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Does power matter?


Jazzjames
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[quote name='dincz' timestamp='1338307063' post='1672485'][b][i](to a specified load)[/i][/b]
[/quote]Amps deliver the same voltage swing irrespective of load impedance. They have to, otherwise every note would be heard at a different volume, as impedance is not the slightest bit a constant. Current varies with load.

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[quote name='Bill Fitzmaurice' timestamp='1338310418' post='1672549']
Amps deliver the same voltage swing irrespective of load impedance.
[/quote]

On this I have to say that's absolutely untrue. Every amp I've ever bench-tested could deliver a higher voltage swing into an open circuit than with a load connected. The power rail voltage and maximum available output voltage swing reduce as you drop the load impedance. No surprises there, just Ohm's Law and real-world imperfect power supplies.

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On a side note, my Harley Benton BA500 (500w at 4 ohms) has a specified power consumption of 170w. I thought it was a typo until Mr Foxen pointed out that other class D amps he'd been looking at had similar power demands.

Anyone have an explanation of why this is? I would have sided with Dincz in saying that this is completely impossible, but now I'm totally confused. :blink:

Something strange going on with the power factor perhaps?

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[quote name='dincz' timestamp='1338311001' post='1672561']
On this I have to say that's absolutely untrue. Every amp I've ever bench-tested could deliver a higher voltage swing into an open circuit than with a load connected. The power rail voltage and maximum available output voltage swing reduce as you drop the load impedance. No surprises there, just Ohm's Law and real-world imperfect power supplies.
[/quote]An open circuit doesn't deliver a load. Within the operating load ranges actually presented by speakers amps must deliver a constant voltage, otherwise they simply won't work.

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[quote name='dincz' timestamp='1338311001' post='1672561']
On this I have to say that's absolutely untrue. Every amp I've ever bench-tested could deliver a higher voltage swing into an open circuit than with a load connected.
[/quote]

How exactly do you measure voltage across an open circuit?

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[quote name='Mr. Foxen' timestamp='1338318423' post='1672731']
How exactly do you measure voltage across an open circuit?
[/quote]

If you want to nitpick, then it's true that a multimeter across the output terminals is not an open circuit, but a megohm or so is near enough.

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[quote name='Bill Fitzmaurice' timestamp='1338317666' post='1672711']
Within the operating load ranges actually presented by speakers amps must deliver a constant voltage, otherwise they simply won't work.
[/quote]

Connect an 8ohm load, measure the maximum output voltage swing and then repeat with a 4ohm load. It'll be different on every single amplifier. Not even debatable, just a demonstrable as well as theoretical fact.

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[quote name='Musky' timestamp='1338317467' post='1672709']
On a side note, my Harley Benton BA500 (500w at 4 ohms) has a specified power consumption of 170w. I thought it was a typo until Mr Foxen pointed out that other class D amps he'd been looking at had similar power demands.

Anyone have an explanation of why this is? I would have sided with Dincz in saying that this is completely impossible, but now I'm totally confused. :blink:

Something strange going on with the power factor perhaps?
[/quote]

Basically RMS isn't putting out the absolute maximum it can forever. Music isn't like that, and that is why RMS is the more relevant value to music.

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[quote name='dincz' timestamp='1338322148' post='1672821']
Connect an 8ohm load, measure the maximum output voltage swing and then repeat with a 4ohm load. It'll be different on every single amplifier. Not even debatable, just a demonstrable as well as theoretical fact.
[/quote]Explain then why a speaker doesn't reproduce every note at a different level, when this is what a typical speaker's low frequency impedance looks like:

Edited by Bill Fitzmaurice
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Better explanation would be where those ideas come from, because I've seen stuff like that posted a few times, and usually claiming some sort of authority from 'experimentation' or a formula that doesn't apply to the situation, whilst ignoring the massive flaws in the internal logic of the situation.

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[quote name='Bill Fitzmaurice' timestamp='1338334423' post='1673073']
Explain then why a speaker doesn't reproduce every note at a different level, when this is what a typical speaker's low frequency impedance looks like:


[/quote]

We're talking at cross purposes here. You're talking about an amp's behaviour under normal operating conditions. I'm talking about maximum available output voltage swing i.e. at clipping.

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[quote name='Mr. Foxen' timestamp='1338322884' post='1672848']
Speakers aren't a resistive load. And you need to have a scope or other means of determining when you are at max clean output, or it doesn't tell you anything relevant.
[/quote]

No they aren't purely resistive and yes you do need a scope or distortion analyser or similar. No disagreement there.

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[quote name='Bill Fitzmaurice' timestamp='1338317666' post='1672711']
An open circuit doesn't deliver a load. Within the operating load ranges actually presented by speakers amps must deliver a constant voltage, otherwise they simply won't work.
[/quote]

This cannot be true. At any specified frequency, the impedance of the speaker is fixed. That's how impedance is defined. If the voltage is constant and the impedance is fixed, then Ohm's Law tells us that the current must also be fixed, meaning that at any frequency, the power from the amp is always the same. This is clearly not the case.

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For some bizarre reason, I always thought of electricity and associated concepts as being pretty, y'know, sort of, well, scientific and pinned down. Like we [i][b]knew [/b][/i]about this stuff. Well some of us knew about it, just not me.

Reading exchanges like this can be a real eye-opener.

FWIW, in any discussion regarding amps/speakers/impedance, I'll always believe the Basschatters who actually [u][b]make [/b][/u]these things - successfully! - for a living (i.e. Bill and Alex).

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[quote name='stingrayPete1977' timestamp='1338360888' post='1673127']
Very good :lol:

And does Norman Watt-Roy count?
[/quote]

I think all bassists should count.....

Just to keep in time with the drummer! :lol:

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[quote name='Happy Jack' timestamp='1338362718' post='1673144']
FWIW, in any discussion regarding amps/speakers/impedance, I'll always believe the Basschatters who actually [u][b]make [/b][/u]these things - successfully! - for a living (i.e. Bill and Alex).
[/quote]

Smart move Jack. I used to teach electrical engineering principles to degree level, but only know the theory :)

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[quote name='Happy Jack' timestamp='1338362718' post='1673144']
For some bizarre reason, I always thought of electricity and associated concepts as being pretty, y'know, sort of, well, scientific and pinned down. Like we [i][b]knew [/b][/i]about this stuff. Well some of us knew about it, just not me.

Reading exchanges like this can be a real eye-opener.

FWIW, in any discussion regarding amps/speakers/impedance, I'll always believe the Basschatters who actually [u][b]make [/b][/u]these things - successfully! - for a living (i.e. Bill and Alex).
[/quote]

But to follow that on logically, you have to put their opnions and thoughts below the producers who sell volumes of the stuff.

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