NC252MP (class D) vs. A250W4R (classAB) burst measurements into 4ohm//2.2uF load

[rwortman]

Ummm ... This Hypex amp is designed to actual use cases. It is the test that wasn't representative of actual use cases.

1. I was asking a general question. 2. I believe that designers (such as those at Benchmark, and perhaps Hypex) specifically design things to get great measurements in a game of one-up-manship that have little to no bearing on how they sound. People in here laud such things, claiming them to be examples of engineering excellence. It’s quite likely that engineers at many audio companies could achieve similar results but it simply isn’t a design goal. One can make the case that wasting time on stuff that doesn’t matter isn’t what great engineers do.

 

[NTK]

1. I was asking a general question. 2. I believe that designers (such as those at Benchmark, and perhaps Hypex) specifically design things to get great measurements in a game of one-up-manship that have little to no bearing on how they sound. People in here laud such things, claiming them to be examples of engineering excellence. It’s quite likely that engineers at many audio companies could achieve similar results but it simply isn’t a design goal. One can make the case that wasting time on stuff that doesn’t matter isn’t what great engineers do.

Buckeye Amps is selling a NC252MP based stereo power amp for USD$575. It outputs plenty of power, no problem driving 2 ohm loads with real life music, runs cool, light weight, performs great. What do you mean by wasting time on stuff that doesn't matter?

 

[MaxwellsEq]

We would need measurements of actual speakers to support that statement. I have seen only measurements of “equivalent circuits” with little validation.
Did I miss those measurements?

Whilst I think the capacitance values that @pma is using is unlikely to represent a real speaker, I can guarantee that no speaker ever made has had a purely resistive load of either 4 or 8 Ohms over the 20Hz to 20kHz range. Perhaps a future speaker will be purely resistive, but based on current known technology, I think it's unlikely.

Speakers with cones usually have coils inside a magnetic field. A coil is largely inductive which means the current will lag the voltage (Lenz's law) The speaker might measure a purely resistive load at one frequency, but there's no saying it will be exactly 4 or 8 Ohms. The motion of the coil in the magnet will change the characteristics (sensitivity to sound level) as will the compression and rarefaction of the air in the cabinet.

Electrostatic speakers are in contrast very large plate capacitors. In this case, the voltage will lag the current. The capacitance changes with signal level as the diaphragm moves. One thing they are not is purely resistive.

 

[rwortman]

Buckeye Amps is selling a NC252MP based stereo power amp for USD$575. It outputs plenty of power, no problem driving 2 ohm loads with real life music, runs cool, light weight, performs great. What do you mean by wasting time on stuff that doesn't matter?

Runs cool, light weight matter more in car audio, and we have established in other threwds that the power spec’s are at best misleading. The stuff I referred to as not mattering is chasing more 0’s in the distortion spec. Check back in 5 years. Reliability is an engineering goal too.

 

[NTK]

Whilst I think the capacitance values that @pma is using is unlikely to represent a real speaker, I can guarantee that no speaker ever made has had a purely resistive load of either 4 or 8 Ohms over the 20Hz to 20kHz range. Perhaps a future speaker will be purely resistive, but based on current known technology, I think it's unlikely.

Speakers with cones usually have coils inside a magnetic field. A coil is largely inductive which means the current will lag the voltage (Lenz's law) The speaker might measure a purely resistive load at one frequency, but there's no saying it will be exactly 4 or 8 Ohms. The motion of the coil in the magnet will change the characteristics (sensitivity to sound level) as will the compression and rarefaction of the air in the cabinet.

Electrostatic speakers are in contrast very large plate capacitors. In this case, the voltage will lag the current. The capacitance changes with signal level as the diaphragm moves. One thing they are not is purely resistive.

How many years have the industry been testing amplifier power using purely resistive load? I can't remember a time that it didn't. Do we have a wide spread problem with amps incapable of driving real speakers which everyone has swept under the rug? Or is it a non-problem?

 

[NTK]

Check back in 5 years. Reliability is an engineering goal too.

Spreading FUD are you?

 

[rwortman]

Testing speakers with resistors is like the EPA fuel mileage test. It may or may not correlate with real world usage.

 

[vicenzo_del_paris]

What I don't really get from that thread is its purpose and motivations.
I am not feeling comfortable with the implicit and explicit extension to a whole amplifier design class.
@pma shown a use case (theoretical or real, that is still to determine) where a given specific class D implementation is not behaving well with some specific load testing. Fine with that.
Now, everybody jump and get crazy about it.
First, we still not have any evidence that this is a real issue in real life with real speakers.
Second, if proven that for a very specific category of speakers that represents an extremely small pourcentage of sold speakers, this specific amplifier has an issue, fine, future buyers are warmed.
Third, let's not generalize from one set of specific tests on a specific implementation to conclude that class D is bad (I am refering to some posts from the thread).
I don't really care in the end about class D.
I just want people to not be fooled by a misleading conclusion resulting from these specifics tests.

