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.

  • \$\begingroup\$ do you want RMS, peak, average or quasi-peak or peak and slow decay? \$\endgroup\$ Mar 10, 2019 at 1:25
  • \$\begingroup\$ I want something like the RMS or peak voltage averaged over ~10ms or more. The DC output will probably have a capacitor so the values do not change much in 10 to 100ms if the noise level is consistent (e.g. a test tone). \$\endgroup\$
    – kewiha
    Mar 10, 2019 at 2:37
  • \$\begingroup\$ for CW use peak then any precision rectifier half or full wave with good SNR with gain to optimize use, such as example given in answer \$\endgroup\$ Mar 10, 2019 at 3:52
  • \$\begingroup\$ But what is expected peak amplitude change dV/dt? \$\endgroup\$ Mar 10, 2019 at 3:58

1 Answer 1


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:

enter image description here

  • \$\begingroup\$ Is there a version of this that works with single supplies? \$\endgroup\$
    – kewiha
    Mar 10, 2019 at 1:46
  • 1
    \$\begingroup\$ Bias the two Vin(+) pins of the opamps to VDD/2. \$\endgroup\$ Mar 10, 2019 at 2:08
  • \$\begingroup\$ What is VDD? If I used a 12V single supply, would I bias to 6V? \$\endgroup\$
    – kewiha
    Mar 10, 2019 at 2:38

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