Precision Audio Millivoltmeter

Measures 10mV to 50Volt RMS in eight ranges

Simply connect to your Avo-meter set to 50µA range

Circuit diagram:

Audio Millivoltmeter

Parts:

R1_____909K    1/2W 1% Metal Oxide Resistor
R2______90K9   1/2W 1% Metal Oxide Resistor
R3_______9K09  1/2W 1% Metal Oxide Resistor
R4_______1K01  1/2W 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__100µF   25V Electrolytic Capacitor
C4_____220µF   25V Electrolytic Capacitor
C5______33pF   63V Polystyrene Capacitor
C6_______2µ2   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

Notes:

  • Connect J2 and J3 to an Avo-meter set to 50µA range:
  • Switching SW2 the four input ranges will 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 sine wave 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 1/4W 1% tolerance