 

[MaxwellsEq]

How many years have the industry been testing amplifier power using purely resistive load? I can't remember a time that it didn't. Do we have a wide spread problem with amps incapable of driving real speakers which everyone has swept under the rug? Or is it a non-problem?

Good engineers design for sensible corner cases. All designers of speaker amps know speakers are complex mixture of inductance and capacitance. Reviews of speakers often show the impedance chart. There needs to be some measurement and people have mostly settled for 8 Ohms resistive and more recently 4 Ohms resistive. You just need to be aware that the whole story is not told in those figures. Real world speakers draw more current at some frequencies than an 8 Ohm resistor and need more voltage at other frequencies than an 8 Ohm resistor.

 

[Buckeye Amps]

Spreading FUD are you?

I always love how “polarizing” Class D can be in the audio circles.

And also that people would come to a site that is trying to spread the importance of science and holding manufacturers accountable for poor performing products and attempt to negatively categorize great measuring/performing products.

And by negative, I mean implying that certain products are only made to "measure well" despite tens of thousands of satisfied customers/positive use cases. Makes zero sense. But there are always those few on any forum...

 

[boXem]

Testing speakers with resistors is like the EPA fuel mileage test. It may or may not correlate with real world usage.

Pro tip from an ex-automotive-engineer: stop amplifier-car comparisons, that is not good for your credibility.

 

[boXem]

Spreading FUD are you?

Ignorance and self confidence are not antagonistic.

 

[tmtomh]

What I don't really get from that thread is its purpose and motivations.
I am not feeling comfortable with the implicit and explicit extension to a whole amplifier design class.
@pma shown a use case (theoretical or real, that is still to determine) where a given specific class D implementation is not behaving well with some specific load testing. Fine with that.
Now, everybody jump and get crazy about it.
First, we still not have any evidence that this is a real issue in real life with real speakers.
Second, if proven that for a very specific category of speakers that represents an extremely small pourcentage of sold speakers, this specific amplifier has an issue, fine, future buyers are warmed.
Third, let's not generalize from one set of specific tests on a specific implementation to conclude that class D is bad (I am refering to some posts from the thread).
I don't really care in the end about class D.
I just want people to not be fooled by a misleading conclusion resulting from these specifics tests.

Oh, I don't think you can separate what @pma is posting about from the step where people "jump in and get crazy about it." He'll never admit it, but IMHO - and really by any reasonable reading of the totality of his posts on this subject - he's got a very clear argument and point of view that he wants to prosecute, and he hides behind "just running tests." If you look at the very clear pattern of facts that he has to ignore in order to keep pounding this drum, I think his agenda becomes very clear.

 

[ThatSoundsGood]

Whoa, for a second there I thought I was going to have to sell all of my Class D amps and buy my old stuff back. I'm glad there are people smarter than me figuring this all out because it's gonna get expensive otherwise..........

 

[amirm]

Whilst I think the capacitance values that @pma is using is unlikely to represent a real speaker, I can guarantee that no speaker ever made has had a purely resistive load of either 4 or 8 Ohms over the 20Hz to 20kHz range.

Magnetic planar speakers are purely resistive although they tend to not produce bass.

That aside, your implication that we are testing the easy stuff is not accurate. I run power tests at 1 kHz whereas real music has bulk of its energy in bass. If you equalize the energy at bass to 1 kHz, you would go deaf!

The crest factor of 1 kHz is also lower than for real music.

Net, net, we are conservative on one side, and aggressive on another. In balance, we get useful data.

I have started to test with reactive loads now so your complaint is no longer valid anyway.

 

[Doodski]

I have started to test with reactive loads now

Very CooL! You rock @amirm.

 

[Deleted member 48726]

How many years have the industry been testing amplifier power using purely resistive load? I can't remember a time that it didn't. Do we have a wide spread problem with amps incapable of driving real speakers which everyone has swept under the rug? Or is it a non-problem?

The "all amplifiers sound the same" doctrine is hiding behind pure resistive testing I bet.

 

[Deleted member 48726]

I have started to test with reactive loads now so your complaint is no longer valid anyway.

I've seen. But you can not relate that power cube data to frequency, can you?

As much as I like the.addition, how does it reflect phase changes?

Would you consider an equivalent speaker test jig. Like it could be three, "typical 2 way" "typical 3 way" "Rough Ruth"

 

[Deleted member 48726]

We would need measurements of actual speakers to support that statement. I have seen only measurements of “equivalent circuits” with little validation.
Did I miss those measurements?

You haven't read the thread in it's entirety. I can tell.
Look at a phase / impedance diagram from a speaker and look at your post again..

 

[Deleted member 48726]

Spreading FUD are you?

Let's not pretend that Class D topology as today has really proven test of time compared to the half of century A/B has.

It's a valid argument because we actually, factually do not know how they will sustain. And with the in-rush of quickly built amps in small enclosures with no cooling I would bet many do not last. But I don't know. So the use of FUD triggers me, as it's used as a bad word to negate whatever it's a response to.

I think it's natural to be Uncertain and Doubtful of the long term reliability of many current Class D's.