Precision Audio Millivoltmeter

Measures 10mV to 50Volt RMS in eight ranges

Simply connect to your Avo-meter set @ 50uA range

Circuit diagram

Precision Audio Millivoltmeter-Circuit diagram


  • R1 909K 1/2Watt 1% Metal Oxide Resistor
  • R2 90K9 1/2Watt 1% Metal Oxide Resistor
  • R3 9K09 1/2Watt 1% Metal Oxide Resistor
  • R4 1K01 1/2Watt 1% Metal Oxide Resistor
  • R5 100K 1/4W Resistor
  • R6 2M2 1/4W Resistor
  • R7 82K 1/4W Resistor
  • R8 12K 1/4W Resistor
  • R9 1K2 1/4W Resistor
  • R10 3K3 1/4W Resistor
  • R11 200R 1/2W Trimmer Cermet
  • C1 330nF 63V Polyester Capacitor
  • C2,C3 100uF 25V Electrolytic Capacitor
  • C4 220uF 25V Electrolytic Capacitor
  • C5 33pF 63V Polystyrene Capacitor
  • C6 2u2 63V Electrolytic Capacitor
  • D1-D4 1N4148 75V 150mA Diodes
  • IC1 CA3140 Op-amp
  • IC2 CA3130 Op-amp
  • SW1 2 poles 5 ways rotary switch
  • SW2 SPDT switch
  • J1 RCA audio input socket
  • J2,J3 4mm. output sockets
  • B1 9V PP3 Battery
  • Clip for PP3 Battery


  • Connect J2 and J3 to an Avo-meter set @ 50uA range
  • Switching SW2 the four input ranges can be multiplied by 5
  • Total fsd ranges are: 10mV, 50mV, 100mV, 500mV, 1V, 5V, 10V, 50V
  • Set R11 to read 1V in the 1V range, with a sinewave input of 1V @ 1KHz
  • Compare the reading with that of another known precision Millivoltmeter or with an The oscilloscope reading must be a sinewave of 2.828V peak to peak amplitude
  • Frequency response is flat in the 20Hz-20KHz range
  • If you have difficulties in finding resistor values for R1, R2, R3 & R4, you can use the following trick:
  • R1 = 10M + 1M in parallel
  • R2 = 1M + 100K in parallel
  • R3 = 100K + 10K in parallel
  • R4 = 1K2 + 6K8 in parallel
  • All resistors 1% tolerance
author: RED Free Circuit Designs
circuit from