VU-meter

Posted on Feb 6, 2014

Quad comparators, type LM339. A flyback DC converter Done. Buck converter Step-up converter Done. that would consume several comparators and maybe there would be some nice LEDs, too. Then it hit me. A VU meter, big, beautiful, professional! From that moment started mythoughts flowing. Those VU-meter ICs that are sold in every electronics shop Maybe ten LEDs. I wanted more, say.

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40 LEDs a channel! 1 dB apart. That could be something. 40 LEDs each accompanied by a comparator, reference voltages generated by a chain of resistors. Linear meter would have been easy to make, but a real VU meter has to be logarithmic. Maybe the meter could be used as a power meter in a power amplifier. VU meter uses formula 20log(V/V0) and power meter formula 10log(V/V0). A spreadsheet for calculating those resistors, easy. Other electronics would contain an input stage, ideal rectifying and RC circuit for smoothing the signal after rectifying. The hit the next problem: The RC circuit. If the VU meter would go down to -40 dB, it would take a long time the meter to decay after an impulse. I did some calculations and noticed, that if the meter would work satisfyingly at 25 Hz input signal, the time constant of that RC circuit would be very long. Decaying after a loud impulse would take seconds. Unacceptable. Well, I included a sample/hold circuit with a digital control. It works as follows: Ideal rectifying finds peaks from the input signal and collects them into a capacitor. That`s the sample part. Then the charge from the capacitor will be transfered into another capacitor, that holds its voltage constant while the first capacitor is discharged and another peak value is collected into it. The LED bar shows the voltage of the second capacitor all the time. That way when a short peak comes into the meter it will be viewed...

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