How true? Class D amps can't sustain their maximum spec'd power

[levimax]

There is no "need" they also work with linear power supplys. But they like regulated voltage.
If you like power without heat( efficience), you go the way with a switching PSU, thats why you see them usually as pairs.

When I see Amir's reviews of class D amps without much "peak headroom" I wonder if it is a disadvantage. When I DIY a linear supply for an AB amp I usually go a little high on the transformer voltage but keep VA down and use some large caps. My theory is this gives me higher peak output for transients without having to worry about burning up the amp. Class D seems like the opposite.... high power but not a lot of headroom for peaks. For music it seems like having some headroom for peaks is more important that continuous power but I do not have any proof of that.

 

[Pdxwayne]

Without knowing anything about your speakers, your room, your listening distance or the nature of dark energy, I can say there is a reason 80hz is the more typical crossover to subs than 50hz. At 50 you are probably at or near the limit of your speakers' best qualities. And a 50hz (or any) crossover is not instant. It doesn't suddenly flip from main speakers to sub. So you're asking your main/satellites to play well below 50hz with diminishing volume. I cross mine at 100hz though my speakers are supposed to play down to 50 without much distortion (at my listening levels, position etc).

You may also have a speaker boundary interference thing going on between your subs & main speakers (search this site for speaker boundary interference). That will vary with position and frequency. But crossing higher will often spare you that.

I guess most likely is speakers related. My surround is KEF LS50, not that easy to drive anyway....

 

[tomtoo]

When I see Amir's reviews of class D amps without much "peak headroom" I wonder if it is a disadvantage. When I DIY a linear supply for an AB amp I usually go a little high on the transformer voltage but keep VA down and use some large caps. My theory is this gives me higher peak output for transients without having to worry about burning up the amp. Class D seems like the opposite.... high power but not a lot of headroom for peaks. For music it seems like having some headroom for peaks is more important that continuous power but I do not have any proof of that.

Thats a good question, with also a lot of points of view.
Wish i would know.

 

[abdo123]

if Class D has thermal constraints then Class A/B is an erupting volcano.

 

[Koeitje]

When I see Amir's reviews of class D amps without much "peak headroom" I wonder if it is a disadvantage. When I DIY a linear supply for an AB amp I usually go a little high on the transformer voltage but keep VA down and use some large caps. My theory is this gives me higher peak output for transients without having to worry about burning up the amp. Class D seems like the opposite.... high power but not a lot of headroom for peaks. For music it seems like having some headroom for peaks is more important that continuous power but I do not have any proof of that.

If class D already hits the headroom numbers of your overdesigned class A/B...what point is it to argue over it.

 

[levimax]

If class D already hits the headroom numbers of your overdesigned class A/B...what point is it to argue over it.

No argument just trying to understand how the advertised numbers relate to real world performance. From my understanding it sounds like a 400 watt Class D is not really comparable to a 400 watt Class AB as the class AB will have more headroom for transient peaks so more like a 400 watt Class D is comparable to a 275 watt Class AB for playing music.

 

[mocenigo]

Yes, I know. It was just my spelling mistake.

I wonder whether this can be changed. Maybe ask the admins?

 

[MakeMineVinyl]

Slightly off topic but related. From reading Amir's reviews it seems like most class D amps use regulated power supplies which limits their "peak" output compared to AB amps using linear supplies. Is there a reason class D amps need regulated supplies?

Class D amplifiers don't "need" a regulated supply and in fact many of our class D amplifiers use linear supplies. Yes a linear supply can provide higher peak output but I don't think this is a major reason for using one.

 

[DonH56]

No argument just trying to understand how the advertised numbers relate to real world performance. From my understanding it sounds like a 400 watt Class D is not really comparable to a 400 watt Class AB as the class AB will have more headroom for transient peaks so more like a 400 watt Class D is comparable to a 275 watt Class AB for playing music.

No. The AB amplifier with unregulated rails may allow peaks over the 400W specification, but that is not the same as under-rating the class D amplifier. Assuming the specs are valid, both are 400 W amplifiers. The headroom is a function of power supply, biasing, and other design choices in amplifier design. Some class amplifiers (AB, D, whatever) with very "stiff" supplies have almost no headroom, while others may provide a dB or two (more for some). The extra headroom only matters if you actually need it, of course. Most people would be shocked at their average power requirements.

