How Much Current Capacity do we Need from our Amplifiers?

[MAB]

> 103dB - 128W (they'll be compressing and distorting badly here)

I believe you, but how did you figure that fact out?
(and destruction?)

A few reasons.
First, they are a tiny low-efficiency speaker. Common sense says a speaker with this size drivers and efficiency won't go above 100dB without damage. And will sound horrible doing so. Amir commented on this specifically in the original review. The physics of small drivers would confirm, preferably in a speaker simulation tool.
Second, the SPL of the Meta version was tested and they distort massively at 96dB, likely not useable above 90dB unless extreme care is taken:

KEF LS50 Meta Review (Speaker)

This is a review and detailed measurements of the KEF LS50 Meta bookshelf coaxial speaker. It was kindly purchased new by a member and drop shipped to me and costs US $1,500 for a pair. The LS50 Meta comes in different colors and I must say, it looks stunning in white: You could sell it as...

www.audiosciencereview.com

Even without the issues reproducing bass, these are not candditaes for pushing the limits. Nice speakers, but not really suited for high volume.

According to multiple questions you are asking, you seem to be looking to use these in a way that isn't consistent with their design. And are trying to test for the limits. Pushing a small inefficient speaker hard is a recipe for bad sound at best, damage gear even more likely, damaged hearing at worst.

 

[john61ct]

Yeah well I don't think you're going to compete with the peak SPL of a tank cannon... even a pistol is louder than any speakers

When my dad conducted the 1812 Overture at Carnegie, he used real cannons.

They did indeed make an (insert superlative) impact!

 

[Thighsofsteel]

Alright mate, let's try and do some simple number crunching for a good estimation.

First, max "safe" SPL: manufacturer specs the LS50 at 106dB max. Let's see how much power that is:

85dB rated sensitivity. They're silly and give that at 8 Ohm which they aren't, so let's say 3 less, 82dB. Means 1W makes them do those 82dB, every 3dB more doubles the power, so it looks like this:

82dB - 1W
85dB - 2W
88dB - 4W
91dB - 8W
94dB - 16W
97dB - 32W
100dB - 64W (this is the point where safety is more about your ears than the speakers)
103dB - 128W (they'll be compressing and distorting badly here)
106dB - 256W (destruction)

They also spec them at "amp power 25-100W". If those 100W are maximum, which is about right for a single small coax driver, you won't even reach those 106dB without destroying them (and your ears).

Look above: 90dB and more is PLENTY loud, but let's say you really want 103 for peaks so it's "safe".

EDPR is 1.7 Ohm, right. Now Ohm's law: Current = squareroot (power / resistance). Makes sqrt(128/1.7) = 4.28 Ampere.

There you have a good estimation of your needed current. The calculation isn't exact but good enough for the purpose: you now know you need not worry at all. If that 3E amp can do 30A peaks, that's super overkill and it'll be able to destroy these speakers without breaking a sweat.

You don't wanna turn them up too much anyway for two reasons: your precious ears, and their drivers are infamous for blowing up under heavy bass load. The membranes crack and all. Don't do it, listen at safe levels

Don’t forget a bit of room gain. Personally I work with 4ohms not 8, but that still gives 97db at 64w add on 2-4db from room gain and it still “blows the bloody doors off” if I may quote Michael Caine.

 

[john61ct]

Even without the issues reproducing bass

As stated, they will be HPF'd at 170Hz or higher.

> these are not candidates for pushing the limits.

> Nice speakers, but not really suited for high volume.

> recipe for bad sound at best, damage gear even more likely, damaged hearing at worst

Exactly why I am seeking to know objectively how to ensure I stay below those limits.

I am NOT trying to get a higher SPL than what is high SQ and safe.

The OG LS50s are a given, it is only THEIR limits I am seeking to work within.

 

[Ropeburn]

Don’t forget a bit of room gain. Personally I work with 4ohms not 8, but that still gives 97db at 64w add on 2-4db from room gain and it still “blows the bloody doors off” if I may quote Michael Caine.

Yeah but we're looking at an outdoor application here so freefield/anechoic estimation is fitting.

 

[john61ct]

Don’t forget a bit of room gain. Personally I work with 4ohms not 8, but that still gives 97db at 64w add on 2-4db from room gain and it still “blows the bloody doors off” if I may quote Michael Caine.

Yes, I realize I will rarely be intentionally hitting this limit when setting the system up in most indoors locations.

But partying kids...

And I do regularly live in remote off grid locations.

 

[Spkrdctr]

When "max peak amps" is used in this context

is there safety / linearity included?

Or is that more a voltage thing? Or SPL?

I have no interest in driving my LS50s past the point of clean SQ output, much less near any danger-of-damage zone.

