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

In my, what is beginning to seam like a never ending, simple decision to which amp to buy, I asked NAD a question regarding their wattage rating on their C272 power amp. Now I’m even more confused. Before sending them a responce, I was hopeing one of you guru’s could help me tailor it better. I’ve found that while directing a technical question via email, I need to be specific. Tech’s like to keep it short and simple.

Their response has me baffled. What do you think?


Submitted Category: Power Amplifiers

2005-04-27
11:23 pm C272 wattage rating
Good day,

I am confused with regards to your watt rating on the C272 power amp. You’re description states 150 watts per channel at 8/4 ohm. I do not understand this. I do not understand why the wattage rating stays the same with less resistance. I read the section regarding your power drive technology, but still, this is contrary to basic electrical theory. With a reduced resistance, wattage should increase.

The reason I am asking, is I have a set of 4 ohm speakers that are rated at 400 watts at 4 ohm continuous power. I want to purchase a new amp / pre amp combination for these speakers, and would like to give NAD a try, but the 4 ohm rating concerns me. I also understand that NAD is very conservative with their power ratings and has a respectable amount of head room and current, but again, please educate me regarding your wattage rating.


2005-04-29
9:09 am re: C272 wattage rating
Thank you for your recent request via the NAD Electronics web-site. I spoke with technical services regarding your questions, and here is their input:

Power Drive works like this:

Power Drive works by constantly monitoring the following conditions
1.Load impedance
2.Power being delivered to the load
3.The heat sink and ambient temperature.

This information is fed to a simple “Analog computer” which controls the power supply between a high voltage and low voltage level.
Switching of the power supply is done on the following basis:

• Load impedance is <4 ohms then switch to low supply. Load is 8 ohms then switch to high supply.

• Heat sink temperature is high switch to low supply.

• Ambient and heat sink temperature is high then switch to low supply.
The switching between two supplies is done when the above conditions are true over a number of cycles of the signal.

Thus you can see that the power is the same for both resistances 4 and 8 because the power rails are switched.

Best regards,

just go with the Rotel...

bigjohn

It almost sounds like NAD has some method of switching power output/supplies for each different load such that the wattage remains the same. Otherwise I've only ever seen these numbers for Class A amps.

Just buy the nad and forget about it. It has way more then enough power to drive whatever speakers you got anyway.

Just think of it like a Mcintosh. Right now i have a MC250 im burining in, Its rated at 50 wpc. Into 4 8 or 16 ohms.. It uses an autoformer system to impedance match the speaker load to the transistors for maximim power transfer.

to me that says:

our amp doesn't have a lot of dynamic headroom and will break or overheat if it gives more than 150 wpc, especially into low impedence speakers, so we've limited the amount of power that can come out of our amp at 150wpc

i'd get a rotel instead. Alan has also stated that NAD has had some QC problems in the past.

I never "just buy" anything without understanding how it works. It's all part of the learning adventure. Plus, I'm not in a big hurry. So why not research and compare?

Unless I’ve forgotten basic electricical theory, the response I got from NAD is basically telling me that the 4 ohm circuit has a lower current capacity than the 8 ohm circuit, which is counter to what they claim (high current amplifiers).

A Watt is a measurement of power in a circuit. Watt = Current squared X resistance ……Ampere is a unit of measurement of current (1 Ampere is = 1 coulomb flow past a point in a circuit every second) 1 coulomb = 6.25 billion electrons). Hence, amperes is king in regards to available power for any electrical driven component.

One Watt is the power measurement of 1 ampere to flow through 1 ohm of resistance. So, in a since, wattage rating is the net available power rating (current) of a given circuit figuring in total circuit resistance. If the wattage rating at 8 ohm is the same as 4 ohm, total current available is lower in the 4 ohm circuit than the 8 ohm circuit.

Am I wrong?

I think where you're getting caught up is in the speaker "power rating". I say that in quotes, because (big secret here) IT DOESN'T MATTER. If you're not dropping a 1000 watt amp on a 200 watt rated speaker, you have nothing to worry about. You're not missing any potential of the speaker by driving it with an amp that does not match the maximum power rating of the speaker. The reason that's on the speaker is so that you don't blow up the thing by driving it with too much power. People get so caught up in the wattage thing, and never stop to think that what they print on the speaker's label has little to do with the actual function of the speaker.

