Complex Load for Power Amplifier torture testing

[March Audio]

Thanks for doing the tests, @March Audio . There is a small difference, as mentioned by @restorer-john , though your modules are about 400W rated power, my KA-5010 clone with only 2x25V power supply is rated at 40W/4ohm. Speaking in power terms, talking about 5W makes sense only for the resistive load. With the R//R-C load according to post #1 and constant output voltage of 4.47V, 5W is true for 50Hz frequency, but at 20kHz the power into 0.5ohm is 40W and output current is 9A. This is just at the limit of the small KA-5010 clone. For your amplifiers, this is deep below the limit and to make a comparable test, you should run it at 14.14V, this would be 50W/50Hz/4ohm and 400W/20kHz/0.5ohm. Output current would be 28A/20kHz. Then the tests would be comparable as of the stress created to the amplifiers.

Just one more comment - testing in REW and STEPS is not equal.
REW sends a sine sweep with duration of several seconds for the whole frequency band. STEPS sends triggered sine wave at each frequency, frequency increment in my case is 1/24 octave and at each frequency the wave is of 200ms duration continued by 200ms pause. The whole measurement takes several minutes, not seconds.

@pma there is another issue here. Again by testing at higher power levels you are straying far away from reality.

Music content at 20kHz is at quite low levels, maybe 50dB lower than at say 1kHz.
So, 14 volts reduced by 50dB is only 44 mV. Let's be generous and say at 20kHz it's only 30dB lower. That equals 0.44 volts which will be only 3.8 watts.

I have already tested at 6 volts which is 72 watts, way more than will ever be required in the real world at 20kHz.

So unfortunately your test has no meaning, its irrelevant.

 

[Wombat]

#142 General comment:

At sixes and sevens

 

[pma]

So unfortunately your test has no meaning.

The result is that your amp passed the test, with the specific complex load, for THD vs. frequency at 6V output voltage.

Have you also tried a square wave test as demonstrated in
https://www.audiosciencereview.com/...r-amplifier-torture-testing.10298/post-282912

to verify unconditional stability of the amplifier? This is not about testing with a "real music signal", because sources triggering possible instability are much wider than just a regular music signal, as was already explained.

 

[restorer-john]

I think that rather than playing tit for tat, it would be better to devise tests that realistically characterize amplifier performance in real world situations. Come together and devise a barrage of tough, but fair, loads and test procedures that can be agreed upon.

And I don't mean giving a free pass, or an easy run for one topology over another. Worse, the converse, where a test can easily demonstrate superiority of one topology over another.

At the end of the day, it's all about typical speakers, typical amplifiers and reasonable stimuli (signals). What needs to be considered is waveform integrity, regardless of the the source limitations. If someone is running an analogue front end with a response out to 50KHz, they are entitled to an amplifier that can reproduce that with integrity. That should be the bare minimum IMO. Whether they can hear it, is irrelevant, just as (0.0015% or 0.00015% is just as irrelevant)

 

[Willem]

I guess the 50 kHz requirement is a personal preference. I have alwasy been in favour of bandwidth limited designs in the Peter Walker tradition. I am not at all interested in frequencies that I cannot hear, and even less if they can cause problems or contain only crap/noise. Peter's designs were certainly stable into difficult loads.

 

[pma]

Come together and devise a barrage of tough, but fair, loads and test procedures that can be agreed upon.

And I don't mean giving a free pass, or an easy run

There is no Free Lunch, John. I am sure that the following plots need no more comments. Both measured in a real-life system with real speakers and cable.

 

[BaaM]

Now, you are welcome to make this task easy for me to implement. I priced the Audiograph Powercube and it costs US $15,000 for a stereo unit with up to 500 watts capacity. Raise the funds for me and I will subject every amplifier to that.

I guess this is potentially an achievable goal if the community could donate for this particular project?

 

[restorer-john]

I guess this is a potentially an achievable goal if the community could donate for this particular project?

Yes it would be. But there's going to screams from Pavel, me and others that 1KHz tone burst testing is not representative of real world loads either.

