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Old 5th May 2006, 07:50 PM   #1
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Default Do amplifiers produce voltage without a load?

I always thought that an amplifier would not produce a voltage at the speaker +/- terminal on the amp unless it had a load available, is this true? I was watching a demo on how to set your gains with a DMM when they set all setting flat hu/amp and put the DMM on the amps output to measure a predetermined voltage for a given load but it was all done without speakers connected, so where did the load come from? Any help would be greatly appreciated here is the link also...Polo.


http://mobile.jlaudio.com/support_pages.php?page_id=143
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Old 5th May 2006, 08:25 PM   #2
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Amps do produce output voltage without a load.
The demo looked correct to me.
I believe you can't do this test with amplifiers equipped with an output transformer.
Experts correct me if I'm wrong.

/Hugo
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Old 5th May 2006, 08:29 PM   #3
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The reason I ask is because the output varries dependant on load so what load are they imposing to get the readings on the DMM? I can see if all speakers are 4 ohm and it doesn't matter but how is the voltage supposed to vary if there isn't a load present? Polo..
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Old 5th May 2006, 09:00 PM   #4
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This is only a way of trimming a preset regardless of the variation in voltage the load will create.

/Hugo
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Old 5th May 2006, 09:47 PM   #5
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The load will not affect the voltage under 'normal' conditions. Only when loaded heavily the maximum available voltage will be limited.

I don't see the point of setting things by meter when the speaker sensitivities will negate all of it. You should set levels by ear or SPL meter/spectrum analyser.
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Old 6th May 2006, 12:45 AM   #6
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Quote:
Originally posted by richie00boy
I don't see the point of setting things by meter when the speaker sensitivities will negate all of it. You should set levels by ear or SPL meter/spectrum analyser.

If you're after sound quality and/or have plenty of self discipline, sure, but some people just want it loud, and setting the sub amp's gain so that you know when it clips and make sure not to exceed that is one of the best ways to save your equipment.
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Old 6th May 2006, 01:39 AM   #7
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Sooo what you guys are saying is that the voltage is a constant and it is the wattage that goes up/down with different loads? Excuse my ignorance but right when you think you know it all there is always someone out there to prove you wrong, lol. Polo..
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Old 6th May 2006, 03:02 AM   #8
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Well, what happens is that for the same voltage, different resistances cause different amounts of current; hence ohm's law.

If you set the volume and play on a four ohm speaker and then switch to an eight ohm of the same efficiency, you'll notice it's a lot quiter.

This is because the eight ohm speaker draws les current at that same voltage than the four ohm one did.

So yeah, power will change with load, but that's not based on any changes in the output voltage of the amplifier.
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Old 6th May 2006, 05:22 AM   #9
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Quote:
Originally posted by Duo If you set the volume and play on a four ohm speaker and then switch to an eight ohm of the same efficiency, you'll notice it's a lot quiter.[/B]
Just for the record and to avoid the typical inherent voluntary DIY auto-confuzzilisation:

The above statement is only true for referring to "power efficiency" (dB/W/m). In case of referring to "voltage sesitivity" (the typical dB/2.83V/m mostly used in data sheets) you should not experience any change of output level.

regards
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Old 6th May 2006, 06:22 AM   #10
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Quote:
Originally posted by Safe_Cracker
Sooo what you guys are saying is that the voltage is a constant and it is the wattage that goes up/down with different loads? Excuse my ignorance but right when you think you know it all there is always someone out there to prove you wrong, lol. Polo..
For most amps yes, for the JL slash amps no, because they actively monitor the subwoofer impedance (only on startup) and adjust the output voltage accordingly, to provide [roughly] the same amount of power at all impedances between 1.5 and 4. PG does the same thing with their Xenon amps.
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