Let's stop calling Class D amplifiers "efficient" when they are not.

[KMM]

Restorer-John or others familiar with Hypex board layout and failure points,

Would a dual mono Hypex NC250mp installed in a more traditional sized case reduce heat density enough to improve longevity? Reasoning being total power draw is spread across 2 boards. And maybe just as importantly, you have only one channel idle loss per board. Not sure if this helps the output filter heat issue and adjacent component heat. Granted the single mono 250 boards are slightly smaller that the 252's but not greatly so.

Thinking something like this: https://www.audiophonics.fr/en/powe...tereo-ncore-nc250mp-2x250w-4-ohm-p-13545.html

Otherwise, the option is a Purifi in a similar case but I don't need the power of even the smaller Purifi and it is approaching twice the price. I am sure the Purifi is technically better and due to better idle heat may still have the edge for reliability. I guess the real question is, will the setup mentioned above in a meaningful way mitigate the 252 series weak points or is it better to just bite the bullet and go all in for the Purifi even though any performance gains are not likely to be audible unless of course the Hypex 250 fails.

I have exactly that one driving two Canton 896.2 full size speakers. They barely get warm to the touch after hours of playing at moderate levels. Great full size amp with no noticeable thermal issues bcs there is practically nothing inside.

 

[Rick Sykora]

Restorer-John or others familiar with Hypex board layout and failure points,

Would a dual mono Hypex NC250mp installed in a more traditional sized case reduce heat density enough to improve longevity? Reasoning being total power draw is spread across 2 boards. And maybe just as importantly, you have only one channel idle loss per board. Not sure if this helps the output filter heat issue and adjacent component heat. Granted the single mono 250 boards are slightly smaller that the 252's but not greatly so.

Thinking something like this: https://www.audiophonics.fr/en/powe...tereo-ncore-nc250mp-2x250w-4-ohm-p-13545.html

Otherwise, the option is a Purifi in a similar case but I don't need the power of even the smaller Purifi and it is approaching twice the price. I am sure the Purifi is technically better and due to better idle heat may still have the edge for reliability. I guess the real question is, will the setup mentioned above in a meaningful way mitigate the 252 series weak points or is it better to just bite the bullet and go all in for the Purifi even though any performance gains are not likely to be audible unless of course the Hypex 250 fails.

I have a couple of NC250MP monoblocks and am quite pleased with them. I suspect this Audiophonics amp is just fine as well.

If you have concerns over the amp's thermal management, should really ask the vendor. They are the designers and anyone here is at best doing an educated guess at the thermal design. Also is a good test of whether they are serious about design or not.

 

[ahofer]

If you have concerns over the amp's thermal management, should really ask the vendor.

Agree, but measurements are always good. Trust, but verify, so to speak.

 

[Rick Sykora]

Agree, but measurements are always good. Trust, but verify, so to speak.

Ok, but if the vendor did the design correctly, he tested or had someone do.

I doubt many members can truly test thermals. Idle current draw is interesting but is only small part of the picture. Notably if you are looking for hot spots that might impact component longevity, you need some serious test rig.

 

[IPunchCholla]

I’m curious about the ultrasonic issue. Is there a reason one couldn’t put the inductors in the filter before the amplifier, thus at least not amplifying them?

 

[fpitas]

I’m curious about the ultrasonic issue. Is there a reason one couldn’t put the inductors in the filter before the amplifier, thus at least not amplifying them?

I think the filter referred to is specifically to reduce the amplitude of the switching waveform from the output devices.

 

[IPunchCholla]

I think the filter referred to is specifically to reduce the amplitude of the switching waveform from the output devices.

Ahh, I was thinking it was passthrough from the PSU.

 

[IAtaman]

I’m curious about the ultrasonic issue. Is there a reason one couldn’t put the inductors in the filter before the amplifier, thus at least not amplifying them?

Are we talking about Class D? If yes, HF is produced by the switching itself. Here is a simplified block diagram and link to Wikipedia to help with basic understanding. If no, and I am stating the obvious, I apologize, then I am not sure which ultrasonic issue and filters we are talking about.

 

[IPunchCholla]

Are we talking about Class D? If yes, HF is produced by the switching itself. Here is a simplified block diagram and link to Wikipedia to help with basic understanding. If no, and I am stating the obvious, I apologize, then I am not sure which ultrasonic issue and filters we are talking about.

Nope you’re good, and thank you! I was having an extended brain fart from switching between topologies and power supply designs as I learn all this stuff.