It is also partly what specs the manufacturers choose to market. With feedback the distortion knee is very sharp, so choosing to spec at 0.001% instead of 0.1% can result in a significant (huge) power difference. Similarly a manufacturer may choose to rate an amplifier to "double down", providing twice the power into a 4 ohm (or lower) load, with the result that the 8-ohm rating is less than it would otherwise be.

Thermal capacity is often cited and that is a problem for many amplifiers. The old "1/3-power for one hour" requirement was reduced to 1/8 power on the theory it better represented the peak-to-average signal content of the sources. Many an amp got very, very hot under the 1/3-power requirement, with some shutting down before it finished (read through old reviews). Whenever discussing maximum power output it is reasonable to ask "for how long?" and be aware of how long transient peaks last in the real world. An amplifier rated at 400 W and meeting the 1/8-power conditioning requirement may not be able to deliver 400 W for a minute, no matter the class of operation. A class D amp will dissipate (waste) far less power as heat, but may also be in a much smaller box with less ability to dissipate the heat. But at some 90% efficiency that's only 40 W, not a huge amount, but not trivial. A class AB amp at around 60% efficiency will burn an extra 166 W or so, more than four times the heat that must be radiated without the amp shutting down. Which amp will last longer? Can't say without knowing a lot more about the amps' thermal management...

There are so many variables, engineering and marketing, that I think sweeping statements and conclusions are bound to be riddled with exceptions. I think the best thing to do is to get a decent estimate of the power you actually need, decide how much extra you'd like (and are willing to pay for), then shop for an amp (whatever the class of operation) that meets your needs. You can start with an online calculator like this one: Peak SPL Calculator

IMO - Don

 

[Geert]

Class D amps were already popular in PA 25 years before they found their way into the living room. Such's amps are cable of delivering multiples of the power the best consumer amps can deliver, and it's not unusual to drive them for 12 hours at max level. Often with multiple speakers in parallel, resulting in impedances below 2 ohm. So what's the problem with class D?

 

[Koeitje]

No argument just trying to understand how the advertised numbers relate to real world performance. From my understanding it sounds like a 400 watt Class D is not really comparable to a 400 watt Class AB as the class AB will have more headroom for transient peaks so more like a 400 watt Class D is comparable to a 275 watt Class AB for playing music.

I am talking about 400W peak vs 400W peak. Both will just do 400W without issue.

 

[MakeMineVinyl]

No. The AB amplifier with unregulated rails may allow peaks over the 400W specification, but that is not the same as under-rating the class D amplifier. Assuming the specs are valid, both are 400 W amplifiers. The headroom is a function of power supply, biasing, and other design choices in amplifier design. Some class amplifiers (AB, D, whatever) with very "stiff" supplies have almost no headroom, while others may provide a dB or two (more for some). The extra headroom only matters if you actually need it, of course. Most people would be shocked at their average power requirements.

It is also partly what specs the manufacturers choose to market. With feedback the distortion knee is very sharp, so choosing to spec at 0.001% instead of 0.1% can result in a significant (huge) power difference. Similarly a manufacturer may choose to rate an amplifier to "double down", providing twice the power into a 4 ohm (or lower) load, with the result that the 8-ohm rating is less than it would otherwise be.

Thermal capacity is often cited and that is a problem for many amplifiers. The old "1/3-power for one hour" requirement was reduced to 1/8 power on the theory it better represented the peak-to-average signal content of the sources. Many an amp got very, very hot under the 1/3-power requirement, with some shutting down before it finished (read through old reviews). Whenever discussing maximum power output it is reasonable to ask "for how long?" and be aware of how long transient peaks last in the real world. An amplifier rated at 400 W and meeting the 1/8-power conditioning requirement may not be able to deliver 400 W for a minute, no matter the class of operation. A class D amp will dissipate (waste) far less power as heat, but may also be in a much smaller box with less ability to dissipate the heat. But at some 90% efficiency that's only 40 W, not a huge amount, but not trivial. A class AB amp at around 60% efficiency will burn an extra 166 W or so, more than four times the heat that must be radiated without the amp shutting down. Which amp will last longer? Can't say without knowing a lot more about the amps' thermal management...

