Axiom Home Page Forums General Discussion Technical Questions Wattage?-responce from NAD, now I'm really confused

This doesn't sound too bad to me... although I always thought PowerDrive had some kind of continuously variable power supply instead of a hi/low switch.

You are absolutely correct that "in the simplest case" the same amplifier will put more power into a 4 ohm load than an 8 ohm load at clipping -- twice as much in fact. It will also drive twice as much current into the load, which is where your extra power comes from, because in the simplest case the amplifier always clips at the same voltage.

Now, the reality is that it's not that simple :

1. Most amps are designed around the 8 ohm "standard", so they run out of voltage, current and heatsinking at about the same time. That gives you the best performance for the dollar. The better amps seem to be designed with a bit of excess current capacity and a bit of excess heatsinking, so they sound good and don't overheat right up to the point where they hit their limits.

With a 4 ohm speaker, you tend to run out of current (power supply size) before you run out of voltage (power supply voltage). When you run out of current you might start to get a thin sound or the amp's protective circuitry might kick in and shut down the amp. When you run out of heatsinking you can probably figure out what happens -- thermal shutdown or crispy critters.

Big-ass amps seem to be designed to be equally happy at 4 or 8 ohms. That costs a bit more and (strictly speaking) wastes some $$ if you only run with 8 ohm speakers but it means the amps will be solid and happy no matter what kind of speaker you throw at them.

What NAD has done seems reasonable as long as their voltage switching doesn't click on and off in time with the music. They essentially let you run an amp optimized for 8 ohm operation with a wider range of loads and gently back down the power supply voltage which pretty much "optimizes for 4 ohm operation" without requiring bigger heat sinks and bigger power supplies.

It means you won't get any more power at 4 ohms than 8 ohms but you need to understand that "getting more power at 4 ohms" only works because you are taking advantage of high current capacity and excess heatsinking built into the amplifier and not used much when running an 8 ohm load.

The extra power isn't totally free... although if you have a receiver or amp designed with some extra headroom (HKs come to mind) you can take advantage of that with 4 ohm speakers.

I just had a thought -- I bet some of the advertising for "high current output stages" is really saying "we designed these to run at 4 ohms but the stupid law requires the stupid switch so we only spec for 8 ohms but there's lots of extra current going unused at 8 ohms". Just a thought...