 

[DonH56]

Here is Hypex's efficiency chart for the NC252MP, the amps in my (Buckeye) class-D amplifier. Notice that efficiency does not exceed 50% until 10 W or so. The Hypex datasheet lists 3.5 W/ch (7 W/module) for the amplifier plus whatever the power supply adds so this seems reasonable. Thus, with respect to @restorer-john's argument, these are probably no better than typical class AB amplifiers at low power levels. And, like class AB amps, if you have higher-power modules the efficiency will be low(er) until even higher power levels. It rises fairly quickly above the idle power knee, exceeding even class B by perhaps 25 W, so at higher power levels is clearly more efficient. But, at low power levels likely in most systems, class D does not offer a lot of benefit efficiency-wise IMO. All system- and listener (volume)-dependent, as usual. It does mean less heat-sinking and thermal management is needed at high output levels, but "less" is not "none".

There is an ASR post about basic class-D operation: https://www.audiosciencereview.com/forum/index.php?threads/class-d-amplifiers-101.7355/ (and probably several more).

 

[AnalogSteph]

As you can see, the NC252MP reaches the maximum theoretical peak efficiency of an AB amplifier (78.5%) at 75ish W into 4 ohms. It's still close to 70% at half that, and still over 60% at 20 W (which is 8% of max output). And it's not too much worse with 2 ohms.

I had a look for a graph for AB amp efficiency vs. output and found this:

https://www.researchgate.net/figure/Dependence-of-the-output-power-on-the-efficiency-for-the-Class-AB-power-amplifier_fig3_371327483

By about 8% of maximum output, you're down to little over 20% of efficiency.

There is absolutely no doubt in my mind that Class D amps have much better efficiency than their AB counterparts (as you would expect), with idle power alone being about a factor of 4 or so lower. Making them multiple times more powerful is obviously eating up a good part of that advantage, but does not fundamentally negate it. And there may still be more to be gained on the power supply front at idle. Looking at the above, a pair of modules would seem to be drawing about 10 W on the DC side, and there is no reason why this should have to be more than 15-20 W AC. Are typical Class D power supplies even using resonant LLC topology?

In the past relatively few people would have considered going for a power amplifier that can deliver 200 W into 8 ohms or 250 W into 4 ohms, and you would have been looking at something with at least an 80 W idle power draw. There's a reason why PA amplifiers in particular went Class G decades ago.

If you want a "traditional" amplifier that is competitive with Class D, it needs to be at least Class G, or better Class H altogether - at which point you are basically using a Class D amplifier to drive the supply rails. That brings with it a set of challenges, of course.

Now the fun part - could you apply the same techniques to a Class D? Maybe? I could certainly see something like a tiered power supply working where the high-power part only kicks in when needed.

 

[IAtaman]

Now the fun part - could you apply the same techniques to a Class D? Maybe? I could certainly see something like a tiered power supply working where the high-power part only kicks in when needed.

Is there such a thing as a rail switching class D amp?

Thank you for that - I was not sure whether I was onto something or beind delusional. Do you know of any actual examples / implementations?

 

[Blumlein 88]

Thank you for that - I was not sure whether I was onto something or beind delusional. Do you know of any actual examples / implementations?

Does it make sense to complicate the design in that area for just a few watts benefit?

 

[IAtaman]

Does it make sense to complicate the design in that area for just a few watts benefit?

I am not sure. Depends on what is the objective I guess?

I was trying to see how much efficiency class G offers over AB, and came across this page fom AnalogDecives. Sounds like they had a rail swithcing Class D amp. MAX98308 chip that offer the functionality is obsolete now so maybe that insinuates it does not make sense. I think it still might though.

Class DG Amplifiers​

The Class DG amplifier uses PWM to produce a rail-to-rail digital output signal with a variable duty cycle. In this respect, a Class DG amp is the same as a Class D amp. The Class DG amp, however, also uses a multilevel output stage to sense the magnitude of the output signal (Figure 6). It then switches the supply rails, as needed, to supply the required signal power more efficiently. A Class DG amp, such as the MAX98308, uses the same dual-power concept as a switching Class D topology for even higher efficiency. For other examples, see Maxim’s Class DG amps.

https://www.analog.com/en/technical-articles/types-of-audio-amplifiers.html

 

[Blumlein 88]

I am not sure. Depends on what is the objective I guess?

I was trying to see how much efficiency class G offers over AB, and came across this page fom AnalogDecives. Sounds like they had a rail swithcing Class D amp. MAX98308 chip that offer the functionality is obsolete now so maybe that insinuates it does not make sense. I think it still might though.

