Amplifiers Which Test Well, Under $1,000 USD

[levimax]

Hypex also take this view and limit their measurements to the audible band. The cynical will say this is just to make the performance look better, however I have yet to hear a convincing argument (or any argument at all for that matter) against the fundamental principle I discuss above.

I understand the argument that US harmonics are not an issue but I am not clear as to what happens to all the US energy when it goes into a tweeter. Assuming there is no mechanical response to dissipate the energy it seems to me the energy would have to either be converted into heat or have some other interaction with the other signal components making some type of change to them or something else? I am fairly certain the energy does not just "disappear without a trace". I have looked but have not found a scientific test showing how exactly a real tweeter acts when being fed an audible signal and US signal compared to just an audible signal. I would be curious not just the sound output but all so the voice coil temp. As good as the Class D amps look in the audio range if the "ideal amplifier" is a "wire with gain" then added US content is not ideal.

 

[levimax]

I did a little more searching on US sound and how it could become audible and found some articles on "beat frequencies" which show how two US tones 1 KHz apart create and audible 1 KHz sound. So to me it seems that if you had a tweeter that can reproduce US harmonic distortion it would be very possible for this distortion to become audible. https://www.researchgate.net/public...ble_Sound_System_using_Ultrasonic_Transducers

 

[andreasmaaan]

I did a little more searching on US sound and how it could become audible and found some articles on "beat frequencies" which show how two US tones 1 KHz apart create and audible 1 KHz sound. So to me it seems that if you had a tweeter that can reproduce US harmonic distortion it would be very possible for this distortion to become audible. https://www.researchgate.net/public...ble_Sound_System_using_Ultrasonic_Transducers

Kind of. The distortion you're talking about is intermodulation (not harmonic) distortion.

There are no dome, cone, ring radiator or ribbon tweeters on the market that can reproduce the frequencies we're talking about here though (to my knowledge, of course).

 

[andreasmaaan]

I understand the argument that US harmonics are not an issue but I am not clear as to what happens to all the US energy when it goes into a tweeter. Assuming there is no mechanical response to dissipate the energy it seems to me the energy would have to either be converted into heat or have some other interaction with the other signal components making some type of change to them or something else? I am fairly certain the energy does not just "disappear without a trace".

That's true Normally the ultrasonic noise is filtered out by an analogue filter on the output stage of the amplifier.

 

[levimax]

Kind of. The distortion you're talking about is intermodulation (not harmonic) distortion.

There are no dome, cone, ring radiator or ribbon tweeters on the market that can reproduce the frequencies we're talking about here though (to my knowledge, of course).

I don't pretend to know if this is a real issue or not but the "harmonic distortion" I am talking about is lets say two tones one at 16 KHz and one at 17 KHz that create 3rd order Harmonic Distortion of 48 Khz and 51 KHz which was then reproduced by some "ribbon tweeters" (Which some apparently can reproduce sound up to 100K). These two ultrasonic tones could then produce a "beat frequency" of 3 KHz.... right in the middle of the audio range.

Again I am not sure but I don't think dismissing US harmonics as "nothing to be concerned with" is the right answer unless there is scientific evidence that this is the case..... maybe it's out there but I have not found it yet.... all I have found are reasons to be concerned. I welcome being educated.

 

[levimax]

That's true Normally the ultrasonic noise is filtered out by an analogue filter on the output stage of the amplifier.

So why don't all these class D amps have such a filter? I think it is not so easy and causes other issues.

 

[andreasmaaan]

I don't pretend to know if this is a real issue or not but the "harmonic distortion" I am talking about is lets say two tones one at 16 KHz and one at 17 KHz that create 3rd order Harmonic Distortion of 48 Khz and 51 KHz which was then reproduced by some "ribbon tweeters" (Which some apparently can reproduce sound up to 100K). These two ultrasonic tones could then produce a "beat frequency" of 3 KHz.... right in the middle of the audio range.

Let's assume a scenario in which the third harmonic distortion of an amplifier at 16-17KHz is around 1% (i.e. orders of magnitude higher than an amp like the NCore - but I'm trying to imagine a worst-case scenario for a very poorly-performing class D amp) and there is no output filter.

This means that the harmonics in your example at 48KHz and 51KHz are 40dB lower in level than the fundamentals.

