On Class D Amplifiers Measurements

[restorer-john]

What 20 kHz? My frequency response measurements go up to 200 kHz:

At a measly 5 watts Amir. Let's not play games here.

 

[JohnYang1997]

What are you going on about??

And I'm sorry, this is just plain rubbish.



What, you want to make it really easy for any DUT?



This is right on the money. It's a concerted attempt to control the narrative and undermine many decades of superlative design and countless luminary engineers' research and development. You will meet opposition here by fanbois, vested interests and sonsumers of the koolaid, but stay the course.

Rule of thumb is used to give reasonable performance without investigating how far each parameters can be pushed. 100khz bandwidth for class A amplifiers will most certainly give pretty good distortion under 10khz. But will including 10th order input filter to filter out any signal over 20khz affect performance? If done right, of course not.

On the second one. Ok then go ahead and play 200khz full power through class AB amplifiers for 24hours. See what happens. Being realistic is not the same as just being easy on DUT.

 

[JohnYang1997]

At a measly 5 watts Amir. Let's not play games here.

Do you really know how to do measurements? Frequency response is generally considered to be small signal measurement. Normally it's 1V at output or 50mV at input or somewhere in that order. Then do the slewrate test for small and large signal.

There is absolutely no need to have high power at high frequency. Flat to 20khz is good enough or including phase then 40khz.

 

[Wombat]

Now there's a challenge.

 

[pma]

I disagree. Here is my challenge

https://www.diyaudio.com/forums/everything-else/344822-hear-difference-2-solid-preamps.html

 

[restorer-john]

Do you really know how to do measurements? Frequency response is generally considered to be small signal measurement. Normally it's 1V at output or 50mV at input or somewhere in that order. Then do the slewrate test for small and large signal.

There is absolutely no need to have high power at high frequency. Flat to 20khz is good enough or including phase then 40khz.

No. Wrong again. Frequency response in amplifiers (we are talking amplifiers here) has been specified at 2.83V (1W@8R) since the dawn of time. Go do some actual research and report back here with your findings.

Do you really know what a power bandwidth test is?

Please go look up, read, and report back here with your answer.

Hint: The frequency range at which an amplifier can deliver its rated power (half power if you want to be kind) at a specific THD (usually 0.1%). Most decent amplifiers in the 1970s rated out to at least 40 or 50KHz...

Here's an ancient Pioneer SA-708 (late 70s design) tested by me way back in 2009 at 76W@8R (rated 65W@8R) FR swept from 10Hz-20Khz just under clipping.

She's rolling off a touch at each end of the spectrum at 61.6W@8R/10Hz, but pretty good in real terms.

 

[pma]

Hint: The frequency range at which an amplifier can deliver its rated power (half power if you want to be kind) at a specific THD (usually 0.1%). Most decent amplifiers in the 1970s rated out to at least 40 or 50KHz...

Here's an ancient Pioneer SA-708 (late 70s design) tested by me way back in 2009 at 76W@8R (rated 65W@8R) FR swept from 10Hz-20Khz just under clipping.

John, just for fun, this are plots from this amplifier
https://www.audiosciencereview.com/...vintage-kenwood-ka-5010-amplifier-clone.9902/

at 1kHz near clipping and then at 100kHz, both wit 4ohm load:

But, you know, these days it is unimportant . Who cares? However, this linear amp is fully competetive even in THD and SINAD . End does not send tons of HF 440kHz rubbish and its multiples into the air. Who cares? EMI pollution is a standard, nowadays.

 

[pma]

And how about this?

132W/4ohm at 100kHz
http://pmacura.cz/pa2.htm

Not so bad, I think.

 

[Wombat]

"These days it is unimportant. Who cares?"

Some more than you, it seems.

 

[restorer-john]

Not so bad, I think.

Understatement.

What I find absolutely hilarious, is the same people who espouse "High Res", high sampling rate, DSD, wide bandwidth, ruler flat sources, and line level processing, are prepared to dismiss the same ruler flat requirements in their power amplification.

