Handling a complex load by power amplifiers

[NTK]

 

[Joe Smith]

You know the Monkees song I'm a Believer, right?

My motto is: Oversimplification leads to false conclusion.
Another one: For every complex problem there is an answer that is clear, simple, and wrong.

Well, all graphs aside, the little A07 I think is a decent $80 amp, sounds good with all the speakers I've thrown at it, all types of music. I feel like this thread is portraying cheaper Class D amps like they are all "secret stealth speaker destroyers lurking in the shadows...!"

 

[levimax]

Well, all graphs aside, the little A07 I think is a decent $80 amp, sounds good with all the speakers I've thrown at it, all types of music. I feel like this thread is portraying cheaper Class D amps like they are all "secret stealth speaker destroyers lurking in the shadows...!"

It is more like some of these amps may self destruct with some speakers. I am grateful for @pma and his work and his perspectives which adds some diversity of opinion to the site and makes for some interesting debate

 

[dlaloum]

When I purchase my latest AVR (Integra DRX 3.4) I didn't expect much form its amp section - I know my speakers are "difficult" with impedance dropping to 1.6ohm.

The ran OK into my speakers, or at least sound came out - but it clearly wasn't happy - the sound was confused / congested sounding - less clear than it should be.

Put my power amps (Crowns rated down to 1 ohm) between the AVR and speakers - and things cleared up instantly with the speakers now sounding as they should.

How the individual amps respond/sound to the complex loads presented by difficult speakers, is highly variable... some just shut down, or go up in smoke... others just sound wrong/bad.

But yes, we need much better measurement methods to be able to accurately be able to predict the suitability of an amp for a specific set of speakers and/or vice versa.

Current situation is a lucky draw.... take your chances.... often mitigated by many of us by purchasing substantially over-engineered and over-powered amplifiers that run at all times at less than 1/10th of their rated power.... just so we can be sure that they will run fine with our speakers of today or tomorrow.

I have 44W@8ohm amps.... when I could probably live with a 75W@8ohm amp .... but one that can handle the difficult complex load.

 

[amirm]

I have expected exactly this kind of reasoning. Unfortunately, it is not that simple. Even amplifier's self-distortion components are able to initiate the erratic instable behaviour and it is enough to have frequencies >=5kHz approx.

That has nothing to do with the frequency response measurement and claim of destructive resonance at nearly 50 kHz. You want to stick to distortion argument, do. But don't try to drag in other unrelated limitations which don't exist in real application of the amplifier.

Below the example of A07 erratic behaviour into complex dummy load, and this load (post #1) is a very good approximation of a real single speaker.

Ok, now logarithmically decay the input signal vs frequency to simulate real music spectrum and you see that is a non-issue as well.

Remember again, no one has put this amplifier on a pedestal and said this is the most perfect amplifier there is. For sure it has limitations and in exchange for that, you get tons of power, very low noise in small, cool, ultra low cost package. Come back to us when you can meet those criteria without the problems you are worried about and then we can talk.

 

[MaxwellsEq]

Music energy at high frequencies is (probably) always lower than low and mid frequencies. We see this when we look at the peaks graphically captured, e.g. at 10kHz they are 40dB lower than lower frequencies:

I wonder what percentage of this energy at 10kHz is composed of fundamental frequencies (e.g. at 10kHz) or is composed of harmonics of lower frequencies (e.g. the 2nd harmonic of 5kHz).

I don't know if there has been a formal study on this. My instinct is that probably more than 50% of sounds above 5kHz are harmonics of lower frequencies. If this is the case, does it have any bearing on whether amps handling difficult loads can be made unstable by harmonics of lower frequencies, as post #38 suggests?

 

[amirm]

It is more like some of these amps may self destruct with some speakers.

Nothing remotely like that has been shown. He continues to create corner cases to complain about. As I said above, if you want the ultimate in amplification, it exists but will cost you money. The value of the Aiyama is that it is able to give you so much for so little space, money and energy use. The IC uses has very robust protection circuits and can stand punishing loads as well. Or else the one that PMA owns would have died already.

