requires dual supplies (+/-5), for concept only, you can play with the values. It's a half-wave precision rectifier followed by a low-pass filter. You will have to consider the reference vs. grounding, many amplifiers are bridging type and do not have a common ground reference (from channel to channel or to earth).
The rectifier amplifies the AC-coupled input signal by -R1/R3 = -47 for positive input.
simulate this circuit – Schematic created using CircuitLab
Output with 100mV 1kHz sine wave input: