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Multichannel amps for active systems

oivavoi

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#21
Not necessarily a misconception, but probably out of date. Most of the issues with early class-D amplifiers have been resolved and the treble (and other) early growing pain issues have been (mostly) resolved. There are still cheap amps that have issue IME/IMO, but designs based on standard modules seem to be fine.
Thanks, Don. I have heard several systems with class d amps which have sounded very good to my years, so class d is clearly capable of creating good sound. For me this is in the same category as the difference between 0.05 and 0.001 of THD - maybe audible or maybe not, but nevertheless an issue of perfectionism and ultimate fidelity.

The things I worry about in that regard is what John Siau from Benchmark writes here:

"The important difference between the AHB2 and class-D amplifiers is that the AHB2 does not produce switching noise. Class-D amplifiers are measured with brick-wall AES17 filters that ignore the switching noise above 20 kHz or 40 kHz. Tweeters can demodulate this ultrasonic noise and fold it into the audible band.

Class-D amplifier switching noise can create problems in a loudspeaker measurement system. More importantly, this ultrasonic noise may become audible when it is demodulated by the non-linearities that occur in all loudspeakers. This potential source of distortion may detract from our musical enjoyment. Class-D amplifiers are compact and cheap, but are not necessarily a good choice for a high-end Hi-Fi system."
https://benchmarkmedia.com/blogs/ap...ory-use-of-the-benchmark-ahb2-power-amplifier

Is this only FUD and marketing fluff, or is there something to it? It's above my technical paygrade. But I do notice that some producers of active monitors still use class AB for the tweeters, and class D for the rest.
 
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#22
The things I worry about in that regard is what John Siau from Benchmark writes here:

"The important difference between the AHB2 and class-D amplifiers is that the AHB2 does not produce switching noise. Class-D amplifiers are measured with brick-wall AES17 filters that ignore the switching noise above 20 kHz or 40 kHz. Tweeters can demodulate this ultrasonic noise and fold it into the audible band.

Class-D amplifier switching noise can create problems in a loudspeaker measurement system. More importantly, this ultrasonic noise may become audible when it is demodulated by the non-linearities that occur in all loudspeakers. This potential source of distortion may detract from our musical enjoyment. Class-D amplifiers are compact and cheap, but are not necessarily a good choice for a high-end Hi-Fi system."
https://benchmarkmedia.com/blogs/ap...ory-use-of-the-benchmark-ahb2-power-amplifier

Is this only FUD and marketing fluff, or is there something to it? It's above my technical paygrade. But I do notice that some producers of active monitors still use class AB for the tweeters, and class D for the rest.
I think there is truth in everything that John Siau says here, so I don't think it is FUD or marketing fluff, but I do think the article needs to be read carefully. He says Class D - "can create problems" not "will create problems" - "may become audible" not "will become audible" and "potential source of distortion". I'm a big fan of Benchmark and their no nonsense approach to engineering (I still own a DAC1 HDR) and the AHB2 is undoubtedly a fine amplifier - it's also a cleverly written piece.
 

DonH56

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#23
Thanks, Don. I have heard several systems with class d amps which have sounded very good to my years, so class d is clearly capable of creating good sound. For me this is in the same category as the difference between 0.05 and 0.001 of THD - maybe audible or maybe not, but nevertheless an issue of perfectionism and ultimate fidelity.

The things I worry about in that regard is what John Siau from Benchmark writes here:

"The important difference between the AHB2 and class-D amplifiers is that the AHB2 does not produce switching noise. Class-D amplifiers are measured with brick-wall AES17 filters that ignore the switching noise above 20 kHz or 40 kHz. Tweeters can demodulate this ultrasonic noise and fold it into the audible band.

Class-D amplifier switching noise can create problems in a loudspeaker measurement system. More importantly, this ultrasonic noise may become audible when it is demodulated by the non-linearities that occur in all loudspeakers. This potential source of distortion may detract from our musical enjoyment. Class-D amplifiers are compact and cheap, but are not necessarily a good choice for a high-end Hi-Fi system."
https://benchmarkmedia.com/blogs/ap...ory-use-of-the-benchmark-ahb2-power-amplifier

Is this only FUD and marketing fluff, or is there something to it? It's above my technical paygrade. But I do notice that some producers of active monitors still use class AB for the tweeters, and class D for the rest.
Without measuring every class D amplifier I could not say how many have ultrasonic content and to what degree. Output filtering is one of the things that has improved over time and with better circuits and devices. I suspect that , like many claims, there is something to it for some amplifiers.

There are plenty of class-D amplifiers I would not consider "cheap" -- some run into tens of thousands of dollars -- and am inclined to doubt (but do not know for sure) they have damaging ultrasonic content.
 

BE718

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#24
The things I worry about in that regard is what John Siau from Benchmark writes here:

"The important difference between the AHB2 and class-D amplifiers is that the AHB2 does not produce switching noise. Class-D amplifiers are measured with brick-wall AES17 filters that ignore the switching noise above 20 kHz or 40 kHz. Tweeters can demodulate this ultrasonic noise and fold it into the audible band.

