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.