Class D amplifier - the real characteristics vs specification

[Ren Kitchener]

I know and love Class A and Class AB amplifiers, where I've always considered Class D to be inferior until now, but I still have a slight reservation that I may be wrong about.

You see the Class D chip manufactures claiming HiFi quality, with distortion figures close to 0.005%. This is true for 20 Hz to 3 kHz (making it past the 1 kHz reference), but for some reason, every chip or hybrid chip/discrete solution seems to have a sharp increase in distortion to around 0.02% to 0.05% at around 6 kHz, which sharply drops back at around 7 kHz. It's still very good, but it's not really 0.005%, 20 Hz to 20 kHz.
Some also see a distortion increase at 10 kHz (but the even and odd harmonics are inaudible at that frequency).

I have little idea why this should be (not being familiar enough with the physics behind PWM switching techniques).

So I wonder if every Class D has this strange anomaly, and it's not really ever published in the amplifier manufacturer's data? Or do they filter that band out? - Which would affect the response curve I guess.

Or, technology may have advanced so much, that the likes of AD, ST and Ti have not yet caught up?

 

[Kvn_Walker]

The Hypex amps all seem to measure incredibly well. I will be thrilled when the price reaches the point where NCores are more mainstream.

I'm not planning to replace the Parasound until it croak, but whenever that day comes I'll be looking for a Hypex-based replacement as long as it has the same features.

 

[Ren Kitchener]

The Hypex amps all seem to measure incredibly well. I will be thrilled when the price reaches the point where NCores are more mainstream.

I'm not planning to replace the Parasound until it croak, but whenever that day comes I'll be looking for a Hypex-based replacement as long as it has the same features.

That is actually really helpful - I think that you read between my lines perfectly - thanks Kvn_Walker

 

[Ren Kitchener]

The Hypex amps are pretty good Kvn_Walker - below is the spec from the NCAS500MP - again. there is this 'blip' at 6 or 7 kHz - which is seems to be typical for Class D technology, but in this case, the blip is still very, very good. I'm waiting to see if the price is 'realistic' enough.

 

[lovinthehd]

Class D "chips"?

 

[Ren Kitchener]

Class D "chips"?

Ah, yes 'lovinthehd' - many Class D amplifiers have chips or chip-sets (integrated circuits), instead of discrete components for the power amplification stage - and some have a chip driver, and discrete switching transistors - so as a consequence, they all will have a similar characteristic, albeit a different enclosure and brand name. I now understand that there are some class D amplifier manufactures that have developed their own discrete versions (that may still have some chips), or use an OEM system which Kvn_Walker has very kindly led me to - which is very exciting... the audio performance of these Class D amplifiers are very good indeed, having super high damping factors, and very low THD+N figures - even with the 'blip' at 7 kHz.

 

[lovinthehd]

Ah, yes 'lovinthehd' - many Class D amplifiers have chips or chip-sets (integrated circuits), instead of discrete components for the power amplification stage - and some have a chip driver, and discrete switching transistors - so as a consequence, they all will have a similar characteristic, albeit a different enclosure and brand name. I now understand that there are some class D amplifier manufactures that have developed their own discrete versions (that may still have some chips), or use an OEM system which Kvn_Walker has very kindly led me to - which is very exciting... the audio performance of these Class D amplifiers are very good indeed, having super high damping factors, and very low THD+N figures - even with the 'blip' at 7 kHz.

Class D doesn't necessarily use a "chip" nor generally are a replacement for the output stage. What 7khz blip?

 

[slipperybidness]

Ah, yes 'lovinthehd' - many Class D amplifiers have chips or chip-sets (integrated circuits), instead of discrete components for the power amplification stage - and some have a chip driver, and discrete switching transistors - so as a consequence, they all will have a similar characteristic, albeit a different enclosure and brand name. I now understand that there are some class D amplifier manufactures that have developed their own discrete versions (that may still have some chips), or use an OEM system which Kvn_Walker has very kindly led me to - which is very exciting... the audio performance of these Class D amplifiers are very good indeed, having super high damping factors, and very low THD+N figures - even with the 'blip' at 7 kHz.

Just for completeness, just because an amplifier is an integrated circuit, that does not make it class D by default. There are certainly class AB IC amps available.

 

[Ren Kitchener]

Class D doesn't necessarily use a "chip" nor generally are a replacement for the output stage. What 7khz blip?

I agree, but to use 100% discrete control and driver components at the nano-second type switching rise and fall times, requires a lot of additional design effort and cost (for little gain in the 'digital' domain). You can see the 'blip' I mentioned on the graph I previously posted in this thread. This seems to be typical with Class D, but I'm sure there may be someone that can prove me wrong (I'm not that familiar with Class D circuits).

 

[Ren Kitchener]

Just for completeness, just because an amplifier is an integrated circuit, that does not make it class D by default. There are certainly class AB IC amps available.

Quite right slipperybidnes - the Ti LM3886 is a rather good Class AB 'chip-amp' - used originally, I think, in the Gain-Clone amplifiers and many DIYer projects - in the right hands, they can achieve remarkable sound quality figures at, and over ,100 Watts.

