Precision Audio Millivoltmeter
Measurement circuits

Precision Audio Millivoltmeter

By internum

Measures 10mV to 50Volt RMS in eight ranges

Simply connect to your Avo-meter set @ 50΅A range.

Precision Audio Millivoltmeter circuit
Precision Audio Millivoltmeter circuit


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__100΅F   25V Electrolytic Capacitor
C4_____220΅F   25V Electrolytic Capacitor
C5______33pF   63V Polystyrene Capacitor
C6_______2΅2   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 @ 50΅A 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 oscilloscope

  • 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

1611 19 September 2009

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