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Why would anyone use an AB amp for a tweeter?

.As to why the AB amp isn't in the LSX range - they are powered according to the description, not active, so that would be a single class D amp?

They are definitely active - separate amplifier for each driver.

The KEF LSX speakers each use a pair of Class D amplifiers:

Midrange/Bass: 70W

Tweeter: 30W
 
The LSX II has amps in each speaker and can be used wireless connection in the KSX II LT amps are only in one speaker with a usb c connection to secondary...
Have you a reference for this? A seller site says The KEF LSX II LT speakers are powered by a 200-watt Class D amplifier, with 30 watts dedicated to the tweeter and 70 watts to the mid/bass driver in each speaker. So that's one amplifier compared to the four in my original KEF LSX speakers!

KEF's site isn't explicit about how many amplifiers are in each variant of LSX.

One sellers site says II has the same driver arrangement as the original, so that makes 4 amps as well.

Several subjective reviewers are saying II LT sounds as good as II. In the LSX set up, can one big amp powering both speakers really be as good as two smaller amps in each speaker?
 
All 4 amps are in the Primary speaker in the LSX II LT. The secondary speaker doesn't have a power cord connection the only connection is the usb c tether cable carrying the audio signal from the primary. They're probably the same amplifiers only difference is the LSX II has them in each speaker so they can be used wireless without the tether connection cable.

Edit: The Amps are most likely in each speaker. I noticed the LT version uses USB C PD 3.0.
 
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So the AB amp on the LS60 is there for a few reasons. Basically, we would prefer to use an AB for everything but in the active products space and therefore cooling as at a premium. We couldn't fit a suitable LF amp into the enclosure so it has to be a class D. As others have pointed to, we did some research that was presented in the whitepaper, this showed that due to the crest factor in the HF of music signals we didn't need too much cooling. This means an AB amp is appropriate.

On the general AB Vs D debate. On ASR and in the worlds of Purifi and Hypex, class D amplifiers are comparable in performance to class to class AB amps. However for most class D designs, especially those unable to use the Hypex designs and knowledge, or if made to a significantly lower price point, the performance (in terms of sinad) of these amps is lower than similarly priced ABs.
Thank you for your explanation! It starts making sense. I never realised before that at the tweeter frequency class AB amps perform very well compared to the best class D.

It even shows (too some extent) in the Hypex Nilai 500 vs the NAD C316 BEE.
Fascinating. It's like my work. When you get to a very deep understanding of the technology, and somebody asks for the best solution, the answer almost always start with: it depends...

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I never realised before that at the tweeter frequency class AB amps perform very well compared to the best class D.
Because they do not. It's quite possible to make a good active speaker with inexpensive 100 W PWM amp for the tweeter and very low distortion of the whole speaker system.

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I think you need to look a little deeper than just the THD content to get a full picture on what's going on, surely?
 
Because they do not. It's quite possible to make a good active speaker with inexpensive 100 W PWM amp for the tweeter and very low distortion of the whole speaker system.

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No worries. I probably cannot hear the difference anyway. ;) Look at this hearing acuity graph. I'm already on the orange curve.

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No worries. I probably cannot hear the difference anyway. ;) Look at this hearing acuity graph. I'm already on the orange curve.

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Yes. The point here is bringing the amp's SINAD argument in when we are talking about transducers is a bit misleading (not to mention the actual availability of high-performing PWM amps at almost any budget; and if we take thermal design into account, class D can be even less expensive to implement in speakers, as was mentioned by the KEF rep themself).
 
Woah, it seems my comment kicked up a little bit of a storm.

While there are valid technical reasons to use a Class AB for a tweeter over a Class D, those that said the reason was "marketing" are certainly not wrong: -

Designing loudspeakers for sale involves assessing the best technical solution, but also the solution that will result in the product being attractive to the most people. Due to the history of the development of the different classes of amplifier, there are implicit biases (or expectations) in the market for each of these types.
Class A has always been as the highest quality, but most reasonable amplifiers were Class AB due to their similar sound quality but much better thermal performance. As others have said, when Class D amplifiers were introduced, they were only for high power/high efficiency applications where sound quality was secondary (or tertiary). Are these "tiers" still applicable today, where we have amplifiers like the AHB2 and LA90 and EIGENTAKT modules? Not really, but in this case, the market technology expectations move slower than the actual technological progress.
 
Woah, it seems my comment kicked up a little bit of a storm.

While there are valid technical reasons to use a Class AB for a tweeter over a Class D, those that said the reason was "marketing" are certainly not wrong: -

Designing loudspeakers for sale involves assessing the best technical solution, but also the solution that will result in the product being attractive to the most people. Due to the history of the development of the different classes of amplifier, there are implicit biases (or expectations) in the market for each of these types.
Class A has always been as the highest quality, but most reasonable amplifiers were Class AB due to their similar sound quality but much better thermal performance. As others have said, when Class D amplifiers were introduced, they were only for high power/high efficiency applications where sound quality was secondary (or tertiary). Are these "tiers" still applicable today, where we have amplifiers like the AHB2 and LA90 and EIGENTAKT modules? Not really, but in this case, the market technology expectations move slower than the actual technological progress.
I'm actually a little disappointed to read this. I've always thought of KEF as primarily an engineering led company, and quite honestly I find it weird that a KEF engineer would state that one AB amp alongside two class D amps would be a marketing led decision in the 2020s.

I think, quite honestly, that you have this the wrong way round. Maybe for the buyers of the passive speakers this thinking would still apply. You are, however, hear selling into a market where your likely buyers are either lifestyle audio purchasers that wouldn't care, or will be comparing this model to products from the likes of D&D, Genelec, Kii and the companies known here for active speakers, and would actually be expecting good class D throughout, and maybe much more sophisticated design and software as wll.