Class-D amplifier switching noise can create problems in a loudspeaker measurement system. More importantly, this ultrasonic noise may become audible when it is demodulated by the non-linearities that occur in all loudspeakers. This potential source of distortion may detract from our musical enjoyment. Class-D amplifiers are compact and cheap, but are not necessarily a good choice for a high-end Hi-Fi system."
https://benchmarkmedia.com/blogs/ap...ory-use-of-the-benchmark-ahb2-power-amplifier

Is this only FUD and marketing fluff, or is there something to it? It's above my technical paygrade. But I do notice that some producers of active monitors still use class AB for the tweeters, and class D for the rest.
I struggle with this as I cant see the mechanism. For example the switching frequency of Hypex amps is around 450kHz. A tweeter will have no response at this frequency so how does demodulation occur and fold into the audible band? It is effectively filtered. In a masurement system yes you need an input filter

I may well be missing something, Thoughts Don?
 

DonH56

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#25
Well, bearing in mind this is not my day job, I tend think it is based mostly on earlier designs with much lower switching frequencies. If the tweeter gets enough energy above its breakup area and yet low enough that it still vibrates then of course distortion is generated and can be modulated with other signals. Excess energy above what the tweeter can follow will be turned into heat, and of course can limit the dynamic range, but I am not sure (mean that literally) it is a problem these days. I am pretty sure amplifier designers are aware of the issue, and an amp that routinely destroys tweeters is unlikely to survive in the marketplace. That said, I have not read many measurements showing just how much ultrasonic energy is generated by modern class-D amplifiers.

Power devices (transistors) capable of delivering amps of current and one hundred volts or more whilst switching at 100 kHz ~ 1+ MHz were not readily available (if at all) ten or twenty years ago. Moving the switching frequency higher allows closing the feedback loop around the output filter, along with other tricks (old and now new again) like feedforward compensation, resolving both the rising output impedance (an issue with all amp topologies BTW) and HF stability issues that plagued (and in some cases really plagued!) early class D designs. A spur at 0.1% and 100 kHz is going to wreck THD measurements but is 60 dB down and thus 0.1 W (using 0.1% relative to FS power output) if the output is at 100 W. Not a large signal, though to be fair it is probably still around 0.1 W when the amp is only putting out 1 W, since it is coming from the switching and not the signal per se. I would like to see more measurements of modern class D amplifiers driving things like ESLs and other speakers that may look capacitive at HF. (Aside: ESLs are tricky; the panel is capacitive and impedance drops at HF, but the signal is driving through a transformer so the actual impedance is not usually a purely capacitive load though is low-Z at HF).

FWIWFM - Don
 

oivavoi

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#26
Thanks guys! I am not sure about this, and as I said it's clear to me that class d is more than capable of providing great sound these days. The question for me is whether it's capable of fidelity in the absolute sense in the highest register. I would also love to see some proper measurements on this - actual measurements of speakers being run full range by class d, compared with class ab - and see whether there is in fact a measurable difference in the output.

Here's a screen shot from the spec sheet of the new 1200As2 from Icepower for example. Can be had through Nord Acoustics as stereo amplifiers.



There is a spike in the intermodulation distortion around 18 and 19 khz. I have seen the same intermodulation spike in virtually all class d amps I've seen measurements for so far, including Hypex (not sure about Anaview though). I don't know why it's there, but I assume it's some residual from the ultrasonic noise? Now 18 khz and 19khz are usually outside the audible band for most of us. The question is whether this may affect frequencies further down, as John Siau seems to indicate. Perhaps not always, and perhaps it's not audible. But I've never understood why this seems to be overlooked when people tout the superior measurements of some class d amps (given that intermodulation distortion is usually regarded as more disturbing to the ear than THD).

Here's also a scientific conference paper about this issue: https://ieeexplore.ieee.org/abstract/document/7513652/ (available here )
According to the authors, intermodulation distortion remains within the audible range for class d amps which are based on pulse width modulation (which is most common), while it may fall below the audible range for class d amps which are based on pulse density modulation (I think anaview uses something like that).
 
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DonH56

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#27
You mean the IMD spurs around -100 dB down? From 1200 W output into 4 ohms? Not sure I'd worry too much about those... In any event the feedback drops off at higher frequencies for any amplifier as loop gain decreases and thus distortion suppression is reduced. That is a different problem than ultrasonic spikes you brought up earlier. For two tones f1 and f2, there will be second-order IMD products at LF (f2-f1) and HF (f1+f2). Third-order products land near the two input tones at (2f1-f2 and 2f2-f1) among other places (further away). The limited bandwidth of an amplifier means there is less loop gain and thus feedback to reduce HF distortion (again, for any amplifier -- audio, RF, microwave -- none have infinite bandwidth and infinite gain). Switching means additional HF energy that could modulate with the signals, of course, and switching outputs means you generally need more bandwidth and more filtering to keep the switching frequency from the output (the ultrasonic problem) and high enough to switch fast enough to generate higher-frequency output. I have often seen IMD rise with frequency because of that reduced loop gain at higher frequencies, be they class A or class D amplifiers. You should look at comparable class AB performance (same tones and power output) ti better judge how much a problem it is for class D relative to class AB amplifiers. It would also be worthwhile to look at output impedance, which rises with frequency for any amplifier but may rise faster for class D.

