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Old 6th November 2014, 10:28 PM   #1
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Lightbulb Simple Clean VU Meter Driver

Hi everyone.

Here's a quick circuit I drew up the other day when I failed to find any VU meter circuits could drive a meter without giving a distorted ground current of the original signal, which is bound to cause problems unless care is taken to prevent it sharing a path with the audio signal. It's also very simple, requiring only 1 op-amp, a resistor, 4 diodes and a capacitor per channel. It does not suffer from poor sensitivity at low signal levels due to diode threshold voltage either. Whats more, the ground current is an undistorted version of the original signal, reducing the chance of distortion in the main signal path. Two meters can be built with 1 TL072.

The circuit consists of a bog standard bridge rectifier driven VU meter, but the bridge is enclosed within the feedback loop of U1. U1 must drive the bridge rectifier to produce a voltage equal to the input voltage across R1 (which can be adjusted for calibration purposes), ensuring the ground current is undistorted.

All in all a simple yet very effective design.
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With perfect linearity, it is impossible to go off on a tangent. Also; My Sziklai pairs are better than your MOSFETs/Darlingtons/Pentodes/Triodes
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Old 9th November 2014, 05:26 AM   #2
PRR is offline PRR  United States
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Yes, effective enough for Boonton many decades ago. (They used a row of tubes.)

I might point out that a true VU meter does not have the Ohms/uA shown in your sketch; but your plan can be trimmed to cover. One gotcha is that if the trimmer is set to zero (oops!), you can have full opamp current into the movement.

You can also avoid ground current with two opamps. At today's prices that is not a big burden. You can also simply take input differentially, which can be handy in some situations.
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Last edited by PRR; 9th November 2014 at 05:29 AM.
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