Audio mV Meter

Posted on Feb 5, 2014

This was my first AS Electronics project which I built way back in 1998! The original report, with diagrams, is shown here and a PDF version of the report is available for download. There are many occasions where I need to be able to measure small audio signals from various pieces of equipment and circuits. A millivolt meter would be very handy on

Audio mV Meter
Click here to download the full size of the above Circuit.

these occasions and therefore my project is an Audio Millivolt Meter. I have looked at two articles in electronics magazines (2, 3) for distortion meters which feature mV meters, and I have also looked at a millivolt meter circuit in an Op-Amp book (2). The designs are all basically the same. They consist of the following stages: The four step attenuator at the input of the circuit gives the meter its four ranges of 10mV, 100mV, 1V and 10V. The gain stage amplifies the output signal from the attenuator. This is then amplified again by the meter drive stage. The output drives a full wave rectifier which drives the moving coil meter. Two gain stages are required due to the gain-bandwidth product of the op-amp. The attenuator stage of the meter gives it its ranges of 10mV, 100mV, 1V and 10V. A capacitor is connected in series with the input to stop any D. C. The basic sensitivity of the meter is 10mV, so the attenuator attenuates the input voltage for full scale for each of the range to 10mV. The input impedance of the meter is 100K © so: Figure 3 shows the meter drive circuit. The diodes rectify the A. C output from the op-amp. With no signal at the input to the op-amp, there is no output signal and so the diodes are turned off` (i. e. not forward biased). This effectively means that the feedback loop of the op-amp is open circuit and so the op-amp will deliver its full gain of the 10 x 1012. This is the typical gain for the...

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