Let's then assume that the tweeter has flat response up to 51KHz (not going to be the case in reality either, but let's make it a by-far worst-case scenario). And that it produces a difference tone at 1% of the level of the harmonics.

That difference tone would be 80dB below the 16-17KHz fundamentals.

So why don't all these class D amps have such a filter? I think it is not so easy and causes other issues.

They all do (to my knowledge)

 

[GoMrPickles]

On this theme, there's this Polish NC500MP supplier I stumbled on; $600/monoblock, and it can come with a wood faceplate for no upcharge:

I'm surprised they aren't selling that case alone for $600. Yowza.

 

[levimax]

Let's assume a scenario in which the THD of an amplifier at 16-17KHz is around 1% (i.e. orders of magnitude higher than an amp like the NCore - but I'm trying to imagine a worst-case scenario for a very poorly-performing class D amp).

This means that the harmonics in your example at 48KHz and 51KHz are 40dB lower in level than the fundamentals.

Let's then assume that the tweeter has flat response up to 51KHz (not going to be the case in reality either, but let's make it a by-far worst-case scenario). And that it produces a difference tone at 1% of the level of the harmonics.

That difference tone would be 80dB below the 16-17KHz fundamentals.

They all do (to my knowledge)

The Lyngdorf TDAI-3400 which Amirm just reviewed https://www.audiosciencereview.com/...nts-of-lyngdorf-tdai-3400-amp-eq-part-1.6738/ apparently has no such low pass filter and the US harmonics were only -25 db.

Beat frequencies "sum" the two fundamentals in amplitude so in the first example you gave of -40 db the beat frequency would be I think -34 db (6 db for double the amplitude?) and for the Lyngdorf -19 db.

Again not sure how much of this is a "real" concern and I don't think the above calculations are realistic but to me US harmonics seem like a lot more potential trouble than the -110 db vs -120 db of the DACs measured.

 

[Ron Texas]

These are the least expensive Hypex amps that I could find:

https://www.nordacoustics.co.uk/nord-one-hypex-ncore-mpxxx-stereo-p

 

[andreasmaaan]

The Lyngdorf TDAI-3400 which Amirm just reviewed https://www.audiosciencereview.com/...nts-of-lyngdorf-tdai-3400-amp-eq-part-1.6738/ apparently has no such low pass filter and the US harmonics were only -25 db.

Ah I hadn't read that review. What a horribly-designed unit. Hadn't realised there were amps out there that bad, nor that used power supplies that switched so low in frequency (50KHz). That's just absurd. I'd treat it as an outlier.

Beat frequencies "sum" the two fundamentals in amplitude so in the first example you gave of -40 db the beat frequency would be I think -34 db (6 db for double the amplitude?) and for the Lyngdorf -19 db.

This is not how it works. The beat frequency is not a separate sound in its own right, it is a pattern of constructive and destructive interference between two (or more) tones. If the tones producing the beat frequency can't be heard (e.g. because they are too high in frequency), the beat frequency can't be heard either.

See this video for an illustration.

The real concern is intermodulation distortion, which is what I assumed you were talking about. All speakers do produce IM distortion, and I addressed this in my earlier post.

Or perhaps I'm misunderstanding what you're getting at?

 

[levimax]

This is not how it works. The beat frequency is not a separate sound in its own right, it is a pattern of constructive and destructive interference between two (or more) tones. If the tones producing the beat frequency can't be heard (e.g. because they are too high in frequency), the beat frequency can't be heard either.

According to this link,
https://www.researchgate.net/public...ble_Sound_System_using_Ultrasonic_Transducers it is possible to have two ultra sonic frequencies create an audible beat frequency which is what I am getting at. I have not tried it myself because I don't have an amp or speakers that can play that high but it is interesting.

I understand about IM distortion.

 

[andreasmaaan]

According to this link,
https://www.researchgate.net/public...ble_Sound_System_using_Ultrasonic_Transducers it is possible to have two ultra sonic frequencies create an audible beat frequency which is what I am getting at. I have not tried it myself because I don't have an amp or speakers that can play that high but it is interesting.

I understand about IM distortion.

Ok, yes. It is very interesting, but such systems rely on the nonlinearity of air at high pressure. Therefore, they don't produce sound in the audio band unless the ultrasonic SPLs are sufficiently high.

