A simple passive logarithmic VU-meter

Posted on Feb 5, 2014

A VU-meter is a very common instrument usually installed on audio Hi-Fi amplifiers and is used to show the instantaneous power provided to the speakers. It is usually connected at the output of the amplifier, in parallel with the speaker. The term `VU` stands for Volume Unit and should be defined with 0VU being equal to +4dBm, or 1. 23RMS across a

A simple passive logarithmic VU-meter
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600 © load. 0VU is often referred to as "0dB" and volume units are very rarely used. In consumer electronics amplifiers this standard is rarely met and the meters only somehow display the power of the amplifier without referring to any physical unit. Anyway, the only thing one probably wants to know is how much power the amplifier is providing to the speakers and nobody really cares about volume units. So a meter that gives a reading between zero and the maximum power output is fine enough (at least, for me). The audio volume is often expressed using a logarithmic scale because the feeling of sound intensity we get from our ears is also logarithmic and a direct power reading in Watts would not be very representative of the perceived volume. For example, a few millivolts on an 8 © speaker are clearly audible. When listening at low volume, the voltage on the speakers is only a few hundreds millivolts (these are only milliwatts). When the volume is raised to a medium setting, the voltage on the speaker is around few volts (the power is around a Watt) and only when the volume is very high that all the watts of the amplifier are required. Non-professional audio amplifiers often have simple (and useless) VU-meters. For example, the circuit diagram shown in the figure below was found in my stereo set (rated 80WRMS on 8 ©) and is composed by a diode rectifier (actually a voltage doubler composed by C1, D1 and D2), a filter...

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