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Old 1st November 2012, 07:25 PM   #1
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Default meters on chipamps

Im getting ready to knock the rust off what little I learned about electronics years ago and build an amp or two. While Ive found some great ideas here, there is one detail Id like to add that Im missing. The analog meters on the front of older amps always intrigued me. Id like to add them to what I end up building, for mostly aesthetic reasons. My question is, how and where? On the power side of the amp monitoring current flowing into the amp, just before the speaker jacks to measure delivered amps, or voltsor ??? Id like not to mess with sound quality in any large way. Any thoughts, resources, etc???

My apologies if this has already been answered. I used search and gave up after a while of not finding what I was looking for.

Many thanks,
Jeff
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Old 1st November 2012, 07:29 PM   #2
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if You want the meters to somewhat reflect the setting of the volume knob, then You add an opamp after the volume controlls of the amplifier, and make that drive the meters.
otherwise You connect the opamp before the volume knob, and it will reflect the volume of the source.
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Old 1st November 2012, 07:35 PM   #3
Mooly is offline Mooly  United Kingdom
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Most meters use a simple passive rectifier and read the "voltage" across the speaker terminals. The meter is calibrated such that this "voltage" equates to "power into 8 ohms". So in practice that means that for 2.83 volts sine wave RMS voltage the meter scale will say 1watt at that point. Yes

Downsides are that this in no way measures real power. If you disconnect the speakers the meter still reads the same because the voltage at the speaker terminals on the amp is the same.

To measure power properly (and I don't know of any amps that do this) would need to take into account the voltage and current and phase angles that a reactive load like a speaker causes. It can be done but the meter circuit would be more complex than the amp.

Disadvantages of the simple meter circuit are that it can be insensitive and being passive there is the volt drop of the diodes to overcome. Germanium can be best here or you can use a simple opamp based rectifier to eliminate the diode drop completely.

Best thing you can do is look up some old manuals of amps (and cassete decks with VU meters becaus ethey are essentially the same and just measure voltage) and see how they are configured.

The loading od such circuits is negligable on any amp and won't alter the sound in any way at all.
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Old 1st November 2012, 09:28 PM   #4
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Real VU meters are either relatively complex, relatively expensive, or (most likely) both. Rod Elliott has an alternative circuit as ESP Project 55 that should satisfy.
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Old 1st November 2012, 09:37 PM   #5
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Thanks everyone!
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Old 1st November 2012, 09:46 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

VU (volume units) meters are useful in radio stations and driving tape recorders.

They are pretty useless on power amplifiers other than as decoration.

rgds, sreeten.
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Old 2nd November 2012, 12:02 PM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Mooly View Post
Most meters use a simple passive rectifier and read the "voltage" across the speaker terminals. The meter is calibrated such that this "voltage" equates to "power into 8 ohms". So in practice that means that for 2.83 volts sine wave RMS voltage the meter scale will say 1watt at that point.
Just label them with Vac instead of W

I would rather know I had maximums around 1V or 5V or 25V, than some power figure that is meaningless.

But, there again I would probably use a dB scale, starting at +1dB = maximum sinewave voltage into a 50r test load.
Then work down from there to maybe -30dB for the lowest green LED.
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Old 2nd November 2012, 12:29 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Just label them with Vac instead of W

I would rather know I had maximums around 1V or 5V or 25V, than some power figure that is meaningless.

But, there again I would probably use a dB scale, starting at +1dB = maximum sinewave voltage into a 50r test load.
Then work down from there to maybe -30dB for the lowest green LED.
Or we can be safe in the knowledge of our designs and use a term coined by Rolls Royce,

Power output is 'undisclosed' but 'entirely sufficient'.

I think meters are more eye candy than anything else... and having said that my amp has them as you know and yet in practice they never move off the end stops for 95% of my listening (that's the lower end if you are wondering )
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Old 2nd November 2012, 12:39 PM   #9
AndrewT is offline AndrewT  Scotland
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I didn't know but thanks for the info.

How have you scaled them, volts, watts or dB?
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Old 2nd November 2012, 01:00 PM   #10
Mooly is offline Mooly  United Kingdom
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They are scaled in watts and calibrated to the "volts into 8 ohm" standard. These were originally a Tandy (Radio Shack) free standing meter assembly that I got as a youngster and always held onto. Seemed an ideal use for them after all these years. I added LED backlighting which doesn't look brilliant in the photo but is very soft and even in reality.
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