To achieve the requisite pressure levels at which this nonlinearity begins to produce audibly loud difference frequencies in the audio band, an ultrasonic transducer must produce well over 100dB SPL at 1m.

This graph comes from an AES paper on ultrasonic transducers and shows the amplitude response of an ultrasonic transducer:

As you can see, the required ultrasonic SPLs are extremely high compared to the audio-band SPLs that are generated (an ultrasonic SPL of [email protected] produces audio band SPLs of around 50dB@4m).

I can't imagine a real-life situation where (a) the amplifier is producing enough output at ultrasonic frequencies and (b) the loudspeakers are reproducing those frequencies at sufficiently high SPLs to cause the air to become sufficiently nonlinear for audible difference frequencies to be generated.

This is an interesting track to go down though, so if I've missed something in your reasoning I'm happy to keep thinking about it

 

[March Audio]

I understand the argument that US harmonics are not an issue but I am not clear as to what happens to all the US energy when it goes into a tweeter. Assuming there is no mechanical response to dissipate the energy it seems to me the energy would have to either be converted into heat or have some other interaction with the other signal components making some type of change to them or something else? I am fairly certain the energy does not just "disappear without a trace". I have looked but have not found a scientific test showing how exactly a real tweeter acts when being fed an audible signal and US signal compared to just an audible signal. I would be curious not just the sound output but all so the voice coil temp. As good as the Class D amps look in the audio range if the "ideal amplifier" is a "wire with gain" then added US content is not ideal.

You are correct, heat would be the result, however in AC circuits you have what is called inductive and capacitive reactance. This adds to the resistance to oppose the current flow. Hence the term impedance.

Inductive reactance is caused by, you guessed it, inductors. A coil of wire (speaker coil) is an inductor. Inductive reactance is calculated by 2*pi*f*l. So increasing the frequency increases the overall impedance and reduces the current flow. It essentially becomes negligeable compared to the in band music power.

The Hypex Nc400 Amir tested showed That with a 20khz fundamental there were low level harmonics at - 70dB. Can't remember the power level this was tested at. This is already really small. You then add the effect of the inductive reactance. I can do some calculations to demonstrate later just how small the heating effect would be. This is also ignoring the effect of any speaker crossover filter that will reduce it further.

Regarding intermodulation distortion, well again it's a non issue. To create those beat frequencies you have actually have acoustic output in the ultrasonic range. If your speakers don't respond at those ultrasonic frequencies you can't generate intermodulation.

So again, with the NC400 example the harmonic distortion at 40 and 60 kHz is already 70 dB down. Let's say you speaker does have some output at those frequencies. Unlikely but for the sake of argument say its another 30dB down. Let's again say that the amp has distortion at 59khz as well so we generate a difference Intermodulation at 1kHz right in the audio band. The signal would be tiny. So we have components at 100dB down which is already inaudible even if you could hear to 60khz mixing to create another signal which would be, even with gross amp distortion, an order of magnitude lower again. Completely inaudible.

 

[March Audio]

I don't pretend to know if this is a real issue or not but the "harmonic distortion" I am talking about is lets say two tones one at 16 KHz and one at 17 KHz that create 3rd order Harmonic Distortion of 48 Khz and 51 KHz which was then reproduced by some "ribbon tweeters" (Which some apparently can reproduce sound up to 100K). These two ultrasonic tones could then produce a "beat frequency" of 3 KHz.... right in the middle of the audio range.

Again I am not sure but I don't think dismissing US harmonics as "nothing to be concerned with" is the right answer unless there is scientific evidence that this is the case..... maybe it's out there but I have not found it yet.... all I have found are reasons to be concerned. I welcome being educated.

It's the levels that you have to put into context.

The tones you mention at 48 and 50 kHz is already massively reduced. Remember dB is logarithmic. Say those tones are 70dB down, which is conservative, they then mix and produce another tone which is even further down.

Also you can't directly hear the 48 and 50 kHz IM.

Most tweeters simply don't have significant response at US frequencies anyway. I have said this before, as you can't hear US, and most high res recordings just have noise there anyway, there is nothing to be gained from US transducers / super tweeters, in fact you better off making sure they don't respond at US.

It's not a case of dismissing, it's a case of thinking through what's happening with understanding.

I will try and get time today to do some real acoustic measurements to demonstrate.

 

[levimax]

You are correct, heat would be the result, however in AC circuits you have what is called inductive and capacitive reactance. This adds to the resistance to oppose the current flow. Hence the term impedance.

