Question about impedence - AVS Forum
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post #1 of 5 Old 05-06-2014, 04:53 PM - Thread Starter
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When an amp is rated at say 300wpc into 8 ohms, and say 450wpc into 4 ohms, does it automatically use the different impedences depending on whether it is feeding the tweeter and midrange, or if it is feeding the woofer? In other words will it run at 8 ohms for the tweeter and mid, and run at 4 ohms for the woofer (at the same time) or should there be some way of switching the amp to pick one or the other?
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post #2 of 5 Old 05-06-2014, 09:18 PM
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Not usually - output impedances are fixed by design. The impedance setting is really a safeguard - it really is there to limit the power delivered to the speaker in order to protect the amplifier. Especially since speaker impedances are not fixed - they vary by frequency. The impedance marked on the speaker is its nominal value, but it isn't unheard of for speakers to have big drops - some 4 ohm ones can drop to 2 ohms or lower for a good part of the frequency band.

If your woofer is 4 ohms, and your tweeter is 8, then set it to 4 to set the limiter.
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post #3 of 5 Old 05-06-2014, 10:13 PM - Thread Starter
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Ok, well I can't set anything.

I am using B&W 802D speakers, rated at 8 ohms and 4 ohms for the woofers. I am also using a McIntosh integrated amp rated at 100wpc 8 ohms, 150wpc 4 ohms. As I said, I cannot switch anything anywhere, So my question was related to exactly what happens if the amp is rated at 8 ohms and 4 ohms but there is no switch or any way of changing the impedance. Does the speaker just draw using the impedance for each speaker as need be?

The reason I am asking is because I am about to upgrade to a higher power amp. I have been considering going with either a 300wpc or a 450wpc McIntosh but each has only one power rating at 16, 8, or 4 ohms (300wpc and 450wpc), or going with a different amp altogether which is rated at 325wpc into 8 ohms and 500wpc into 4 ohms. Might this amp be a better way to go considering the woofers are rated at 4 ohms? Would the amp supply 500wpc into those woofers while supplying 325wpc into the mid and tweeters?
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post #4 of 5 Old 05-07-2014, 09:35 PM
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Those power ratings are really all the same - the reason is the 4 ohm speakers are easier to drive, and overdrive without limiting. It really represents a total load. In fact, if it was a better amp, the power into a 4 ohm speaker should be double that of the 8 ohm one, but instead something else is limiting the output power.

In your case it'll be a combined power - probably around 400w into the entire speaker system rather than 325 or 500.
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post #5 of 5 Old 05-07-2014, 10:56 PM - Thread Starter
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So am I right? The 325/500wpc amp may be a better choice because it will work out to be around 400wpc? As opposed to 300wpc with the McIntosh lower powered amp?
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