The power ratings are even more deceiving. There's a box system sitting at the local Wal-Mart that offers something like 850 watts total power. Of course, that's based on a single channel driven into *3* ohms at 10% distortion at 1 kHz and then adding up the number of channels.

An amp will produce more power at 10% distortion than at some reasonable number. However, the high frequency harmonics will kill a tweeter in short order.

Power goes up with decreasing impedance (resistance) because the amplifier is basically a voltage source. As we all know, power is voltage squared divided by resistance, so as the resistance drops the power goes up. However, as power (and current) rise, the heat load on the output transistors goes up and the demand on the power supply also goes up.

This rating is for driving a single channel. That means that a shared power supply that's capable of maybe 250 watts total output is only feeding one set of output transistors and does not have to share with multiple channels. The rating for all channels driven will be substantially lower.

For various technical reasons, an audio amplifier, especially a cheaply constructed and designed one, is capable of producing somewhat more power in the center of the bandwidth than at the extremes. This also adds to number.

That 850 watt unit, measured while being driven into 8 ohms, all channels driven, 20 Hz to 20 kHz, at less than .1% THD (Total Harmonic Distortion), after a 20 minute warmup at 40% power, *might* make as much as 200 watts. Total.

The 7500 under the same conditions pushes 1000 watts easily.

Back in older days (late 1970's) there were quite a few different power ratings for amplifiers, starting with the most restrictive as above and progressing through IEC, IPP (Instantaneous Peak Power) and ILS (jokingly, If Lightning Strikes). You've got to have the same conditions for the ratings to compare power, and high power sells.

I watched I Am Legend the other night and when Will Smith fired the Claymores I felt the sofa move under me. It's sitting directly on tile laid on the concrete slab foundation.

Power is good.