However, I do want to push up close to maximum SPL within that context

As one of the resident experts on ASR on testing speakers to let out the smoke, I will tell you to not get crazy in your testing. Speakers generally will take a lot of abuse but you never know when the smoke will release. It even goes by the specific driver as build variances of the drivers can make a difference. Of course time is a big factor. So, Just use your speakers within their stated parameters and you will have a long life and great sound. Push then hard and you can find the smoke limit. I say why bother? This entire thread is the best thread I have read in a long time here on ASR. Fantastic help posts and lots of great knowledge. This is why I like ASR.

 

[-Matt-]

The discussion and misdirection in the B200 thread is unfortunately all too common.
Thanks for presenting this so clearly.

I hope you do not include my post in the B200 thread under the category of misdirection. If you do, would you please explain where you think any errors or problems occur?

 

[john61ct]

This entire thread is the best thread I have read in a long time here on ASR. Fantastic help posts and lots of great knowledge. This is why I like ASR.

Yes, thanks to all.

...

Just use your speakers within their stated parameters

So, you think going to max SPL of 106dB is OK for longevity?

Or is measured 100W with a high current power amp safer?

I want to verify myself, and was thinking determining a repeatable objective upper limit

at which high SQ linearity is fully preserved

would be LOWER than mfg published specs, or testing based on the possibility of physical damage.

To me, this is the opposite of "going crazy", the point of the testing is to stay conservative.

 

[-Matt-]

By the way, here is what can happen if you try to push the LS50 too hard.

 

[john61ct]

By the way, here is what can happen if you try to push the LS50 too hard.

yes I've seen many like that

Exactly my concern

...

If you do, would you please explain where you think any errors or problems occur?

To help keep this thread well focused, could that explanation take place in that original thread?

 

[Spkrdctr]

By the way, here is what can happen if you try to push the LS50 too hard.

Now that is a photo I can like! Total destruction........as long as I don't own them. LOL

 

[Ropeburn]

Now that is a photo I can like! Total destruction........as long as I don't own them. LOL

LS50 = Less than Suitable beyond 50W

 

[Spkrdctr]

According to Amir's testing and looking at those LS 50s, I just don't think they are even close to what anyone should be using in a large room at all. They are just too small and the testing showed they don't play well at higher spl. If anything a large but not super expensive pair of large Cerwin Vegas might do the trick.....

 

[john61ct]

Again

I am NOT trying to get a higher SPL than what is high SQ and safe.

The OG LS50s are a given, it is only THEIR limits I am seeking to work within.

The system (or parts of it) will be to some extent portable, mobile.

There is no "usual" room type, in fact not designed for ANY "room",

cabin of a boat, back of a box truck, converted skoolie are examples.

Plus outdoors. Whether these speakers are "appropriate" for a given space is not the issue. I am learning how to make sure to use them appropriately.

 

[Endibol]

By the way, here is what can happen if you try to push the LS50 too hard.

Only the nice color scheme survived…

 

[Morinor]

In my opinion amplifiers should do their job relative with the load to be more stable. This means double power capabilities at 4 ohm and quadruple at 2ohm. Only AB amplifiers lower than 1000-1500€ can be justified to not be such capable. Otherwise the opposite way of calculation takes place to evaluate an amp that will not sweat. Meaning divide the capability of 2ohm by 4 and 2 in order to find how much power it can handle at 8ohms and 4 ohms. Above those prices there are few AB amplifiers that can truly follow this rule. Cambridge Edge A (or W) are one of those examples. Some Rotel around the 3000-5000€ can do that and I think NAD 40n is also capable. Topping B200 although very clear at 4ohms is very limiting reaching lower impedances. Class D from Purifi or Hypex are the better options for not caring about current. 500 watts at 2ohm mean that the amplifier can drive any 8 ohm nominal speaker with 125 clear watts or any 4ohm nominal speaker with 250 watts. Purifi 1ET9040 mostly at 1200 watts at 2ohms means equivalently 300 watts for 8 ohm and 600 watts for 4 ohm.
One comment about the example used on the original post, we would wished to had such easy speaker. On most frequencies it doesn't reach below 6ohm and at high frequencies although dropping there is usually less need for power/current there. Only few expensive speakers can do that and most of them are two way. Usually there can be drops at around 3-3.5 ohm and even 2.5ohm at low frequencies that are more demanding.