It's a no-win situation without a programmable active load IMO and that is beyond my pay grade, especially one that can sink the sorts of power outputs we are going to be dealing with.

 

[ajawamnet]

Yes it would be. But there's going to screams from Pavel, me and others that 1KHz tone burst testing is not representative of real world loads either.

It's a no-win situation without a programmable active load IMO and that is beyond my pay grade, especially one that can sink the sorts of power outputs we are going to be dealing with.

Not sure if this would help:
https://quantasylum.com/collections/frontpage/products/qa450-programmable-8-4-ohm-load
$269
The mfg's on the list somewhere...

 

[March Audio]

The result is that your amp passed the test, with the specific complex load, for THD vs. frequency at 6V output voltage.

Have you also tried a square wave test as demonstrated in
https://www.audiosciencereview.com/...r-amplifier-torture-testing.10298/post-282912

to verify unconditional stability of the amplifier? This is not about testing with a "real music signal", because sources triggering possible instability are much wider than just a regular music signal, as was already explained.

Yes the square wave was fine I will post it later

You need step back and grab a dose of reality mate. It's stable, unaffected by your capacitive low impedance load and very low distortion.

If you want to go on a quest to show that if you get extreme enough you can cause an amp a problem that's up to you but you need to do it on your time at your expense.

Of course it's about using real music signals. What do you listen to? What's the purpose of an amp otherwise? Going unstable is an utterly rare issue that you are exaggerating the possibility of for reasons unknown. It's tunnel vision that's informative to no one.

Find a report on the net of a Hypex/Purifi amp going unstable and we can investigate specifics. Until then there is nothing to see here.

 

[pma]

Yes the square wave was fine I will post it later

You need step back and grab a dose of reality mate. It's stable, unaffected by your capacitive low impedance load and very low distortion.

If you want to go on a quest to show that if you get extreme enough you can cause an amp a problem that's up to you but you need to do it on your time at your expense.

OK, we may have different view of stability.

Thank you for your suggestion to make a test on my time and my expense - I already did that (in the year 2014) and measured the NC400 professionally built module when I tested step response into 6.8uF capacitive load.

1) capacitive load connected to NC400 output, no input signal for NC400. NC400 output was filtered by a double RC low pass filter with amplitude response as follows

This filter was used to get rid of 440kHz carrier frequency which messes all measurements. However, amplitude response of this filter must be taken into account in further measurements.

At the output of NC400, with no input signal, and the filter connected to the NC400 output, oscillations of 53.48kHz and quite high amplitude were measured. Once again - there was no input signal, so these are system oscillations.

On step response, this can be seen as oscillations on bottom and top lines

however the step was not needed to excitate these oscillations.

At 53.48kHz, the 2RC filter used has -6dB attenuation, so the amplitude of oscillations is in fact twice bigger.


I am sorry but to me this is the unstable system (with capacitive load of 6.8uF as tested) and oscillation frequency is to close to audio band to be neglected and falls into frequency range of SOTA tweeters and will be radiated as an acoustical wave and may cause potential intermodulation distortions in the ESL speaker.

This is my contribution to

if you get extreme enough you can cause an amp a problem that's up to you but you need to do it on your time at your expense.

 

[March Audio]

Do we?

As stated previously no real world load is a 6.8uf cap without any other resistance or inductance.

Try putting that cap directly across the output of any number of class A/B amps and see what happens.

The NC400 is *NOT* unstable. There are thousands of them out there and I have yet to see any complaint about instability. In fact we have demonstrated further up the thread just how load tolerent the nCore and Bruno Putzeys designs are, ironically with your design of pathological load.

You are simply wrong. You are creating artificial circumstances that just do not exist in any real load. Your test above is meaningless.

 

[fredoamigo]

interesting what AP says for its instructions for use with Audiograph Powercube

https://www.ap.com/technical-library/measuring-power-amplifiers-with-reactive-loads/

 

[pma]

KA-5010 clone, step response into 6.8uF capacitive load:

 

[March Audio]

KA-5010 clone, step response into 6.8uF capacitive load:
View attachment 42409

You still dont get it do you?

