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Old 13th September 2008, 02:38 PM   #1
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Default Movig Coil VU Meters

Hi All

I've found a pair of moving coil VU meters from an old cassette deck and would like to use them in an amp I'm building.

What would I need to measure on them, and where abouts in the circuit would be best to fit them.

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Old 14th September 2008, 10:22 AM   #2
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You need to determine how much current it takes for full deflection, which will usually in the microamp or very low milliamp range, so do be careful when doing this and use something like a 1.5V AA battery and large limiting resistors (300K or some such) and work your way down from there till you find the current value.

At this point, you can take the output of the power amplifier and knock it down with a voltage divider, and use the voltage to drive a simple voltage-to-current converter opamp circuit.
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Old 14th September 2008, 11:17 AM   #3
Mooly is offline Mooly  United Kingdom
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You can make it as simple or as sophisticated as you want.
Simple = a passive network using a couple of gemanium diodes and caps and a preset to calibrate.
Sophisticated = Active driver + rectifier, maybe with fast attack and slow decay.
There must be dozens of meter drive circuits around.
Such as
First one out of the hat
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Old 14th September 2008, 01:43 PM   #4
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Good grief! I forgot about rectifying the signal...

Good link.
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Old 14th September 2008, 01:54 PM   #5
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Default The rectifier can be Silicon or Germanium... to very low signals

It is prefered to use Germanium, as it will conduct starting into 100 milivolts...while the Silicon will start around 500 milivolts.

The trimpot will allow you to adjust the 100 percent indication, or Zero VU, or even S9 radio signal into your undistorted audio power threshold of clipping.

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Old 14th September 2008, 07:48 PM   #6
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Nice one, thanks everyone. I'll look into these and let you know how it goes.

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