Class D FAQ

[antcollinet]

unfortunately the science of evaluating amplifiers strictly from measurements and what my ears tell me are often two different things.
many things come into play by the time an amplified audio signal reaches one's ears, not the least of which is the electrical interaction with the loudspeaker's impedance curve and the resulting performance of the speaker.

This is where the measurement of that thing called damping factor (or output impedance) comes into play. Get output impedance low enough (high damping factor) and there is almost no interaction with the speakers impedance curve.

Pretty much all solid state amps (other than some "boutique" "no feedback" designs) have a low enough output impedance for all but the most exreme speakers.

Interestingly this is where the good class D designs (think hypex/purify) tend to score particularly well.

 

[mhardy6647]

On the other hand, extreme loudspeakers (in terms of their impedance and phase curves) have become ever more fashionable, for reasons that completely escape me, over the past two or three decades.

 

[antcollinet]

On the other hand, extreme loudspeakers (in terms of their impedance and phase curves) have become ever more fashionable, for reasons that completely escape me, over the past two or three decades.

Truedat. All the more reason for an ultra low impedance class D amp (as long as it has plenty of headroom)

 

[EJ3]

Truedat. All the more reason for an ultra low impedance class D amp (as long as it has plenty of headroom)

And unfortunately, headroom is an area in which most class D amps have an issue. Take a look at the "headroom" that AMIRM got on one of my NAD 2200's in the NAD 2200 Vintage Amp Review. Incidentally, the spec listed by NAD for the 2200 for Dampening Factor is >100.. Well, we don't know exactly what that means (and I have seem it stated >200 on the later versions (of which mine are), so who knows. But, if it were what was considered to be too low, they would likely have not listed it at all.
I also believe that both "Dampening Factor" and. "Slew Rate" should be part of the specifications. IMO, using these, in conjunction with the other things that we check, can give us a better overall view of an amp (and likely some other devices we use in our audio systems).

 

[Matias]

And unfortunately, headroom is an area in which most class D amps have an issue. Take a look at the "headroom" that AMIRM got on one of my NAD 2200's in the NAD 2200 Vintage Amp Review. Incidentally, the spec listed by NAD for the 2200 for Dampening Factor is >100.. Well, we don't know exactly what that means (and I have seem it stated >200 on the later versions (of which mine are), so who knows. But, if it were what was considered to be too low, they would likely have not listed it at all.
I also believe that both "Dampening Factor" and. "Slew Rate" should be part of the specifications. IMO, using these, in conjunction with the other things that we check, can give us a better overall view of an amp (and likely some other devices we use in our audio systems).

That is why I sound like a broken record and always recommend the NC502MP as low as 750 usd from Buckeye, and while it has high SINAD and 500W in 4 ohms by the specs, Amir measures 600W in 4 ohms. This can drive pretty much all floorstanding speakers in the market, low impedance and variable phase included.

 

[EJ3]

That is why I sound like a broken record and always recommend the NC502MP as low as 750 usd from Buckeye, and while it has high SINAD and 500W in 4 ohms by the specs, Amir measures 600W in 4 ohms. This can drive pretty much all floorstanding speakers in the market, low impedance and variable phase included.

One of the few (maybe the one & only) that have the headroom that we look for. And, while I haven't heard it, it is the one CLASS D that, if I where in the AMP buying mode right now, I would absolutely make sure to find a way to audition it.

 

[antcollinet]

And unfortunately, headroom is an area in which most class D amps have an issue. Take a look at the "headroom" that AMIRM got on one of my NAD 2200's in the NAD 2200 Vintage Amp Review. Incidentally, the spec listed by NAD for the 2200 for Dampening Factor is >100.. Well, we don't know exactly what that means (and I have seem it stated >200 on the later versions (of which mine are), so who knows. But, if it were what was considered to be too low, they would likely have not listed it at all.
I also believe that both "Dampening Factor" and. "Slew Rate" should be part of the specifications. IMO, using these, in conjunction with the other things that we check, can give us a better overall view of an amp (and likely some other devices we use in our audio systems).

Headroom (even in class D) is something you can purchase in the form of a higher power rating than you need.

