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.