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#325447 - 10/13/10 01:38 AM amp power rating @ 8 and 4 ohms
pinoy168 Offline
regular

Registered: 09/22/10
Posts: 8
hello.

in evaluating amps for 4ohm speakers like the m80, do i look at amp's 4ohm power rating instead of its 8ohm power rating? conversely, do i look at the same amp's 8ohm power rating when matching it with 8 ohm speakers like the m60?

for instance, a stereo amp is rated 250w/550w in 8/4 ohms per channel, respectively. for the m80 (max amp power of 400w at 4ohms), can i say that the amp is more than sufficient to drive the m80's since the amp rated at 550w in 4ohms? for the m60 (max amp power of 250w at 8ohms), do is say that the same amp is just enough?

in relative terms, however, does it mean i have more reserve power using the same amp with m80's than the m60's (550w/400w at 4ohms against 250w/250w at 8ohms).

please enlighten. thanks.

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#325450 - 10/13/10 03:15 AM Re: amp power rating @ 8 and 4 ohms [Re: pinoy168]
JohnK Offline
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Posts: 10634
P, it appears that you're paying too much attention to nominal speaker impedance ratings and also the maximum suggested amplifier power. Speakers vary widely in impedance as frequency varies; for example, the M80 is a 4 ohm speaker over a bit less than half its frequency range. So yes, at those frequencies where it happens to be around 4 ohms, amplifiers face lower resistance and have a higher maximum output capability. The 250w/550w example you give isn't very realistic though, since the theoretical maximum difference is double, and more typical would be about 50% higher into 4 ohms.

Also, the maximum power rating given to a speaker has nothing to do with how much it actually needs, but rather indicates how much it can handle continuously without going up in smoke. Speakers such as the M80s use about 1 watt at a comfortably loud average sound level. Brief split-second peaks in material with a wide dynamic range, such as some classical material, take much more of course, but rarely would need more than the typical 100 watt receiver ratings.

"Just enough" power might be only a few watts for some material, and certainly wouldn't be the maximum advertised suggested amplifier power.
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#325452 - 10/13/10 06:11 AM Re: amp power rating @ 8 and 4 ohms [Re: JohnK]
pinoy168 Offline
regular

Registered: 09/22/10
Posts: 8
thanks for the reply, JohnK. i'm sure your observations are correct.

but would you a version that's less "nuanced"? :-) just want a better handle on how "nominal speaker impedance ratings" and "maximum suggested amplifier power" interact, in practical terms.

fyi, the example i gave is for the w4s st-500 as posted in www.wyred4sound.com website.

thanks!

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#325463 - 10/13/10 11:48 AM Re: amp power rating @ 8 and 4 ohms [Re: pinoy168]
Ken.C Offline
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Ignore max suggested amplifier power. It's nonsense, in pretty much all cases. Axiom has tested 'M80s with waaaaaay higher power than 400w.
As far as impedance and amplifier power go, assuming ideal conditions, 4 ohm ratings would be double 8 ohm ratings. But like John said, you're rarely, if ever, going to get close to that max power in real life. If you are, you probably have
A. An extraordinarily large room
B. A need for a larger amp.
C. Hearing damage
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#325465 - 10/13/10 12:08 PM Re: amp power rating @ 8 and 4 ohms [Re: Ken.C]
bdpf Offline
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Registered: 06/28/08
Posts: 769
Loc: Toronto, Canada
Out of curiosity, how big is your room?
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#325466 - 10/13/10 12:24 PM Re: amp power rating @ 8 and 4 ohms [Re: pinoy168]
BlueJays1 Offline
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Registered: 09/19/08
Posts: 4116
Loc: Porch,enjoying Bombay Sapphire
Originally Posted By: pinoy168
hello.

in evaluating amps for 4ohm speakers like the m80, do i look at amp's 4ohm power rating instead of its 8ohm power rating? conversely, do i look at the same amp's 8ohm power rating when matching it with 8 ohm speakers like the m60?

for instance, a stereo amp is rated 250w/550w in 8/4 ohms per channel, respectively. for the m80 (max amp power of 400w at 4ohms), can i say that the amp is more than sufficient to drive the m80's since the amp rated at 550w in 4ohms? for the m60 (max amp power of 250w at 8ohms), do is say that the same amp is just enough?

in relative terms, however, does it mean i have more reserve power using the same amp with m80's than the m60's (550w/400w at 4ohms against 250w/250w at 8ohms).

please enlighten. thanks.


This is a well informed article on speaker impedance and its interaction with common receivers.

http://www.audioholics.com/education/amplifier-technology/impedance-selector-switch-1
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#325469 - 10/13/10 12:33 PM Re: amp power rating @ 8 and 4 ohms [Re: Ken.C]
jakewash Offline
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Registered: 12/26/03
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Originally Posted By: kcarlile

As far as impedance and amplifier power go, assuming ideal conditions, 4 ohm ratings would be double 8 ohm ratings.
Just to clarify, 4 Ohm amp power is closer to 1.5 times that of the 8 Ohm rating in the real world.
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#325520 - 10/13/10 09:29 PM Re: amp power rating @ 8 and 4 ohms [Re: pinoy168]
JohnK Offline
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Registered: 05/11/02
Posts: 10634
P, Ken's reply should be "less nuanced" enough, so I'll just point out that impedance ratings and suggested maximum amplifier power don't "interact"; they have nothing to do with each other. The maximum amp number is essentially meaningless for home use; quality speakers such as Axioms can continuously handle many hundreds of watts without suffering permanent damage, but your hearing can't.

Incidentally, I did take a look at that ST-500 site link. Unless they operate in a world where Ohm's Law doesn't apply, the 550 watt 4 ohm spec is a typo. As was said, the theoretical maximum increase is a double, so possibly the spec was intended to be 500 watts.
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#325538 - 10/14/10 05:36 AM Re: amp power rating @ 8 and 4 ohms [Re: JohnK]
pinoy168 Offline
regular

Registered: 09/22/10
Posts: 8
many thanks johnk and you guys for patiently explaining things. much appreciated.

@bruno - room size is about 6,000 cu. ft.

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