 

[DonH56]

Amplifier headroom to me is the short-term power available above the long-term specification. I do not consider a low number good or bad, just something to include when making a purchase decision. Conventional amplifiers are all over the map. Some amplifiers specified up to 6 dB (4x) of headroom, while others essentially 0 dB. The former had a very "weak" power supply or insufficient thermal management to sustain that power for a long time. The latter typically had very "stiff" or regulated power supplies and thermal management (e.g. heat sinks, fans) to sustain the power long-term. Then you decide based upon your budget, listening habits, speakers, etc. whether you want a 100 W amplifier with 6 dB of headroom or a 400 W amp with 0 dB headroom. Neither amp is good or bad, and the choice is not right or wrong, just pick the one most appropriate for your system and meeting your other parameters (like cost, size, weight, etc.)

Of course, manufacturers may choose to rate to emphasis different parameters, and that can affect headroom as well. Derating the 8-ohm power to achieve a doubling in power at 4 ohms, often deemed desirable by audiophiles, can lead to greater headroom. The exact same amp could be rated for more power at 8 ohms and thus have lower headroom. Similarly, a manufacturer may want to specify very low distortion at rated power, leading to greater headroom than if they chose to specify higher distortion and (higher) rated power. All sorts of variables.

Class D output stages switch between power rails so a tightly-regulated supply reduces the chance of amplitude modulation with pulse width (distortion). High sampling rates and feedback control that issue, but the way they work headroom is not usually a desirable trait in a class D amplifier. Arguably not in any amplifier, but there are trades to be had, though by and large class D amps achieve greater power and performance than many (most?) class AB amps. Choice can be good.

FWIWFM - Don

 

[Everett T]

And unfortunately, headroom is an area in which most class D amps have an issue. Take a look at the "headroom" that AMIRM got on one of my NAD 2200's in the NAD 2200 Vintage Amp Review. Incidentally, the spec listed by NAD for the 2200 for Dampening Factor is >100.. Well, we don't know exactly what that means (and I have seem it stated >200 on the later versions (of which mine are), so who knows. But, if it were what was considered to be too low, they would likely have not listed it at all.
I also believe that both "Dampening Factor" and. "Slew Rate" should be part of the specifications. IMO, using these, in conjunction with the other things that we check, can give us a better overall view of an amp (and likely some other devices we use in our audio systems).

Wouldn't the input impedence indicate damping factor quality? As for head room, once you're at 400wpc at 4ohms, there is enough power to get past most consumer headroom issues, especially considering what real world average SPL needs are.

 

[antcollinet]

Wouldn't the input impedence indicate damping factor quality? As for head room, once you're at 400wpc at 4ohms, there is enough power to get past most consumer headroom issues, especially considering what real world average SPL needs are.

Damping factor is output impedance related. It is calculated as speaker impedance divided by amplifier output impedance.

So for 4 ohm speakers and 0.04ohm output impedance would give a damping factor of 100

I've seen specifications for some of the hpex and purify amps in the 1000's.

 

[Everett T]

Damping factor is output impedance related. It is calculated as speaker impedance divided by amplifier output impedance.

So for 4 ohm speakers and 0.04ohm output impedance would give a damping factor of 100

I've seen specifications for some of the hpex and purify amps in the 1000's.

Thanks, I knew I was in the park but just couldn't remember which corelate and this is why I have to relie on EEs so much.

 

[mhardy6647]

So for 4 ohm speakers and 0.04ohm output impedance would give a damping factor of 100

My "problem" with damping factor (not that anyone asked) is just that.
'cause, you know... speaker impedance...

JBL Stage A130 Review (speaker)

This is a review and detailed measurements of the JBL Stage A130 bookshelf speaker. I purchases these online but can't find the receipt. They normally cost US $300/pair but the place I bought them from has them for $209 right now. The A130 looks plain until you take off the grill and then...

www.audiosciencereview.com

 

[Everett T]

My "problem" with damping factor (not that anyone asked) is just that.
'cause, you know... speaker impedance...

JBL Stage A130 Review (speaker)

This is a review and detailed measurements of the JBL Stage A130 bookshelf speaker. I purchases these online but can't find the receipt. They normally cost US $300/pair but the place I bought them from has them for $209 right now. The A130 looks plain until you take off the grill and then...

www.audiosciencereview.com

Wouldn't a very damping factor/ output impedence negate the worry about the impedence plot?

 

[antcollinet]

My "problem" with damping factor (not that anyone asked) is just that.
'cause, you know... speaker impedance...

JBL Stage A130 Review (speaker)

This is a review and detailed measurements of the JBL Stage A130 bookshelf speaker. I purchases these online but can't find the receipt. They normally cost US $300/pair but the place I bought them from has them for $209 right now. The A130 looks plain until you take off the grill and then...

www.audiosciencereview.com

Well yes - but that is the whole point.