Class DG Amplifiers​

The Class DG amplifier uses PWM to produce a rail-to-rail digital output signal with a variable duty cycle. In this respect, a Class DG amp is the same as a Class D amp. The Class DG amp, however, also uses a multilevel output stage to sense the magnitude of the output signal (Figure 6). It then switches the supply rails, as needed, to supply the required signal power more efficiently. A Class DG amp, such as the MAX98308, uses the same dual-power concept as a switching Class D topology for even higher efficiency. For other examples, see Maxim’s Class DG amps.

https://www.analog.com/en/technical-articles/types-of-audio-amplifiers.html

The Tact amps I have actually in effect did the digital conversion in the output stage by varying the output voltage levels. Maybe I should check the quiescent power draw of those. Wadia made an amp that also did this.

 

[blueone]

Does it make sense to complicate the design in that area for just a few watts benefit?

IMO, only if it benefits product longevity, but that seems better attained with adequate ventilation and heat sinking.

 

[Chr1]

Am I ok powering on a class H without a speaker load connected?
Curious to check it's idle draw but obviously don't want to risk potential damage...

 

[DonH56]

As you can see, the NC252MP reaches the maximum theoretical peak efficiency of an AB amplifier (78.5%) at 75ish W into 4 ohms. It's still close to 70% at half that, and still over 60% at 20 W (which is 8% of max output). And it's not too much worse with 2 ohms.

I had a look for a graph for AB amp efficiency vs. output and found this:

https://www.researchgate.net/figure/Dependence-of-the-output-power-on-the-efficiency-for-the-Class-AB-power-amplifier_fig3_371327483

By about 8% of maximum output, you're down to little over 20% of efficiency.

There is absolutely no doubt in my mind that Class D amps have much better efficiency than their AB counterparts (as you would expect), with idle power alone being about a factor of 4 or so lower. Making them multiple times more powerful is obviously eating up a good part of that advantage, but does not fundamentally negate it. And there may still be more to be gained on the power supply front at idle. Looking at the above, a pair of modules would seem to be drawing about 10 W on the DC side, and there is no reason why this should have to be more than 15-20 W AC. Are typical Class D power supplies even using resonant LLC topology?

In the past relatively few people would have considered going for a power amplifier that can deliver 200 W into 8 ohms or 250 W into 4 ohms, and you would have been looking at something with at least an 80 W idle power draw. There's a reason why PA amplifiers in particular went Class G decades ago.

If you want a "traditional" amplifier that is competitive with Class D, it needs to be at least Class G, or better Class H altogether - at which point you are basically using a Class D amplifier to drive the supply rails. That brings with it a set of challenges, of course.

Now the fun part - could you apply the same techniques to a Class D? Maybe? I could certainly see something like a tiered power supply working where the high-power part only kicks in when needed.

My old notes list about 70% as the maximum efficiency for a class B amplifier, so an AB should be a little below below that. Class A is something like 27% for single-ended and 50% for push-pull designs. The analysis I have (hand written notes from a grad class long ago) puts class B push-pull around 67% max. Around 78% max is what I have seen for class C. Not saying you (or the references) are wrong, just that's what I have based upon grad-school and later work (real-world) analyses. That is for the output stages; biasing, driver stages, etc. add power and reduce efficiency, natch.

There have been designs for both class G and H using a class-D core amplifier but I am not familiar with them (and too lazy to go research them). I piddled with one for RF but it was not practical at the time. The ones I have seen have targeted low-power applications; I do not know (literally) if there are any audio power amps using switched (G) or tracking (H) power rails and a class D core. Keeping high performance and stability whilst adjusting the rails of a class-D amp gets tricky, but with enough gain-bandwidth and feedback I don't see why it could not be done -- at least theoretically.

Benchmark's AHB-2 is class G IIRC, as are amplifiers from Emotiva and probably others.

 

[DonH56]

Am I ok powering on a class H without a speaker load connected?
Curious to check it's idle draw but obviously don't want to risk potential damage...

Class H generally uses a class AB core amplifier so should be fine. You could always check with the manufacturer to be sure. Tube amps usually do not like to run unloaded and can be seriously damaged. SS amps are usually OK, though there have been (and probably are, and will be) those rare ones that oscillate out of control when unloaded.

If you just want to measure idle draw, all you need do is turn it on and measure with the volume set to zero, no need to disconnect anything.

 

[Chr1]

Thanks. Yes, a quick search suggests it's ok. The amps in storage, so just want to quickly check it's idle draw with minimal faff. Will check tomorrow when I get home.