Handling a complex load by power amplifiers

[levimax]

If one were correcting the in-room response with DSP would it render the “problem” solved from a listening experience standpoint?

Since whether there is a problem or not is not settled it is hard to say but if you assume there is HF oscillation (instability) DSP would not work because "the problem" would not be consistent and would be source dependent so not really correctable with DSP.

 

[Sokel]

Yeah, or stop praising bad stuff because it's cheap.
Either the reviews should be 100 % objective and the price just stated at the time of review. And the reviewer should not be recommending. -How could he anyway, as he does not know either budget or usecase for the individual??

I'm a nerd when it comes to measurements so I can be a subjective when I'm listening,so...
Ok,real life question:
What happens to it with loud REW sweeps?The results will be ok or should be discarded?

 

[solderdude]

Note the parts in the red ellipses. Fit them and the resonance peak goes away. Specifically the 3R3 + 1uF.

Yep, it seems the designer was focussing more on reducing emitted RF than with peaking.
To those not understanding what Thorsten meant here I will explain.

We see 'nice' behavior with a 4 ohm load. We see less nice behavior with an 8 ohm load. Namely peaking.
So the goal is to ensure a (close to) 4 ohm load for high frequencies even if there is no load or a high impedance load.

This is exactly what the 3.3Ω and 1μF does. In fact, it already 'works' a bit with a 4Ω load as well.
That RC filter (also called Boucherot filter often in combination with a small inductance) provides an extra load for frequencies above 50kHz.
Even when there are none in the music because of the amp being a switching amp.

Some (very basic) calculations at 20kHz , 50kHz and 100kHz will show what the 'real world' (seen by the amplifier) is with a 4Ω(resistive) and 16Ω (resistive) load will be.
Why 16Ω and not 8Ω... well at 100kHz the impedance of a tweeter will be higher (and mostly inductive) so even 16Ω may well be conservative for 100kHz.

A 4ohm resistive load would seen by the amp as:
20kHz 3Ω
50kHz 2.4Ω
100kHz 4.5Ω
Note that in music there will not be a voltage level at 20kHz and most certainly not at 50kHz or 100kHz so while the 'seen' impedance is low the currents (with music) won't but will using a sine-sweep test at full power. One could even make a 1W resistor (that would be used) to go up in smoke.


A 16ohm resistive load would be seen by the amp as:
20kHz 6.5Ω
50kHz 4.4Ω
100kHz 3.5Ω
So... lower than 8Ω and would show hardly any peaking. Problem solved.
Another problem then arises... and that is 'testing' that might occur at say 40kHz at full power.

You see there already is a 1μF in parallel with the output and that Boucherot would just be a second one in parallel. But the rub lies in the resistor.
A fairly large portion of the current will be passing through that resistor. Let's assume a 2W resistor would be used in the Boucherot.
At 40kHz the output is down -2dB opposite 20V (in 8Ω) so about 16V.
At 40kHz about 1/3 of the current will flow through the resistor so about 2A and that resistor will heat up as it gets way more than 2W. In fact... if that testing would be done a bit too long in duration the magic smoke would come out and a burning smell would come out. Don't worry the amp will still work afterwards though.

So... that's probably why no Boucherot is placed.
In practice, when using music even at loud levels, the resistor would not blow up ever but with testing using a slow sweep or tones near full power above 20kHz a burning smell would be there.

Now... is the peaking at 20kHz an issue. With Pavels load we see about +1dB at 14kHz. Let's assume most budget audiophiles can hear this. At 18kHz were up +2dB.
At the same time there will be higher harmonics (inaudible) and some IM for sure but again... with music those levels that will be small as well.

I don't think that is not so much as an audible issue. Certainly not for 'budget hifi'.
Any sane audiophile with higher end speakers will have bought a decent amp anyway and won't be using <$100.- amps.

