Capacitor C4 couples the output of U1 to a simple attenuator, which is used to provide a loss of 0 dB, 20 dB, or 40 dB, depending on the setting of range switch Sl. The circuits sensitivity is 10-V rms for fullscale deflection, so the attenuator gives additional ranges of 100-mV and 1-V rms. The attenuator output is connected through capacitor C5 to common-emitter amplifier Q1, which has a high-voltage gain of 40 To get linear scaling on the meter, we have to use an active-rectifier circuit built around U2.
Audio-millivoltmeter - schematic

That IC is connected so that its noninverting input is biased to the 0-V bus via R7. Capacitor C6 couples the output of Q1 to the noninverting input of U2; C7 is the compensation capacitor for U2. The voltage gain of .U2 is set by the difference in resistance between the output and the inverting input, and between the inverting input and the ground bus. One resistance is made up of the diode-bridge rectifier D1 through D4, the other by resistor RS. This circuit has a nearly flat frequency response to about 200 kHz.

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