 

[PENG]

I agree, but to use 100% discrete control and driver components at the nano-second type switching rise and fall times, requires a lot of additional design effort and cost (for little gain in the 'digital' domain). You can see the 'blip' I mentioned on the graph I previously posted in this thread. This seems to be typical with Class D, but I'm sure there may be someone that can prove me wrong (I'm not that familiar with Class D circuits).

I don't see that in the Purifi or Hyped amps. May be as you said it's the characteristics of a particular chip used in certain chip amps?

Review and Measurements of Hypex NC400 DIY Amp

This is a review and detailed measurements of a DIY power amplifier based on Hypex NC400 amplifier modules. It was built and is on kind loan from a member. I asked him to list the parts and this is what he supplied: $980 Amp modules, SMPS, internal cables (AC, DC, and shielded input) with...

www.audiosciencereview.com

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www.audiosciencereview.com

 

[lovinthehd]

I agree, but to use 100% discrete control and driver components at the nano-second type switching rise and fall times, requires a lot of additional design effort and cost (for little gain in the 'digital' domain). You can see the 'blip' I mentioned on the graph I previously posted in this thread. This seems to be typical with Class D, but I'm sure there may be someone that can prove me wrong (I'm not that familiar with Class D circuits).

Sorry, but you need to prove your point...you don't seem to have a clue.

 

[Ren Kitchener]

Sorry, but you need to prove your point...you don't seem to have a clue.

Here is the 'blip' I often see for class D amplifiers lovinthehd. This actual graph is typical, but not necessarily universal, and in this case, Kvn_Walker has fully answered my question (and more). Yes, there is a predicted THD rise in a specific frequency band for this very good brand, but in this case, it's insignificant. So, I'm going to try this brand instead of stressing over a class AB amplifier.

 

[ryanosaur]

Anybody can paste a graph. Where is that from? Why should we believe it?

 

[lovinthehd]

Here is the 'blip' I often see for class D amplifiers lovinthehd. This actual graph is typical, but not necessarily universal, and in this case, Kvn_Walker has fully answered my question (and more). Yes, there is a predicted THD rise in a specific frequency band for this very good brand, but in this case, it's insignificant. So, I'm going to try this brand instead of stressing over a class AB amplifier.

Sorry, cannot read the parameters, too blurry

 

[Ren Kitchener]

Sorry, cannot read the parameters, too blurry

If you came across in a more polite or respectful way, and was genuinely interested in my thread, then maybe I'll make the effort in getting you a better graph.

 

[PENG]

If you came across in a more polite way, and was genuinely interested in my thread, then maybe I'll make the effort in getting you a better graph.

I am sure he meant to be polite, he's a great guy always willing to help, probably get frustrated..

Did you see the curve in the review I linked, for the Hypex DIY one I think? It looks almost like a straight line, no inexplicable peaks in the 20-20,000 Hz range.

 

[Ren Kitchener]

I am sure he meant to be polite, he's a great guy always willing to help, probably get frustrated..

Did you see the curve in the review I linked, for the Hypex DIY one I think? It looks almost like a straight line, no inexplicable peaks in the 20-20,000 Hz range.

Yes I did, thank you PENG - In fact, I've been in touch with Hypex and I'm buying a module next week. The graph I posted above, is taken form the Hypex module data sheets - they all show the same characteristic peak at roughly the same frequency - but it's so low in distortion, it doesn't matter.

 

[lovinthehd]

If you came across in a more polite or respectful way, and was genuinely interested in my thread, then maybe I'll make the effort in getting you a better graph.

Just trying to understand your view/reasoning about class d amps....and the graph I cannot read....

 

[Ren Kitchener]

Just trying to understand your view/reasoning about class d amps....and the graph I cannot read....

Stating - "you don't seem to have a clue" - is not really 'trying to understand' my view about class D amplifiers. Nevertheless:
The Graph below shows this anomaly I first spoke about, in a different way, which may be more helpful.

This is taken from an Infineon chip at random, but all chips seem to have a similar anomaly. Being a chip, does not automatically make it worse or better than having discrete components (or a combination for higher power) - so the 'chip' term is not important - as you may well know, there are two typical ways to design a class D - one is PWM using a triangle wave and a comparator, and the other is free running without the triangle wave. There may be others, or some with added signal processing. I'll take a wild guess that some really high speed DSPs or MCUs (ARMs etc.) can do the same job simply by inputting an analogue signal into an 24 or 32 bit ADC, then outputting it as a PWM (PWM DAC), using it's internal timer, and some MOSFET Drivers.

So, many (not necessarily all) non-processor based class D configurations are going to have a similar characteristic.
The graph clearly shows that at 6 kHz, the distortion is remarkably higher. You could claim for <0.02% at 1 kHz, 1 to 20 Watts in the amplifier specification, whereas at 6 kHz, the distortion is close to 0.1% - this is a big difference. Seeing as the harmonics of a 6 kHz sine is at and beyond 12 kHz, for many of us, this is not that much of a problem. This is why I wasn't that keen on Class D, even if the specs claimed 0.001% THD at 1 kHz.

But now I have seen the Hypex range, with the help of this thread, my view is a little different - these do still have this increase in distortion at around 6 kHz/7 kHz [with an 8 Ohm load], but it's low enough not to worry about. So I'm going to get one of their products to try out. On the other hand, is this sudden increase in distortion indirectly noticeable or influential? - that's another question - I'll soon find this out for myself.