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Is 200w at 4ohm the same as 400w at 2ohm?


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#1 of 5 OFFLINE   Matt Odegard

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Posted November 20 2003 - 04:48 PM

Someone told me this today, and I don't believe it. Although at the time I could not explain why he was wrong. I just never heard of such a thing in the time i've been in car audio. Also, why would manufacturer's give statistics of their amp power rating at 4ohm, 2ohm and 1ohm? If there is no difference in power besides THD. Can anyone tell me why he is wrong? Or why I am? I know amps give out more power at 2ohm because there is less resistance than at 4ohms. But that doesn't meen its the same 200w being pushed to the subs. Correct?

#2 of 5 OFFLINE   VinhT

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Posted November 21 2003 - 03:02 AM

[quote] Also, why would manufacturer's give statistics of their amp power rating at 4ohm, 2ohm and 1ohm? If there is no difference in power besides THD. [quote] The reason they state power ratings at various impendences is because customers run a variety of configurations and it allows consumers to make a more informed purchase. Additionally, the power does not automatically double when the impedence is halved. On cheaper amps, the power increase between four and two ohms will be marginal, and the amp itself will not even be one-ohm stable.
Vinh Tran

#3 of 5 OFFLINE   brentl

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Posted November 21 2003 - 03:10 AM

"On cheaper amps, the power increase between four and two ohms will be marginal, and the amp itself will not even be one-ohm stable." I wouldn't that's true. Amps with well regulated power supplies aren't always the "cheap" amps. Not only Cheap amps are unable to run into 1Ohm! Brent

#4 of 5 OFFLINE   Matt Odegard

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Posted November 21 2003 - 05:17 AM

[quote] The reason they state power ratings at various impendences is because customers run a variety of configurations and it allows consumers to make a more informed purchase. [quote]

Of course, I know that. But that doesn't mean that the same amp configured at 2ohms is pushing the same power at 4ohms.

I didn't mean to say that by cutting the impedance in half, that you effectively double the wattage. That was just the example he gave me.

I had an old Sony ES amp that was 1ohm stable. Pretty nice but by no means competition equipment, and it was fairly cheap. I never had it wired down to 1ohm since I never had a 2ohmDVC sub, but I could run that baby hot!

#5 of 5 OFFLINE   Sean^M

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Posted November 24 2003 - 05:04 AM

simple answer:
Ohm's Law.

disregarding the loss to inefficiency, as you halve the impedance, or load on the amp, the power output will double within the range the amp is able to operate.
If the amp isn't a "high current amplifier" then that range is usually 2 to 16 Ohms.

high current amps can go as low as 0.5 Ohms stereo and even .67 Ohms bridged.

Now, some amps, like JL Audio slash series amps use a regulated output that keeps the same power output regardless of load. This way using a 2 Ohm load gives the same power out as a 4 Ohm load.

if an amp is:
300 watts x 2 @ 4 Ohms
then it will be:
600 watts x 2 @ 2 Ohms and
1200 watts x 2 @ 1 Ohm
bridged, the amp will produce:
1200 watts x 1 @ 4 Ohms.

this is just an example, and again, not counting a slight decrease in power output as the impedance drops due to loss in heat or efficiency.
Also note this applies to class A, AB, B, and D solid state amps.
this isn't the case for tube amplifiers which gain very little in output by lowering the load.

more info can be found here:
http://www.eatel.net...ptech/elecdisc/




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