So is all of this a technical 'issue' (from a design standpoint) ... sure. It does not get our hands together.
Should/would such a cheap amp be inserted in a high-end system ? I don't think so... but who knows.
Is this a non-issue for home and DT speaker usage... no of course not. In fact 1dB extra a 14kHz will not be an issue.
Even +15dB at 40kHz (with music which will have virtually no signal there anyway) will not blow up tweeters and probably still sound 'good enough' for most (IM products will be higher in level) people with most music.

Is this desirable from an engineering P.O.V. ? No... is it of practical concern in the majority of real world cases ? I don't think so.

When you want to be safe... buy a 'better' class-D or AB. When you don't need top notch 'certainty' and are on a budget... buy these devices and enjoy them for as long as they work and buy the newest 'best budget' thingy a few laters later (as an 'upgrade') or when it inexplicably stops working after a few years.

 

[valerianf]

In electronic it is a mistake to pretend that something could never happened.
The 49khz resonance could be triggered by the 3rd harmonic of a 16.33khz or worst by the non controlled breaking resonance of a tweeter.

What worries me more is that some curves are showing an increased noise in the high frequencies of the audio spectrum when the amplifier is loaded with a complex load.
More and more I believe that a class A/B is more suited to drive the high frequencies.
Best configuration would be a class D amp driving the woofer(s) and a class A/B driving the midrange and the tweeter.
In that case bi-amping is perfectly justified.

 

[pma]

I investigate amplifiers up to their limits to find possible weak points and possible reasons of eventual failure. I am aware of the fact that the plots shown may not be easily understandable to unqualified reader. To prevent unwanted fear, I have investigated if the resonant behaviour with load that is inductive at high frequencies may be induced by the input signal limited to 20kHz.

The output BTL filter of A07 can be modelled by 20uH + 0.5uF LC circuit. Its resonant frequency is 50kHz. In previous measurements, it was shifted down to 40kHz due to damping by approx. 4.5kohm input impedance of the audio analyzer.
The next set of measurements is done with my Picoscope with voltage probes with 10Mohm impedance, so the resonant frequency would be unaffected and resonance undamped.

Again the load from post #1 is used. Sine burst 2kHz with Gaussian shaping is used as a test signal. We can see that with the complex dummy load the resonance is induced when the signal amplitude reaches clipping level.
With 4.7ohm resistor the resonance is not induced, as expected, because the LC filter of A07 is damped by the 4.7ohm resistor.

This resonance would not appear with 4.7 ohm resistor load. But the resistor value is critical, due to LC Q damping. Simply - above 6 ohm we shall see frequency response peaking, higher peak with higher resistor value.

Conclusion

So, in a real world use with real speakers, the resonance excitation is unlikely to happen as long as the amplifier works below output clipping level.

 

[MaxwellsEq]

So, in a real world use with real speakers, the resonance excitation is unlikely to happen as long as the amplifier works below output clipping level.

Given the choices regarding power supplies, this suggests that a more powerful PSU than the stock one normally sold would be beneficial.

 

[JktHifi]

I investigate amplifiers up to their limits to find possible weak points and possible reasons of eventual failure. I am aware of the fact that the plots shown may not be easily understandable to unqualified reader. To prevent unwanted fear, I have investigated if the resonant behaviour with load that is inductive at high frequencies may be induced by the input signal limited to 20kHz.

The output BTL filter of A07 can be modelled by 20uH + 0.5uF LC circuit. Its resonant frequency is 50kHz. In previous measurements, it was shifted down to 40kHz due to damping by approx. 4.5kohm input impedance of the audio analyzer.
The next set of measurements is done with my Picoscope with voltage probes with 10Mohm impedance, so the resonant frequency would be unaffected and resonance undamped.

Again the load from post #1 is used. Sine burst 2kHz with Gaussian shaping is used as a test signal. We can see that with the complex dummy load the resonance is induced when the signal amplitude reaches clipping level.
With 4.7ohm resistor the resonance is not induced, as expected, because the LC filter of A07 is damped by the 4.7ohm resistor.

View attachment 280609

View attachment 280610

View attachment 280611

This resonance would not appear with 4.7 ohm resistor load. But the resistor value is critical, due to LC Q damping. Simply - above 6 ohm we shall see frequency response peaking, higher peak with higher resistor value.

