Precision Audio Millivoltmeter

Measures 10mV to 50Volt RMS in eight ranges
Simply connect to your Avo-meter set to 50µA range

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