The paper shows IMD products and discusses various architectures. I have some issues with the terminology and methodology of the paper but don't have the time to read it thoroughly now and this isn't the thread to debate it anyway (I would normally respond to the authors directly but don't usually bother, and been years since I was an IEEE editor and that was JSSC not the consumer journal so generally at a lower level). There are trades in PDM and PWM with pros and cons of each. A lot of regulators (do not know as much about the latest audio power amplifiers) vary among PFM/PWM/PDM to maintain efficiency and minimize noise under varying load conditions.

I do not know how much better or worse a given amplifier will be than a comparable class D design but if you are concerned about class D then stick with class AB and end the second-guessing and "what-if" dilemma that may plague you (buyer's remorse).
 
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oivavoi

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#28
Thanks for your good and thorough reply, Don! Appreciated.

You are right, this is a detour in this thread on multichannel amps. And sticking with class AB for the treble in my active system may be a smart choice, just to stop obsessing about it!

(just one thing: Does that figure show IMD at 18 and 19 khz 100 db down...? Doesn't it show a baseline of IMD 120-140 db down, and then two spikes which are only 10 db down or something? Maybe I misread it completely. But you may be right that it would look the same for many AB amps. I'm out of my technical league here)
 

DonH56

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#29
The two spikes at 18 and 19 kHz are the input signals for the IMD test. They would normally be about -6 dB because when you take two sine waves and add them the result is peaks that are twice the amplitude (+6 dB) when both add in-phase. That is, take one signal, add another of the same amplitude, and the voltage doubles when they add "together".
 

oivavoi

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#30
The two spikes at 18 and 19 kHz are the input signals for the IMD test. They would normally be about -6 dB because when you take two sine waves and add them the result is peaks that are twice the amplitude (+6 dB) when both add in-phase. That is, take one signal, add another of the same amplitude, and the voltage doubles when they add "together".
Ah. Thanks! Then I had this exactly and very fundamentally backwards. I assumed that those were actual IMD spikes, not input. Thanks for enlighetening me! No, IMD 100 db down doesn't look like too much to worry about.
 

BE718

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#31
Ah. Thanks! Then I had this exactly and very fundamentally backwards. I assumed that those were actual IMD spikes, not input. Thanks for enlighetening me! No, IMD 100 db down doesn't look like too much to worry about.
I use hypex amps in my active design. Sounds fine, I wouldnt be concerned.

Hypex nc500 datasheet, measurements inside

https://www.hypex.nl/product/nc500-oem/50
 
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#34
Thanks for your good and thorough reply, Don! Appreciated.

You are right, this is a detour in this thread on multichannel amps.
No, not at all - it's great you raised this @oivavoi - I suspect a good many folks using active systems will be looking at multichannel class D because of their cost, size and SQ. And I don't think it is as simple as sticking a bunch of OEM modules in a case without thinking through layout etc.

I started out using mixed class A, class AB and tube gear in active systems - too messy, different gains, tonally different - much multichannel gear at that time was looked down on by the audiophile community as only suitable for the home cinema market. There's plenty of good class AB and class D multichannel amps out there now, perfectly capable of good SQ for audio use. All IMO, of course.
 

DonH56

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#35
Layout and power supply are arguably more important in class D designs due to the wider frequency range of switching harmonics and because noise on the voltage rails can be directly transmitted to the output. EMI can also be a bigger issue, of course. Moving the switching frequency well above the audio band means power supply fundamentals are no longer the problem they are for linear amplifiers but they will tend to find anyplace they can sneak back in to be rectified/mixed back into the audio band by other circuits. And of course all the usual issues that can happen with high-gain circuits do not go away (ground loops, stability, noise, etc.) Starting with pre-built and qualified modules makes life a lot easier (though takes some of the fun away for those who want to do everything themselves).
 
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#36
I finally got my Apollon NC800SL delivered today !
First of all, my impression to this amplifier is just like arts! The build quality is extraordinary, I choose all black aluminum and feet, it just looks cool and high end.
Then I quickly plug in everything and connect to my speaker. When I turn on the sound, wow, that's like I am buying another system, everything is so bright. The most obvious difference is the deeper bass, and very neutral sound coming from the speaker . I heard that this amp needs some burn in time .I will definitely wait and see how much it can improve further.
I am looking forward to give you guys an update afterwards.
 
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