Conclusion

So, in a real world use with real speakers, the resonance excitation is unlikely to happen as long as the amplifier works below output clipping level.

Very good and comprehensive conclusion

 

[thecheapseats]

Yep, it seems the designer was focussing more on reducing emitted RF than with peaking.
To those not understanding what Thorsten meant here I will explain.

We see 'nice' behavior with a 4 ohm load. We see less nice behavior with an 8 ohm load. Namely peaking.
So the goal is to ensure a (close to) 4 ohm load for high frequencies even if there is no load or a high impedance load.

This is exactly what the 3.3Ω and 1μF does. In fact, it already 'works' a bit with a 4Ω load as well.
That RC filter (also called Boucherot filter often in combination with a small inductance) provides an extra load for frequencies above 50kHz.
Even when there are none in the music because of the amp being a switching amp.

Some (very basic) calculations at 20kHz , 50kHz and 100kHz will show what the 'real world' (seen by the amplifier) is with a 4Ω(resistive) and 16Ω (resistive) load will be.
Why 16Ω and not 8Ω... well at 100kHz the impedance of a tweeter will be higher (and mostly inductive) so even 16Ω may well be conservative for 100kHz.

A 4ohm resistive load would seen by the amp as:
20kHz 3Ω
50kHz 2.4Ω
100kHz 4.5Ω
Note that in music there will not be a voltage level at 20kHz and most certainly not at 50kHz or 100kHz so while the 'seen' impedance is low the currents (with music) won't but will using a sine-sweep test at full power. One could even make a 1W resistor (that would be used) to go up in smoke.


A 16ohm resistive load would be seen by the amp as:
20kHz 6.5Ω
50kHz 4.4Ω
100kHz 3.5Ω
So... lower than 8Ω and would show hardly any peaking. Problem solved.
Another problem then arises... and that is 'testing' that might occur at say 40kHz at full power.

You see there already is a 1μF in parallel with the output and that Boucherot would just be a second one in parallel. But the rub lies in the resistor.
A fairly large portion of the current will be passing through that resistor. Let's assume a 2W resistor would be used in the Boucherot.
At 40kHz the output is down -2dB opposite 20V (in 8Ω) so about 16V.
At 40kHz about 1/3 of the current will flow through the resistor so about 2A and that resistor will heat up as it gets way more than 2W. In fact... if that testing would be done a bit too long in duration the magic smoke would come out and a burning smell would come out. Don't worry the amp will still work afterwards though.

So... that's probably why no Boucherot is placed.
In practice, when using music even at loud levels, the resistor would not blow up ever but with testing using a slow sweep or tones near full power above 20kHz a burning smell would be there.

Now... is the peaking at 20kHz an issue. With Pavels load we see about +1dB at 14kHz. Let's assume most budget audiophiles can hear this. At 18kHz were up +2dB.
At the same time there will be higher harmonics (inaudible) and some IM for sure but again... with music those levels that will be small as well.

I don't think that is not so much as an audible issue. Certainly not for 'budget hifi'.
Any sane audiophile with higher end speakers will have bought a decent amp anyway and won't be using <$100.- amps.

So is all of this a technical 'issue' (from a design standpoint) ... sure. It does not get our hands together.
Should/would such a cheap amp be inserted in a high-end system ? I don't think so... but who knows.
Is this a non-issue for home and DT speaker usage... no of course not. In fact 1dB extra a 14kHz will not be an issue.
Even +15dB at 40kHz (with music which will have virtually no signal there anyway) will not blow up tweeters and probably still sound 'good enough' for most (IM products will be higher in level) people with most music.

Is this desirable from an engineering P.O.V. ? No... is it of practical concern in the majority of real world cases ? I don't think so.

When you want to be safe... buy a 'better' class-D or AB. When you don't need top notch 'certainty' and are on a budget... buy these devices and enjoy them for as long as they work and buy the newest 'best budget' thingy a few laters later (as an 'upgrade') or when it inexplicably stops working after a few years.

thanks for taking the time to type that all out... very clear...