There's no free passes in high fidelity, no matter how thick and sweet-smelling your smoke screen is.

 

[pma]

"These days it is unimportant. Who cares?"

Some more than you, it seems.

I hope so!

 

[March Audio]

Because I ask for the product, being it instrumentation or audio, to work flawlessly in the frequency range at least one order higher (10x) than is the supposed bandwidth of the signal processed. .

For what reason?

 

[March Audio]

Understatement.

What I find absolutely hilarious, is the same people who espouse "High Res", high sampling rate, DSD, wide bandwidth, ruler flat sources, and line level processing, are prepared to dismiss the same ruler flat requirements in their power amplification.

There's no free passes in high fidelity, no matter how thick and sweet-smelling your smoke screen is.

I dont, I think high bandwidth audio is a waste of time.

 

[March Audio]

Re your question of squares, I have already explained that the step response completely describes transfer function of the linear system, because impulse response is a derivative of step response and amplitude and phase response can be calculated by Hilbert transformation of Fourier transformation of impulse response. This forum calls itself "science", right? Step response quickly shows possible issues and possible stability issues, as it does here. 20kHz amplitude spectrum shows a little.

There are no stability issues with these amps.

 

[pma]

Understatement.

What I find absolutely hilarious, is the same people who espouse "High Res", high sampling rate, DSD, wide bandwidth, ruler flat sources, and line level processing, are prepared to dismiss the same ruler flat requirements in their power amplification.

There's no free passes in high fidelity, no matter how thick and sweet-smelling your smoke screen is.

The reason is that good class AB is expensive - you cannot go without big transformers, big case and heatsinks etc. On the other hand, some NC400 implementations are not cheap as well. However, you do not need to know much. Buy OEM modules, put it into nice case and you may start production with fast assembly at reduced cost. The price counts and dictates, dictates the rules.

 

[March Audio]

I find all this defense of narrow band 20kHz measurements as manufacturer's protection only and a part of marketing strategy. This is a must for contemporary class D technology.

You have missed the point. This isnt about narrow band measurements, its about the context and real world impact of things shown in measurements.

Happy to look at wide band measurements, however that doesnt mean anything seen has any real world significance.

 

[pma]

There are no stability issues. with these amps.

How about capacitive load, like some ESL speakers.

HYPEX NC400, no LPF before the oscilloscope, 52.3kHz oscillations, 3.41Vp/p, about 1.2Vrms

 

[March Audio]

At a measly 5 watts Amir. Let's not play games here.

There is something wrong with that amp if thats what it does, thats not how the NC500 behaves.

 

[March Audio]

How about capacitive load, like some ESL speakers.

View attachment 40087
HYPEX NC400, no LPF before the oscilloscope, 52.3kHz oscillations, 3.41Vp/p, about 1.2Vrms

What about it? I have never heard of people having problems with decent class D amps going unstable and oscillating.

So, just to be clear, in your measurement above youve put a 6.8uF cap across the output with no resistive or inductive component? Really? . That's a realistic load... Not...

Arent ESLs driven through a step-up transformer? Inductive?

I think I will reproduce your measurements, with perhaps a more realistic scenerio

 

[March Audio]

John, just for fun, this are plots from this amplifier
https://www.audiosciencereview.com/...vintage-kenwood-ka-5010-amplifier-clone.9902/

at 1kHz near clipping and then at 100kHz, both wit 4ohm load:

View attachment 40084

View attachment 40085

But, you know, these days it is unimportant . Who cares? However, this linear amp is fully competetive even in THD and SINAD . End does not send tons of HF 440kHz rubbish and its multiples into the air. Who cares? EMI pollution is a standard, nowadays.

Haha

No they dont create huge amounts of RF pollution.

I have a Purifi 1ET400 on the bench in front of me. Its a lash up, not in a case, driving a speaker via 2m of cable.

Here is the RF spectrum, antenna is just 1m away. You can see broadcast radio stations at 630kHz and 782 kHz. Scope is measuring the amp switching frequency at 507kHz.

Amp off

Amp on

switching frequency is about -100dBm