 

[Ropeburn]

In my opinion amplifiers should do their job relative with the load to be more stable. This means double power capabilities at 4 ohm and quadruple at 2ohm. Only AB amplifiers lower than 1000-1500€ can be justified to not be such capable. Otherwise the opposite way of calculation takes place to evaluate an amp that will not sweat. Meaning divide the capability of 2ohm by 4 and 2 in order to find how much power it can handle at 8ohms and 4 ohms. Above those prices there are few AB amplifiers that can truly follow this rule. Cambridge Edge A (or W) are one of those examples. Some Rotel around the 3000-5000€ can do that and I think NAD 40n is also capable. Topping B200 although very clear at 4ohms is very limiting reaching lower impedances. Class D from Purifi or Hypex are the better options for not caring about current. 500 watts at 2ohm mean that the amplifier can drive any 8 ohm nominal speaker with 125 clear watts or any 4ohm nominal speaker with 250 watts. Purifi 1ET9040 mostly at 1200 watts at 2ohms means equivalently 300 watts for 8 ohm and 600 watts for 4 ohm.
One comment about the example used on the original post, we would wished to had such easy speaker. On most frequencies it doesn't reach below 6ohm and at high frequencies although dropping there is usually less need for power/current there. Only few expensive speakers can do that and most of them are two way. Usually there can be drops at around 3-3.5 ohm and even 2.5ohm at low frequencies that are more demanding.

Your numbers for Hypex/Purifi are flawed because at 2 Ohm, power is usually current limited by the PSU while at 4 and 8 it isn't.

Hypex NCx500 Class D Amplifier Review

This is a review and detailed measurements of the Hypex NCx500 NCOREx reference design amplifier. It was sent by the company and kindly assembled by our forum expert, @Rick Sykora. As you see the sample is a "monoblock" configuration (one channel) driven by Hypex SMPS1200 switching power...

www.audiosciencereview.com

As you see, 700W at 2 Ohm doesn't mean 350 at 4, rather you get 670.

 

[CDMC]

In my opinion amplifiers should do their job relative with the load to be more stable. This means double power capabilities at 4 ohm and quadruple at 2ohm. Only AB amplifiers lower than 1000-1500€ can be justified to not be such capable. Otherwise the opposite way of calculation takes place to evaluate an amp that will not sweat. Meaning divide the capability of 2ohm by 4 and 2 in order to find how much power it can handle at 8ohms and 4 ohms. Above those prices there are few AB amplifiers that can truly follow this rule. Cambridge Edge A (or W) are one of those examples. Some Rotel around the 3000-5000€ can do that and I think NAD 40n is also capable. Topping B200 although very clear at 4ohms is very limiting reaching lower impedances. Class D from Purifi or Hypex are the better options for not caring about current. 500 watts at 2ohm mean that the amplifier can drive any 8 ohm nominal speaker with 125 clear watts or any 4ohm nominal speaker with 250 watts. Purifi 1ET9040 mostly at 1200 watts at 2ohms means equivalently 300 watts for 8 ohm and 600 watts for 4 ohm.
One comment about the example used on the original post, we would wished to had such easy speaker. On most frequencies it doesn't reach below 6ohm and at high frequencies although dropping there is usually less need for power/current there. Only few expensive speakers can do that and most of them are two way. Usually there can be drops at around 3-3.5 ohm and even 2.5ohm at low frequencies that are more demanding.

Except there are very few amplifiers that actually double when impedance is halved. Even huge Krells and Levinsons didn't, instead they underrated them at 8 ohms which would show up when actually tested. The only amps I recall actually doubling were the Sunfire amps, but they were hampered by other issues (i.e. low sn ratios and moderate distortion).

 

[-Matt-]

According to Amir's testing and looking at those LS 50s, I just don't think they are even close to what anyone should be using in a large room at all. They are just too small and the testing showed they don't play well at higher spl. If anything a large but not super expensive pair of large Cerwin Vegas might do the trick.....

Again



The system (or parts of it) will be to some extent portable, mobile.

There is no "usual" room type, in fact not designed for ANY "room",

cabin of a boat, back of a box truck, converted skoolie are examples.

Plus outdoors. Whether these speakers are "appropriate" for a given space is not the issue. I am learning how to make sure to use them appropriately.

In a "scientific" use of language, outdoors is roughly equivalent to an infinite (or very large) sized room.

The LS50s are suitable for indoor near field listening, at modest levels, with room reinforcement and with an amplifier power up to the maximum specified value of 100W. (You can use an amplifier with higher maximum output capability if you are happy to limit power via the volume control. In your case you don't want to entrust the kids with this, so use a lower wattage amp).

Regarding crossovers and max useable SPL you should refer to the distortion graphs and pick a distortion level that you will accept (e.g. 1%). We have graphs for the LS50 Meta, original LS50 will likely be slightly worse.

You can see from the above that if you plan to produce 96dB SPL, at 1m from the speakers, and keep below 1% distortion, then you would already want the crossover to be higher than 600 Hz.

Each time you double the distance the peak SPL will reduce by 6dB. So 96dB at 1m will yield:
90dB @ 2m
84dB @ 4m
78dB @ 8m
72dB @ 16m
66dB @ 32m

Just not a good candidate for high SPL outdoor listening at all, even if you have a ridiculously complicated system of mid woofers and subs.