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Old 3rd June 2004, 02:23 PM   #1
PatPet is offline PatPet  Hong Kong
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Default Audio VU meter connected to speaker output?

Guys,
If an audio power meter is connected in parallel with the speaker to the speaker output of the amp circuit, would the amp pull out with less power as compared with occasion that the meter is not connected?
I'm confused with the circuits on the web but I'm interested to add the meters to decorate the front panal. So I must be sure that adding the meter wouldn't affect the sound quality or the power output of the amp itself.

Thank you.
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Old 3rd June 2004, 03:49 PM   #2
azira is offline azira  United States
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It depends on the type of meter/circuit you use. If you are using a voltage measuring circiut then it should have a fairly high impedance. In this case, putting it in parallel with the speaker is fine and the right thing to do.
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Old 3rd June 2004, 04:59 PM   #3
PatPet is offline PatPet  Hong Kong
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Ah yes I almost forgot

If the circuit has an impedance like 5k ohm and the speaker measures 8 ohm norminal,

then the total impedance is (1/8 + 1/5000) ^ (-1) which is approximately 8 ohms also.

Correct me if there is any mistakes.

Thank you
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Old 3rd June 2004, 11:28 PM   #4
azira is offline azira  United States
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Nope, dead on. Basically you are dealing with "orders of magnitude" as EEs work with them. If you're concerned with 1% tolerance, then you just need the parallel resistance to be 100* your load. or 800 ohms. Some speakers rise to up to 30 ohms in their usable range, so to be safe, 30*100 = 3k, 5k is still plenty fine... if you're using 1% parts. Personally I use 5% parts so my tolerance levels are 5%.
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Old 5th June 2004, 01:02 AM   #5
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We are of course assuming that the meter is designed
to accept an AC input. Mechanically, meters are DC
creatures, so it requires some circuitry to drive them.

And of course, most VU meters are calibrated for
lower levels than the average speaker will see, so you
might want to consider some sort of voltage divider.
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Old 5th June 2004, 07:33 AM   #6
PatPet is offline PatPet  Hong Kong
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Thank you Nelson

But the point is i have to make sure that adding the potential

divider wont change the total load impedence in great amount so

that the output to the speaker is not affected much
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Old 7th June 2004, 07:06 PM   #7
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Using a divider will actually isolate the amplifier more from
the meter, and it wouldn't be much of a concern anyway
as the meter will have a much higher impedance than the
speaker.
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Old 7th June 2004, 07:16 PM   #8
PatPet is offline PatPet  Hong Kong
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Thank you
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Old 11th June 2004, 01:36 PM   #9
PatPet is offline PatPet  Hong Kong
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Another thing is, if the impedence peaks at about 100 - 200ohm at certain frequency(i think that is the internal crossover frequency), it is still no need to consider about the effect on the amp circuit with the presence of such VU meter with potential divider?
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