Inductive reactance is caused by, you guessed it, inductors. A coil of wire (speaker coil) is an inductor. Inductive reactance is calculated by 2*pi*f*l. So increasing the frequency increases the overall impedance and reduces the current flow. It essentially becomes negligeable compared to the in band music power.

The Hypex Nc400 Amir tested showed That with a 20khz fundamental there were low level harmonics at - 70dB. This is already really small. You then add the effect of the inductive reactance. I can do some calculations to demonstrate later just how small the heating effect would be. This is also ignoring the effect of any speaker crossover filter that will reduce it further.

Regarding intermodulation distortion, well again it's a non issue. To create those beat frequencies you have actually have acoustic output in the ultrasonic range. If your speakers don't respond at those ultrasonic frequencies you can't generate intermodulation.

So again, with the NC400 example the harmonic distortion at 40 and 60 kHz is already 70 dB down. Let's say you speaker does have some output at those frequencies. Unlikely but for the sake of argument say its another 30dB down. Let's again say that the amp has distortion at 59khz as well so we generate a difference Intermodulation at 1kHz right in the audio band. The signal would be tiny. So we have components at 100dB down which is already inaudible even if you could hear to 60khz mixing to create another signal which would be, even with gross amp distortion, an order of magnitude lower again. Completely inaudible.

Thank you and what you say makes sense for the NC400 ..... I am not so sure about the Lyngdorf TDAI-3400 though.... -25 db at 50KHz seems like trouble... especially with an amp that can put out so much power. I am kind of surprised these early generation Class D amps aren't know as tweeter killers....

 

[March Audio]

Oh totally agreed, the lyngdorf is fundamentally flawed. The US noise is too high and too close to the audio band. We do need to look at what's going on in the ultrasonic region, it must not be ignored. Do so with an Fft which is informative. My issue is with using a simplistic wideband thd + n measurement and its implied correlation with audible distortion.

As with all things there will be good and bad amp designs. The problem is when they all get tarred with the same brush, ie class d is bad.

 

[levimax]

Ok, yes. It is very interesting, but such systems rely on the nonlinearity of air at high pressure. Therefore, they don't produce sound in the audio band unless the ultrasonic SPLs are sufficiently high.

To achieve the requisite pressure levels at which this nonlinearity begins to produce audibly loud difference frequencies in the audio band, an ultrasonic transducer must produce well over 100dB SPL at 1m.

This graph comes from an AES paper on ultrasonic transducers and shows the amplitude response of an ultrasonic transducer:

View attachment 22285

As you can see, the required ultrasonic SPLs are extremely high compared to the audio-band SPLs that are generated (an ultrasonic SPL of [email protected] produces audio band SPLs of around 50dB@4m).

I can't imagine a real-life situation where (a) the amplifier is producing enough output at ultrasonic frequencies and (b) the loudspeakers are reproducing those frequencies at sufficiently high SPLs to cause the air to become sufficiently nonlinear for audible difference frequencies to be generated.

This is an interesting track to go down though, so if I've missed something in your reasoning I'm happy to keep thinking about it

Thanks.... obviously Ultrasonic harmonics can't be that much of a problem as even amps like the Lyngdorf apparently sound OK. Still bugs me though to see all that added energy (which is certainly a form of distortion) and makes me hesitate to jump in and buy a Class D amp even though everything else looks good.

 

[andreasmaaan]

Thanks.... obviously Ultrasonic harmonics can't be that much of a problem as even amps like the Lyngdorf apparently sound OK. Still bugs me though to see all that added energy (which is certainly a form of distortion) and makes me hesitate to jump in and buy a Class D amp even though everything else looks good.

It's also been interesting to nut out whether/how this might be a problem. I'm still not 100% sure you're not onto something, but I just can't see it. Personally I have no real fear of class D amps - although I wouldn't buy the Lyngdorf obviously

 

[Ilkless]

I'm surprised they aren't selling that case alone for $600. Yowza.

The heatsinks in the shape of the company's name are very distinctive. Would love to have 4 or 6 of these on a rack driving an active setup.

Also, on the discussion about ultrasonics, there's a NC100HF dedicated for tweeter use. Anyone know anything about that? Rolls off below about 500Hz, but I wonder if there was any work done to it in the ultrasonics.