*4ohm speakers can be used with an 8ohm amp*

There is no such thing as an *8 Ohms amp*. Amps have their power rated at 8 ohms so that the power of various amps can be fairly compared. If you test the power to a lower impedance, the power comes out higher. But the true limiting factor on real power is the Power Supply Voltage.

For example, here is the voltage limit on a 50w/ch and a 100w/ch amp (power to 8 ohms) -

50w = 20 volts

100w = 28 volts

If we assume the amps have sufficient power supplies, then to a 4 ohm load the 50w amp appears to be a 100w amp, but the voltage is still limited to 20 volts.

So, some manufacturers would inflate their power ratings by simply saying 100w/ch with no qualifying impedance. So, the government and audio users got together and set a standard for rating power in Hi-Fi amps. This is sometimes referred to as FTC or RMS. In this standard, power is always measure to an 8 ohm load, so that all amps can be fairly compared to each other.

Now, while most real Hi-Fi components must adhere to this standard, they can post or print other power ratings; rating to a range of impedance and *Dynamic* power in addition to the standard 8 ohm power.

Most quality amps can handle impedance loads in the range of 4 ohms up to 16 ohms, which covers a vast majority of speakers out there. As the impedance goes down, the amount of current goes up, and correspondingly, as current goes up, heat goes up. If the heat gets too high, the amp will (hopefully) shut down to protect itself.

So, the fact that the amps power is rated to 8 ohms, does not make it an 8 ohm amp. It simply makes it an amp. Most amps can handle loads from 4 ohms up to 16 ohms.

Now the High/Low impedance mode switch found on Yamaha, and other amps, it there to reduce heat for low impedance loads. But this is something of a cheat. From what I've been able to gather, it simply drops the voltage on the output stages, in effect, lowering the potential power, and thus making the amp run cooler.

I have a Yamaha amp and always use the HIGH setting on the Impedance switch because it gives the most uncompromised sound. But it does require that I use the amp with a degree of common sense, which I have always done. I've never blown a speaker or an amp, and I've thrown some raging parties in my time. Though sadly that was years ago.

Just a few thoughts.

Steve/bluewizard