If you have a high damping factor then the impact of that is much less, compared with a low damping factor.

Damping factor of say 20 (0.2 ohm), then that 20ohm to 4ohm shift will cause an aprox 4% change in voltage accross the speaker.

Damping factor of 100 - that drops to less than 1% - possibly still audible.

Get your damping factor to 1000, and then the change is less than 0.1% and inaudible.


Of course you have to consider your speaker cable impdedance in that calculation as well.

 

[DonH56]

Damping factor (output impedance) and speaker examples: https://www.audiosciencereview.com/...amping-factor-and-speakers.23968/#post-807327

 

[mhardy6647]

Well yes - but that is the whole point.

If you have a high damping factor then the impact of that is much less, compared with a low damping factor.

Damping factor of say 20 (0.2 ohm), then that 20ohm to 4ohm shift will cause an aprox 4% change in voltage accross the speaker.

Damping factor of 100 - that drops to less than 1% - possibly still audible.

Get your damping factor to 1000, and then the change is less than 0.1% and inaudible.


Of course you have to consider your speaker cable impdedance in that calculation as well.

Exactly -- so why don't "we" just stick with output impedance, which depends solely on the amplifier (and whatever tricks are used to make it as low as possible, at least for the past five or six decades) and not on the vicissitudes of the load?

Damping factor is, at best, misleading (as the other post above illustrates via the link contained therein). There are hordes of knuckledraggin' hifi nuts out there bragging about the amount of control their amp exercises over their wily woofers, all based on how many zeros there are after the "1" on the "damping factor" spec they read on the blurb from the amp's manufacturer.

Sorry to go all Andy Rooney on this -- but DF gets me goin'...

 

[antcollinet]

Ok now i understand. I agree. Never understood the need for two terms where one is the inverse of the other.

 

[DonH56]

Ok now i understand. I agree. Never understood the need for two terms where one is the inverse of the other.

Probably because bigger numbers are always better.

 

[EJ3]

Wouldn't the input impedence indicate damping factor quality? As for head room, once you're at 400wpc at 4ohms, there is enough power to get past most consumer headroom issues, especially considering what real world average SPL needs are.

With separate bass cabs in my system that I can run at 4 0hms or 2 ohms (supposedly 3 Db more efficiency) and having a large (living) room with a cathedral ceiling, a curved bay window that has a great enough extension toward the front lawn that 2 adults with a child by each ones side can sit in the bay window without touching each other, that opens into 2 other large rooms (dining room at one end, a staircase to the second floor and off set, three brick steps (this whole area has the cathedral ceiling going to join the ceiling of the the second floor]). (hint: I am good at run on sentences) Having lived at this location since I was 8, (& I will be 66 in 3 month)s, I have a pretty good idea of the power I need, at what ohms I need for which speakers that I am using, how much headroom I need and how it needs to be matched in a room that has very little in the way of standing waves.
Not having studied electronic engineering more than a bit (12 volt and 48 volt battery and alternator systems are something I became quite familiar with), I don't know the relationship between impedance and dampening factor.
As to how much power and headroom, those things depend on if there are 5 friend or 20 friends at the house, whether the 2 sets of sliding glass doors are open or closed, whether one side of the front oversize double doors are open or both or not. And whether I like the amps that I am using or not. Since the 80's it has been 3 to 5 NAD 2200's running through the LAB INPUTS (95 SINAD). I have started looking as to a possible replacement. To me, the best candidates seem to be 3 to 5 Benchmarks AHB2's or The Buckeye Hypex NC502 Amplifier, 8 channel. My circuits are all USA 125 V 20 amps and I run the NAD's on 2 separate circuits. The Benchmark's would be done similarly, But the Buckeye presents the problem of: can it hit the full momentary headroom spikes using the one 30 amp circuit? This is something 2 years to the future but the planning has started already. And then, who knows what we will have in 3 years (or not have). The Buckeye has the potential to reduce my amp footprint but they need to substantially exceed the NADS (97 SINAD score is, uhm? [when I already have 95]) But there are other factors at play, too. We'll see what the landscape looks like in 2 years.

 

[Holmz]

With separate bass cabs in my system that I can run at 4 0hms or 2 ohms (supposedly 3 Db more efficiency) …

Hope does dropping the impedance by half increase efficiency?