 

[theREALdotnet]

More and more I believe that a class A/B is more suited to drive the high frequencies.

So, to a prospective buyer of an A07, which $100 class AB amplifier would you recommend they buy instead?

 

[JktHifi]

So, to a prospective buyer of an A07, which $100 class AB amplifier would you recommend they buy instead?

Currently the price of Aiyima A07 is $60

 

[tmtomh]

Yeah, or stop praising bad stuff because it's cheap.
Either the reviews should be 100 % objective and the price just stated at the time of review. And the reviewer should not be recommending. -How could he anyway, as he does not know either budget or usecase for the individual??

Please identify reviews where @amirm has praised equipment that is objectively bad. Only then can we properly guage the validity of your position on this.

 

[Deleted member 48726]

Please identify reviews where @amirm has praised equipment that is objectively bad. Only then can we properly guage the validity of your position on this.

I don't need your gauging.
As you were.

 

[Deleted member 48726]

So, to a prospective buyer of an A07, which $100 class AB amplifier would you recommend they buy instead?

A used one.

 

[tmtomh]

I don't need your gauging.
As you were.

Oh, nice try! The high hat doesn't work if there's nothing underneath it.

 

[Deleted member 48726]

Oh, nice try! The high hat doesn't work if there's nothing underneath it.

Okay.

 

[sarumbear]

I don't need your gauging.
As you were.

But I do. You are talking about bad practice in ASR reviews but cannot show examples.

 

[IAtaman]

I think the complication is caused by the fact that "value for money" and "good" attributes gets entangled when a product is classified as recommended.

It is kind a funny how opinionated we can all be about someone else's personal opinions and recommendations - but here we are.

Anyway, what I wanted to say was this : I suppose when you want your recommendations to have utility beyond philosophical, and help people make purchase decisions, you need to take the price and value into consideration. However those two parameters are inevitably subjective and time bound, depends on what is the price, what other options are available etc. Someone's good value for money is another person's trash, and vice versa. This creates the subjectivity of recommendations in my opinion.

I think there is no reason to think Amir didn't consider how to approach this and made a decision that is probably the best given his personal objectives - after all it is his time and money invested into these reviews. Nevertheless though, given the status of ASR and the weight of his recommendations, I think it might be a good time to maybe revisit the recommendation scheme and introduce some sort of nuance so as to avoid these discussions perhaps .

 

[Deleted member 48726]

When you want to be safe... buy a 'better' class-D or AB. When you don't need top notch 'certainty' and are on a budget... buy these devices and enjoy them for as long as they work and buy the newest 'best budget' thingy a few laters later (as an 'upgrade') or when it inexplicably stops working after a few years.

You are right, of course. This style of buy and throw away just grinds my gears. We should aim higher somehow. Pay for quality not pay the lazy ones making quick bucks by selling stuff that isn't made to last.

 

[IPunchCholla]

I am aware of the fact that the plots shown may not be easily understandable to unqualified reader.

I’ll certainly raise my hand as one of those unqualified readers, but in this thread and in several others people who are seemingly qualified raise questions about some posts that seem like they are missing information or point out questions about presentation and are simply never addressed. I would also point out that this is not (as far as I understand, an audio engineering forum, so you are looking at a very high probability that lay people are going to access the posts. I certainly change how I present information based on the audience I am talking to. I don’t expect people to have my domain knowledge outside of certain settings. Lastly, I am trying to become “qualified”. Not that I am looking for actual certification, but more in the sense of having an understanding. While I am also (slowly) going through online EE lessons, this is another place I try and learn as it seems like a good place to come to see how theory meets application.

I really appreciate you making these posts, and clearly you have a very high level of expertise, which is why I read them.

 

[sarumbear]

Quoting Amazon review star numbers as gospel is nearly as bad as using wikipedia as a reference...

A cynic is incapable is unable to benefit from progress.