I doubt that (despite this thread) anyone who has got the ASR message would really care what class of amplifier is in this speaker, as long as the engineering and the results are good and that in the event of failure it can be easily repaired at reasonable cost (if out of warranty).

The AHB2 is not class D.
 
More interesting than disappointing ? We will see what the future brings . If we look at the bigger picture.

Many of the traditional speaker brands are still producing passive speakers for the home hifi market , pro market has been mostly active speakers for the last 30+ years where the amps and DAC’s inside are uninteresting commodities as long as they are fit for purpose serviceable and reliable. That’s not the HiFi market :)

KEF now has 4 speakers in the active category !? The LS60 is thier boldest proposition . It’s at a price point where many traditional audiophiles would only consider passive speakers and is misunderstood as an expensive lifestyle speaker by others ( all thier actives are hifi speakers imho ).

So it’s interesting to see KEF gradually treading into active speaker territory and test the market

Fanboy disclaimers: I have both LSXii and LS60 and KC92’s .

If we would have a ”nitpicking with KEF” tread :) running i i have a long list of other topics regarding thier actives that’s more problematic than the amp class of the tweeter amp in one model . I still bought thier products as I saw them as the best for my purposes, but these are not blameless implementations and it’s a bit off a leap of faith to do so .
 
Currently my spare thoughts are occupied with the kef ls60.

Reading through the specs I noticed they use class D for the bass and mids and an AB amplifier for the tweeter.
I would expect class D to be superior in sinad, efficiency, heat management, cost...basically anything that counts.

My question is therefore: why would the engineers at kef choose for an AB amp, specifically for the tweeter ?
What technical benefit does it have?

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I'm using an old Rane 6 ch. amp to drive a pair of 2405 tweeters, 2420 CDs and 10" of the week with class AB. Not a sexy amp but it has 19" mounting. It's portable that way and works great going on 10 years with BSS dsp.
 
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Woah, it seems my comment kicked up a little bit of a storm.

While there are valid technical reasons to use a Class AB for a tweeter over a Class D, those that said the reason was "marketing" are certainly not wrong: -

Designing loudspeakers for sale involves assessing the best technical solution, but also the solution that will result in the product being attractive to the most people. Due to the history of the development of the different classes of amplifier, there are implicit biases (or expectations) in the market for each of these types.
Class A has always been as the highest quality, but most reasonable amplifiers were Class AB due to their similar sound quality but much better thermal performance. As others have said, when Class D amplifiers were introduced, they were only for high power/high efficiency applications where sound quality was secondary (or tertiary). Are these "tiers" still applicable today, where we have amplifiers like the AHB2 and LA90 and EIGENTAKT modules? Not really, but in this case, the market technology expectations move slower than the actual technological progress.
Thank you for being so honest. I appreciate that more than anything.

Given my experience with the Genelec 8351, which sound amazing with quite simple amplifiers, this makes sense to me.

I will demo the ls60 soon. I'm curious to experience if they sound so right I can't describe their sound, just give up judging them and enjoy the music.
 
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Would not Genelec and Neumann and JBL be good benchmarks for anyone making active speakers ?

Suppose KEF already have some teared to pieces :)

Genelec would be a good case study for reliability ( and competitive coax drivers ) . I think longevity and reliability is where ( nudge , nudge ) more action is needed as an all in one solution also is an all in one failure point. And it would help the acceptance of active speakers if they where reliable and repairable.

@AOR can scrutinise this tread https://www.audiosciencereview.com/forum/index.php?threads/kef-lsx-teardown-and-repair.55697/
 
Genelec has been doing this for a long time so I guess you can study the movement from class AB amps to class D ?

Have class G or H been used in actives . My old Meridian 5200DSP speakers had some “clever” bias scheme to keep their AB amps relatively cool
 
Agreed, although the question of “today” is worth thinking about
My comment was meant to be a more general one, not a specific one about KEF models.

And yes, "today" means "recently" in engineeering / product design cycle terms, so let's say designs developed in the last two years or so....

... however:
Not really, but in this case, the market technology expectations move slower than the actual technological progress.
 
Because they do not. It's quite possible to make a good active speaker with inexpensive 100 W PWM amp for the tweeter and very low distortion of the whole speaker system.

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Yes and we call it class H then and similar can be done on AB design, class G. It improves performance in every aspect and doesn't cost all that much (voltage pump) with some cost in efficiency but not much. And finally we are starting to have such (cheap and good). But never the less it still costs and AB passes without it. We (or at least I) aren't into this regarding possibly the best implementations possible but really transparent and up to possible useful limits.
AHB2 is class G but you have a lot of class H SoM amp's with or without Bruno and in forms suitable even for DIY activities. Given mentioned price difference most manufacturers aren't exactly transitioning to class H. You get a decent class D in most cases. How did Neumann menage to have quietest self noise active monitor's in the past and much better than Genelac which whose average at best? Simple they used AB class for HF and now they don't in most cases and big suprise the self noise went up. Now some smart guy (wanted to write something else) will tell me how that's not important (in a near field) yet we have flood of people complaining about it every month hire and elsewhere. Meanwhile no one is complaining regarding any amplifier that can keep the SINAD above 75 dB in audible range.
 
All 4 amps are in the Primary speaker in the LSX II LT. The secondary speaker doesn't have a power cord connection the only connection is the usb c tether cable carrying the audio signal from the primary. They're probably the same amplifiers only difference is the LSX II has them in each speaker so they can be used wireless without the tether connection cable.
How do you know this? Do you have a reference, or did you perform a tear down?
 
Are these "tiers" still applicable today, where we have amplifiers like the AHB2 and LA90 and EIGENTAKT modules? ...
Does the KEF LSX amps use the AHB2 or LA90 or EIGENTAKT modules?
 
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