There are so many variables, engineering and marketing, that I think sweeping statements and conclusions are bound to be riddled with exceptions. I think the best thing to do is to get a decent estimate of the power you actually need, decide how much extra you'd like (and are willing to pay for), then shop for an amp (whatever the class of operation) that meets your needs. You can start with an online calculator like this one: Peak SPL Calculator

IMO - Don

Thermal management is a huge concern with multi-channel amplifiers. Realistically no class A/B multi-channel amplifier could pass a 1/3rd power pre conditioning test for 1 hour - it just ain't gonna happen. Yes, such an amplifier could be made which would pass this threshold, but in the real world, who in their right mind would be willing to pay the gigantic cost penalty in much larger heatsinks, larger chassis to accommodate those heatsinks, the shipping of the much larger, heavier amplifier, and the chiropractor bills of the end user lugging the beast into a rack.

Stereo amplifiers have an easier time with a 1/3rd power test since the thermal density is so much less.

Multi-channel class D amplifiers aren't off the hook either! Multiply the heat dissipation by 7 or 8 (for a 7 or 8 channel amplifier) and even the 10% inefficiency of a class D amplifier's overall heat starts to add up. It is for this reason that we use heatsinks even on class D amplifiers and don't just bolt them to the bottom of the chassis and rely on chassis dissipation.

The typical thermal cutoff is 85º C, which is too hot to touch. Some amplifiers use a bi-metal switch to cut power at this temperature and some use thermal ICs, the output of which is monitored by a microcontroller. Heat buildup is also a major concern where power amplifiers are stuffed in a rack or shelf somewhere where there is little ventilation.

 

[oupee]

I have an excellent Class B 100W / 4Ω 1kHz / 1% amplifier and Ncore NC250 monoblocks (Audiophonics). I don't hear any difference in strength with nCore, and on the contrary, I lack a physical feeling of bass for some songs. The heights at nCore are generally weaker, but I don't want to deal with it so that the battle doesn't happen again. Interestingly, a potentially stronger amplifier strikes me as weaker. Both measure frequency straight. It has a different input sensitivity with precisely aligned relay volume control and the preamplifier has an output of 8V eff. Due to the sensitivity of the speakers (91db) and the relatively small size of the room, there can be no question of distortion or limitation.

 

[MaxBuck]

Its temperature limited. In practice it doesn't matter, because no content uses full power all the time.

Exactly this. I don't understand the fascination with "maximum power continuously." Even if one had an amplifier capable of that, you'd either have to flee the room because of the heat produced, or take a second job to pay the electric bill. Or both.

 

[AdamG]

Yes, I know. It was just my spelling mistake.

You can send a PM directly to Amir @amirm and request the name change due to spelling error. As long as the new name is not already taken, and from a quick check it is not, he will be happy to make the change for you!

 

[HighImpactAV]

This video by QSC does a good jobof explaining peak vs continuous power in Class D.

 

[mocenigo]

You can send a PM directly to Amir @amirm and request the name change due to spelling error. As long as the new name is not already taken, and from a quick check it is not, he will be happy to make the change for you!

Seems to have happened. Now I do no longer cringe when I see it (I am Italian after all...)

 

[MaxBuck]

So glad we have our cheeses properly characterized now.

 

[mocenigo]

No argument just trying to understand how the advertised numbers relate to real world performance. From my understanding it sounds like a 400 watt Class D is not really comparable to a 400 watt Class AB as the class AB will have more headroom for transient peaks so more like a 400 watt Class D is comparable to a 275 watt Class AB for playing music.

I always got the impression that the power of Class D amplifiers tends to be measured with some more generosity than other typologies.

 

[MakeMineVinyl]

I always got the impression that the power of Class D amplifiers tends to be measured with some more generosity than other typologies.

In any amplifier, if a switching supply and a linear supply both have exactly the same output voltage under no-signal conditions, the linear supply will sag to a lower voltage than the switching supply under heavy load. How much the linear supply will sag has to do with the amount of capacitance on its output. So in this instance, the linear supply is at a disadvantage and will supply fewer watts under sustained heavy output than the switching supply.

If the linear supply has a higher output voltage than the switching supply but sags to the same voltage as the switching supply under sustained heavy load conditions, then the linear supply will have the ability to deliver higher output wattage for a short period (in other words transient output), and will have the same ability to output sustained wattage as the switching supply. This is typically how a linear supply is implemented (at least in our amplifiers) so that there is transient ability above what a switching supply can offer.