AV Amps - 4 and 8 Ohms Compatibility

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A question from a friend that is buying an AV amp. As a hifi buff it doesn't make complete sense to me. Why would an AV amp specify 4 and 8 Ohm output compatibility for speakers? Is this a way for the amp to limit power output to protect speakers? An example is here
 

scene

Well-known member
igglebert:A question from a friend that is buying an AV amp. As a hifi buff it doesn't make complete sense to me.

Why would an AV amp specify 4 and 8 Ohm output compatibility for speakers? Is this a way for the amp to limit power output to protect speakers?

An example is here
Yes, it is about limiting voltage.

In a lot of cases speakers provide 4 ohm / 8 ohm switches (sometimes even 4/6/8). The effect of this switch, in most cases, is to reduce the rail voltage, i.e. when set to 4 ohms position, a lower voltage is used on the power transformer so that it can deliver higher current current at the same power. Basically, only 71% of the voltage required to deliver the same power to an 8 ohm speaker compared to a 4 ohm one. Equally, 141% of the current is required to deliver the same power to an 8 ohm speaker as is required for a 4 ohm one.
 

scene

Well-known member
Sorry, I should have added, that in a lot of cases the 4/8 ohm switch is included for safety certification purposes and with a good amp and sensible speaker matching, you can safely leave the setting at 8 ohm in the majority of cases.
 
A

Anonymous

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Nice one scene, thanks for that. I suspected what you said, including the fact that it probably doesn't matter all that much! I didn't want to advise a friend to spend a stack of cash without confirming with someone who knows about these things!

Cheers!
 

scene

Well-known member
I'm not an expert, per se, I just looked into the issue a while back when I got my MA RS-LCR, as it's rated 6 ohm and my Silver 8is are rated 8 ohm, and my AVR250 is only switchable 4/8 ohm... Lost a bit of sleep over that one.

Especially after my amp died.
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(Unrelated, to the setting, btw)
 
A

Anonymous

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I was trying to find out about his speakers. He said they dip to 4 Ohms but I'd want to see an impedance curve before judging their load.
 

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