 

[amirm]

My motto is: Oversimplification leads to false conclusion.

So does overcomplication to the point where you lose the plot. I already test amplifiers way past the point of how people use these devices. I run harmonic tests with bandwidth of 45 kHz for example whereas no one can hear such. I push amps to full power at 15 kHz which never, ever happens in real life (if it did, you would go deaf instantly). To keep piling on tests simply serves to confuse people about what the story of an amplifier really is. Yes, it warms the cockles of old timers who want to cling on to class AB amps that they can design and understand. But in reality it doesn't advance the understanding of the musical fidelity of an amplifier.

Class D amplifiers have proven themselves to be highly performant. They provide tons of power in small packages. Without them, many people would be using underpowered class AB amps which would easily clip at any frequency with even resistive loads. And produce tons and tons of distortion.

 

[IAtaman]

we need much better measurement methods to be able to accurately be able to predict the suitability of an amp for a specific set of speakers and/or vice versa.

I think speakers need much better measurements much more desperately than amps. Maybe amp specs can use one of two additional parameters, maybe not. Speakers specs on the other hand are useless.

Hypex NC252 discussion where all this started, first 5 pages of that thread is a discussion on how realistic the load is because there is so little information on the actual electrical characteristics of speakers on their datasheets. You look up NC252 specs, is says min 2 ohms. You measure output vs freq for 2 loads, you know its output impedance, I think that is pretty much all you need to know about the amp. As en engieer or a hobbyist you might of course be interested in the interaction of the load and the amp, whether oputput transistors are pushed outside of their SOA for wonky loads, what are the out spec characteristics of the amp etc, but those concerns are mostly academic in my opinion.

Trying to measure amps for all possible scenarios because we don't know what kind of loads speakers might throw at them makes no sense to me.

 

[okok]

how far the measurements can prove your theory thoroughly?

 

[MaxwellsEq]

In another thread, there is a link to a 2019 post by Bruno Putzeys talking about his Eigentakt design and how his previous design (presumably referring to NCore but deliberately not named):
"Then there is the loop structure that allows better control of the closed loop frequency response. My previous amp* has approximately a 1st order roll-off. Now, since the output filter naturally has a second order roll-off it means that this amp could be overdriven with out of band noise from e.g. DSD recordings (in fairness, only when you cranked a quiet recording high)."

@pma appears to be exploring the circumstances where Bruno's predicted behaviour seems to happen (e.g. though higher frequencies and difficult speaker loads).

Bruno raises an interesting point - quite a lot of HiRez Flacs are actually just DSD ports which included the ultrasonic noise. They shouldn't be like this, but they are. I've got about three of them. These sorts of files played into difficult speaker loads could viably cause some of the instability discussed here.

Ncore design refinement

News-Pre-launch of Lyngdorf MXA-8400 | Lyngdorf Audio https://lyngdorf.com/wp-content/uploads/2019/02/MXA-8400-fact-sheet.pdf Its a refinement of Ncore. No digital direct drive or a numerical amp.

www.diyaudio.com

 

[restorer-john]

Bruno raises an interesting point - quite a lot of HiRez Flacs are actually just DSD ports which included the ultrasonic noise. They shouldn't be like this, but they are. I've got about three of them. These sorts of files played into difficult speaker loads could viably cause some of the instability discussed here.

It just goes to prove again that "high res" and DSD brought more trouble to the table than good old brick-walled PCM.

Even some high powered, very fast amplifiers in the early 1980s came with switchable (or extra) inputs for HF filtering, in order to reduce any chance of sampling frequency (44.1) issues as the CD was still a year or two off and the effectiveness of the LPFs was unknown at the time. Denon is one. Their power amps had FRs (-3dB) out to 300kHz without the filter. Two sets of inputs, one rolling off to be several dB down at 40kHz, the other wide open.