Well lets put it this way. I know all the ohms law mumbo jumbo too but i use a c370 amp to push my M80's so i think i can safely say from personal experience that it would blow your ears out before it runs out of power. Not to mention the 272 is even more powerfull than the 370

You are wrong. Take a look:

According to NAD, the watts when driving a 4 ohm or an 8 ohm load are constant, so we have

x^2 * (4 Ohms) = y^2 * (8 Ohms) -- divide by 4 Ohms
x^2 = y^2 * (2 Ohms) -- take a square route
x = y * sqrt(2 Ohms)

x is the amp's current when driving a 4 Ohm load
y is the current when driving an 8 Ohm load

As you can see, x is greater than y.

This is what we expected, since more current flows into a less resistant load.

I agree with both of you. I’m not so much caught up in the wattage required debate, just more curious now than anything else. And, while comparing spec sheets of comparably priced amps, why buy an amp that has a significant lower current supplying capacity than another? I’m just mystified why NAD would do this. They are reputed to be one of the best amps in their price range for producing amps with tremendous power capabilities. I’ve got to be misunderstanding something here. Hopefully one of the EE’s here can chime in.

I dunno.. I like the fact they're honest. Think about their system as protection as opposed to current cutting. You get an amp that doesn't do that, and then you've got a melted pile of slag when it thermally overloads. (worst case, of course)

You lost me (granted that’s not hard to do, it’s been a few years since my apprenticeship - that I didn’t finish). Current is the unknown value. We are only given wattage ratings. What’s the formula for converting watts at a known resistance to amperes?

The general formula for watts is
watts = volts * current

The general formula for current is
current = volts / resistance

if we know the watts and the resistance, we can solve these equations for current like so:

current = sqrt(watts/resistance)

Agree with HAOLEB, I have a NAD 352 integrated at 80wpc into the M60's. Believe me, sitting between 10-15 feet away. This amp delivers at ear splitting, but very pleasent loudness. How much becomes to much?? This is plenty for me and my listening pleasures........

dammit....I was wrong.
150 watts / 8 ohms = 4.33 Amps and 34.6 V
150 watts / 4 ohms = 6.12 Amps and 24.49 V

I found a cool link to on line audio and power calculators.

http://www.sengpielaudio.com/calculator-ohm.htm

For those copy/paste challenged

http://www.sengpielaudio.com/calculator-ohm.htm

Hey now! Mine looks just like yours, except no underline....and well, it doesn't work. What'd I do wrong?

you need to add before the address and after the address, a few sites do this automatically, Axiom's board doesn't.

I'm a racing fan and I wish I had a dollar for everytime I've done [irl] [/irl]

I've been looking at the NAD T773 as well. I would hope that the swapping of output sources or levels for load, power and heat sensitivity, would not be audible. I guess they must be balancing the workload, so the receiver contributes equally to either speaker type. Based on the heat output required and resistance it feels, it achieves similar balance.

I had the impression that you could always get a little more power into the 4 ohm speaker, from the same amplifier if the amp could handle it. I would hope this design doesn't limit the power into 4 ohm as much as it increases the power into 8 ohm.

From my research so far the Nad T773 is pretty much one of the best out there, without going to seperates. Simple to operate, with easy remote and programming functions, as compared to the Harman Kardon. The honest power ratings to all channels running, is only met by a couple of companies out there. Sure there is better out there, but you might have to spend a thousand or more to get it, as in the case of Rotel.

Good luck,
Troy

Mike, what NAD is saying is that their amp has a automatic circuit to accomplish what some others do with either a manual 4ohm switch or menu setting. In either case the rail voltage available is reduced, which in turn reduces both the maximum current(I=E/R)and therefore power(P=ExI, or P=I(squared)xR)available at four ohms. That's why their amp isn't rated for the higher wattage that is normally available at 4ohms if the full voltage is still supplied at that impedance.

Despite that, it's highly likely that the NAD would be more than adequate at 4ohms. I'll repeat my previous suggestion that if your 7200 is plenty for for the M80s, so would the similar channels of the 3480 be for stereo M80s.

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.