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Why would anyone use an AB amp for a tweeter?

Marc v E

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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?

1000027005.jpg
 
The high frequency switching may lead to IMD or heat that wouldn’t be a problem with the traditional AB design.

Edit. The LS60 is rated to 36 kHz. So the 15 kHz SINAD might be better with their Class AB amp than their Class D amp.
 
The tweeter needs less power. As such a class AB amp may be more cost effective or reliable for the same performance. KEF are a company you can trust to make an engineering decision in this regard.
 
It is all reasoned in their LS60 Wireless White Paper chapter 3.4.1 Amplification, exemplary from it:

The HF section has very different power statistics. Only
low continuous output is required but the amplifier
must have the ability to deliver sudden large peaks.
High bandwidth is also an important consideration for
true High-Res support. For this reason a class AB
amplifier was selected. Class AB amplifiers have a much
lower efficiency than Class D, but the thermal power
generated is reasonably low under these signal
conditions. Class AB has the added advantage of wide
HF bandwidth and of not requiring an output filter.
The reason behind the high-power specification for
both MF and HF amplifiers, especially the HF one, is due
mainly to the crest factor of music at the intended
frequency passband. Ensuring the amplifiers have the
rail voltage available to supply the power to amplify a
sudden sharp peak in the signal is crucial to avoid
harmonic distortion. The KEF Class AB design was
revised from LS50 Wireless II to further reduce
distortion.
 
The tweeter needs less power. As such a class AB amp may be more cost effective or reliable for the same performance. KEF are a company you can trust to make an engineering decision in this regard.
If it's more cost effective then why isn't it used in the KEF LSX range? Why would you think a profit making company designs products based exclusively on engineering decisions? Maybe they used AB in this context as a marketing ploy aimed at audiophiles wedded to "AB is better"? There's an interesting split in the KEF LSX range with the KEF LSX II LT aimed at the non-audiophile streamer, and the KEF LSX II aimed at audiophile types (Roon capability, aux input for turntable, etc.,...) One wonders if KEF might introduce an LS60 LTD at a much lower price point?
 
Probably not audible but KEF seems to have an idea that hf response should go well above 20kHz and few off the shelf class modules seems to offer this ? And also maintaining low output impedance possibly maybe the tweeter driven directly without any series component, the one case where you may want to control output impedance in the tweeter region.
 
But does any of this engineering produce audible results? Are there any blind test comparisons?
I personally doubt so, like also most electronics measured here with three digit SINADs, but its one engineering approach to do better than the minimum needed.
 
I have the same preference (and my personal policy?) of "right-person-in-right-place" for amplifier selection in my multichannel multi-SP-driver multi-amplifier fully active audio setup, as shared in my posts #311 and #931 on my project thread.

This post would be also of your interest and reference, I assume;
- Even Greg Timbers uses "reasonable and budget" Pioneer Elite A-20 for compression drivers (super tweeters) in his extraordinary expensive multichannel stereo system with JBL Everest DD67000 which he himself designed and developed: #435
 
I think it's because "every audiophile knows Class D sucks at treble" (tongue in cheek). That's a common belief (misconception) and the sales department knows that, and can easily make a large ROI (markup) on the "engineering" investment.

Good practice? Likely. Increased margins? Also very likely. Audible? Undetermined...
 
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?

View attachment 415641
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.
 
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.
(Edited)
Sounds more like marketing than a technical or economic reasoning.
 
Last edited:
  • Amplification
    • Woofer
      • Amplifier Type: PWM
      • Amp. Power RMS: 500 W
    • Midrange
      • Amplifier Type: PWM
      • Amp. Power RMS: 300 W
    • Tweeter
      • Amplifier Type: Class A/B
      • Amp. Power RMS: 50 W
 
I'm not sure in which department of KEF @AOR works but it seems that is not the marketing one.
I'm not targeting the person or their job description, I'm targeting the provided arguments. It's ridiculous to say that class D cannot provide comaparable performance at a given price level.

So for the Neumann KH 120 II (€613) 100 W class D was performing well enough (not even a Hypex nor a Purifi, AFAIK), but for the LS60 ($5000) it was not? Something is not aligning here.
 
KEF knows how to do marketing. So called audiophile reviewers are still telling their audiophile followers that A/B amps, in the end, are the better amplifiers. It doesn't fit any profit oriented marketing strategy that a $150 or even cheaper D-amp sounds better than any $1500 A/B thing. So, to keep the legend alive and mark the D-amp to be some green enforced, lower quality substitute, there must be some advantage of the power and resource hungry A/B construction. You can see the same patern with Class A or valve amps. They are, in the final oppinion, the only really audiophile worthy class of amplifiction. For KEF it is only a $1 investment in production cost to fit a tiny A/B inside their overpriced active speaker, but a great marketing advantage. "Oh look, they took all the gigantic problems of developing a special A/B amp, just for better sound!" BS, indeed. There is no problem to implement some 20 Watt A/B amp inside an active speaker. Any car audio head unit has 4 of them integrated.
On the other hand, if you are more of a skeptical mind and know KEF and their hyped products for decades, you may guess that they are simply not able to build a decend Class D amp for driving a tweeter.
 
KEF knows how to do marketing. So called audiophile reviewers are still telling their audiophile followers that A/B amps, in the end, are the better amplifiers. It doesn't fit any profit oriented marketing strategy that a $150 or even cheaper D-amp sounds better than any $1500 A/B thing. So, to keep the legend alive and mark the D-amp to be some green enforced, lower quality substitute, there must be some advantage of the power and resource hungry A/B construction. You can see the same patern with Class A or valve amps. They are, in the final oppinion, the only really audiophile worthy class of amplifiction. For KEF it is only a $1 investment in production cost to fit a tiny A/B inside their overpriced active speaker, but a great marketing advantage. "Oh look, they took all the gigantic problems of developing a special A/B amp, just for better sound!" BS, indeed. There is no problem to implement some 20 Watt A/B amp inside an active speaker. Any car audio head unit has 4 of them integrated.
On the other hand, if you are more of a skeptical mind and know KEF and their hyped products for decades, you may guess that they are simply not able to build a decend Class D amp for driving a tweeter.
No speaker company build their own amps,they are all of the self.
Here's one of what Genelec uses:


Tottaly of the self ones.
Look at the suggested applications


Gen.PNG


So?
 
Let's try to be a bit less curt please. We want the presence of manufacturers and their reps here. Call out and challenge where warranted but your follow up comment might have been better deployed in the first instance . Thanks
Rephrased the post.
 
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