MakeMineVinyl
Well-Known Member
As always, its a numbers game.
It is physically impossible in the known universe to have any amplifier exactly double its power at half the impedance if you use the "full power" wattage as at the point just before clipping at the higher impedance. For instance, if you have an amplifier that just starts to clip at 200 watts into an 8 ohm load, it is physically impossible to expect that amplifier to produce 400 watts into a 4 ohm load at the clipping point. This is because of voltage losses in the driver stage mostly and voltage sag on the rails which power the output stage.
Therefore manufacturers who want to have a rating which neatly "doubles" at each halving of impedance pick a wattage rating which is somewhat less than the point of clipping as the wattage rating at the higher impedance. So the above amplifier would be "rated" at perhaps 175 watts; it would be able to produce 350 watts into 4 ohms at clipping. Bada bing, an amplifier which "doubles its power exactly into lower impedances".
The SAE amplifier comes closer to the "ideal" than other amplifiers by having a separate power supply which only powers the driver stage which has a higher voltage than the power supply which powers the main rails. But even with this extra power supply, it only lowers the differential between losses at different impedances; it doesn't erase the difference.
That is why you don't often see amplifiers advertised as exactly doubling their power into halving impedances; manufacturers would rather rate their amplifiers at a higher wattage overall, and take the hit at lower impedances. Customers are more impressed at an overall higher wattage rating than if an amplifier can double its power.