The little Aiyima amp is ridiculously good value for what it is. But it doesn't deserve being put on a performance pedestal either. It's a cute little garage amp for playing on a pair of old rat speakers while you work on the lawnmower or change the oil in your car. Maybe as a teen's 'my first college room system' it's ok- just.

I've had a few SMSL small amps and a Topping or two. They are fun for about 5 minutes. Then I give them to my Dad to play with. He gives them a workout and gives them back. Then they are either given away or in the cupboard someplace. None come close to a real amplifier in performance, usability, quality or enjoyment.

 

[pma]

Some more with complex load from post #1

 

[JktHifi]

I've had a few SMSL small amps and a Topping or two. They are fun for about 5 minutes.

Why is it only 5 minutes? After 5 minutes, what happen?

 

[Thorsten Loesch]

To get that resonance you need source content that had sample rate of 96 kHz to have bandwidth of nearly 50 khz.

Or excitation from the switching frequency... I would not consider this acceptable.

Ultimately this is an $80 amp with some load dependency. This is known and repeatedly stated in my reviews. You are only confusing the issue with these misleading tests.

Ultimately any amplifier that measures like this is an Amplifier any competent designer and / or engineer measuring it would describe as "faulty by design".

If I was you, I would call that out and state under which circumstances this problem may not be a big one.

Yes, it's cheap. And?

Thor

 

[PHD]

Real-world loudspeakers are even more complex than this RLC dummy load model. They also produce back EMFs and non-linear distortion due to magnetic saturation, as well as harmonic oscillations due to acoustic resonances. There are some (quite expensive) electronic loads that can emulate a loudspeaker more precisely.

 

[IPunchCholla]

It just goes to prove again that "high res" and DSD brought more trouble to the table than good old brick-walled PCM.

Even some high powered, very fast amplifiers in the early 1980s came with switchable (or extra) inputs for HF filtering, in order to reduce any chance of sampling frequency (44.1) issues as the CD was still a year or two off and the effectiveness of the LPFs was unknown at the time. Denon is one. Their power amps had FRs (-3dB) out to 300kHz without the filter. Two sets of inputs, one rolling off to be several dB down at 40kHz, the other wide open.

The little Aiyima amp is ridiculously good value for what it is. But it doesn't deserve being put on a performance pedestal either. It's a cute little garage amp for playing on a pair of old rat speakers while you work on the lawnmower or change the oil in your car. Maybe as a teen's 'my first college room system' it's ok- just.

I've had a few SMSL small amps and a Topping or two. They are fun for about 5 minutes. Then I give them to my Dad to play with. He gives them a workout and gives them back. Then they are either given away or in the cupboard someplace. None come close to a real amplifier in performance, usability, quality or enjoyment.

Maybe, just maybe, those SMSLs and Toppings are real amps that give their owners just as much pleasure as your “real” amps give you, since they perform very well within the parameters the owners need them to? I don’t need to own the best in any category. I actually get a ton of pleasure from getting the device that meets my desired parameters for as little money as possible. You might as well. But you seem to argue that your parameters should be everyone’s. The question isn’t whether these are good or bad amps. It is simply to characterize them to an extent that people can decide if they meet their use case.

 

[pma]

@NTK said

Yes, they correspond to the resonant peak at 39kHz, but it does not make the situation any better. Plot harmonics at harmonic frequency display method may be used

 

[IPunchCholla]

@NTK said

Yes, they correspond to the resonant peak at 39kHz, but it does not make the situation any better. Plot harmonics at harmonic frequency display method may be used

View attachment 280406
View attachment 280407

View attachment 280408

Given your tagline in your signature, it might be nice to state the voltage level these were done at. Maybe also explicitly state the bandwidth since that seems particularly relevant.

Is there a reason for not plotting the fundamental?
Any chance of a comparison at 48kHz at the same voltage?

 

[DonH56]

@pma What is different about the test conditions in the three plots above?