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...

It's only an issue if that last 3db of SPL is making all the difference for you. And that would be if the NAD put out twice the power at 4ohms (i.e., max. voltage stayed the same). Many manufacturers do not rate their amp at TWICE for four ohms, but some reduced rating instead.

It seems NAD is (also) saying, "No point in doubling the power at 4 ohms, it's a barely noticeable difference, and even then only when wide open. This way, the 4/8 ohm speaker decision is not a factor when using this amp."

Thanks for the replies. Some interesting reading, for sure. I’m certain the 272 will have enough power to make my ears bleed, no argument there. I’ve asked their tech department for more clarification and will post what they tell me, if anything.

I guess it all comes down to how a person prefers their music to sound. After numerous hours searching with Google, the general consensus from about a gazillion posts, is NAD is a “neutral” amp with excellent mid range. I’ve also read that they tend to tame down the highs and don’t have as much punch with low end bass at low volume. Rotel on the other hand is most often described as “bright”, with excellent punch on the low end bass at low volume and those cymbal crashes at high volume. Seeing how I wouldn’t mind warming up the 80’s a little, the NAD’s characteristics might just be the way for me to go.

And NAD isn’t exactly cheap. They’re right about in line with Rotel. The only difference is that there are a few NAD dealers on line that will give you a bit of a break. Rotel doesn’t sell on line and I have found no dealers that will sell under MSRP. But even with the discount, the NAD combo is only $175 less than the Rotel when shipping is figured into the mix.

I am also interested in the C272 for the M80's. I am planning to run them from the 3805 preouts. Please keep us updated with the NAD respoces.

I did consider the Rotel, however, for some reason in Oman a similar Rotel will cost double the price of a C272.

Their response to my last silly question………

Speaker impedance is complex at 8ohms or 4 ohms at 400Hz. Across the audio band 20Hz - 20K it will vary by quite a bit. Also, this is excluding the crossover network, which makes it a complex load.
The impedance has peaks and troughs. Certain speakers can drop to below 1 ohm deepening on the frequency.
So doing straight ohms law doesn’t quite apply as for resistive loads. Phase angle isn't accounted for in your ohm law.

Powerdrive will rarely activate with music signal, as the transient isn’t long enough to trigger it.
With sinewave testing the powerdrive will activate after a number of cycles.
So with music it will deliver the power as for normal 8 and 4 ohm load.
Also our power amp output stages are designed to deliver very high currents, typically they can deliver anything up to 100A peak into 0.1 ohms load. Again measure with 1mS single cycle.
Even in our budget amps like 320BEE the output current drive is 35Amps.

………..I think I’ll shut the hell up now.

So are you buying the NAD then?

Yes, I ordered the NAD 162 / 272 combo yesterday.