Show me a speaker that is a pure 6.8 uF load with no resistance and no inductance.

I dont know what you think you are trying to prove, but as I said you need to get a dose of reality.

BTW put your LP filter in circuit.

 

[March Audio]

interesting what AP says for its instructions for use with Audiograph Powercube

https://www.ap.com/technical-library/measuring-power-amplifiers-with-reactive-loads/

No one is denying the usefulness of reactive load testing, we did some above. However PMA is is doing things that are not in any way representative of the real world.

 

[March Audio]

For info, step response of P701 (Hypex NC1200) with simple filter in series with the scope to remove switching frequency which would mess up the measurement. Bits I had on the shelf, not ideal.

4 ohm load + the same 30uF cap in series with 0.56 ohm resistor as previous tests.

 

[pma]

It is the similar filter as I use.

With a "normal" amplifier, it totally degrades rise time of the step response, to some 11.2 us. Yours would be twice more.

But I understand it must be used in case we measure the modulated power oscillator.

Without the filter, the same, normal amplifier (KA-5010 clone) measures rise time about 1.2 us, to 2.5 ohm resistor.

 

[March Audio]

Yes, expected, just bits that were to hand.

So as you can see above just a very small resistance in series with a 30uF cap and the amp is stable.

Your test, putting a high value cap straight across the output is not correct. This will never happen. There will always be some series resistance and inductance.

If you have been testing amps this way Im afraid you have been misleading yourself and drawing false conclusions about stability.

 

[Willem]

Does anyone have an up to date opinion on the more modern Quad ESL incarnations that came out of IAG?

My first serious speakers were the ESL57s, bought in the 1970's, and driven by a Quad 33/303 amplifier combination. I still have both the speakers and the amplifier. The ESL57 was a very hard speaker to drive, and at the time the Quad 303 amplifier was effectively the only safe option. Some other amps would do it as well, but it was a hit and miss thing, and you could fry either the speakers or the amp, or both. The good news was that for all its limitations it nevertheless sounded very good indeed. My current main speakers recently had to go in for repairs, so I brought the old ESL57 and 33/303 out of storage, and they were still a joy to listen to.
About a decade ago I decided that it was worth getting an update for the second half of my audio life, so I bought a pair of modern 2805s. The difference is by no means night and day - you are still listening to the same style of reproduction. At the same time the 2805 is a lot better in many respects. Stereo image is altogether improved, and the sound is as spatial as you could possibly wish for. The music is quite simply in the room rather than coming from the speakers. Low frequency response has also been improved significantly, just as power handling. Conversely, the 2805 is less efficient and though it will play louder with suitable amplification, it will still not play very loud in a larger room. My old 2x45 watt 303 was clearly not up to the job (we had also moved to a rather bigger house), so I replaced it with a refurbished 2x140 watt Quad 606-2, and my hunch is the 2805s can do with even more power.
One way to address the speaker's limitations is to get the larger 2905/2912, because it can handle a bit more power, and has two additional bass panels for deeper bass (it is not even that much more expensive than the smaller 2805/2812). However, they were just too imposing to my taste, and they would have blocked the view from a large panorama window. Also, they need more space behind them.
In the old vinyl days I never felt I needed a subwoofer, but even though the new speakers have rather better (deeper and tighter) bass response I felt the time had come for a subwoofer, so I bought a B&W PV1d. This was a disappointment at first, because the sound was fairly woolly unless I turned down the sub to pretty useless levels. I then discovered the discussion about room modes, and bought an Antimode 8033 dsp room eq unit. This cleaned up the sound enormously to the extent that the lowest frequencies (up from about 13 Hz) now seemed to emanate from the stats without any integration problem (therefore, a second sub is on my wish list). My conclusion is that the common reservations about Quads with subs originate in the fact that the Quads' dipole character was responsible for far fewer room modes from the main speakers and hence made the room modes generated by subs stand out even more.
With regard to amplifier speaker interaction, the 2805s are not nearly as hard to drive, and impedance never dips below 4 Ohm:
https://www.stereophile.com/content/quad-reference-esl-2805-loudspeaker-measurements