Krell KSA-100 MK2

[anthonyTD]

Some further thoughts on the class issue, and the Lux amps in paticular;
From what I have experienced with these paticular amps over the 15 years or so of listening to them in various systems, here are some personal observations;
In class A mode these amps can be biased hot, [around 2 Amp] or quite cold, [around 900 MA] somewhere in the middle there is a sweet spot, [which will be dependant on the rest of the system, But personaly, I would say around 1.2 Amp] ] the following descriptions are somewhat extreme but hopefuly it will get my point accross; If the amp is biased too hot, it can sound slow,Fat, and ploddy, if its biased too cool, it will sound quicker, but lighter, and more forward sounding etc, to be honest,as a starting point; these amps sound very good in the A/B set up, [as stated in an earlier post, the class is determined by the Bias, and the selected transformer windings etc]I guess what I am saying is; Just make sure the amp is set up properly in its chosen class, before evaluating it properly.
Good listening.
A...

[Bigman80]

The KSA 50, 100 and 200 all run high fixed bias of between 700mV and 900mV across the 1 ohm emitter resistors in the output stage. That puts them into class A for their full rated power. Every model that came after these 3 the KSA-S versions and all the other numbered designs KSA250, FPB models and everything else used a much lower bias and are just big class AB amps. The KSA 250 for example runs 110mV or thereabouts over the emitter resistor and as such can only provide class A power for the first 25 watts into an 8 ohm load, so it's very definitely a Class AB amp.

We could argue until the cows come home what one person thinks the definition of class A is but the fact remains that those three KSA models have sufficient standing bias to operate in class A for all of their rated output power, they are all complimentary designs and the output devices conduct for the full 360 degrees of the input waveform. Where I'm sat that makes them class A, no ifs or buts.

I couldn't care less what anything thinks about the brand, their history, their marketing, how they did business or anything else that they'd emotionally like to attach to the brand. I'm not interested in their baggage and none of it alters the fact that these first three amps were all class A, all the time. You could be of the opinion that a 'real class A amp' has to be single ended, but the literature disagrees. A single ended amp has to be class A, but a Class A amp does not have to be single ended.


KSA 50s, that's a class AB design.

Thanks for this. It's nice to know that it's not just twisted marketing.





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[Bigman80]

Some further thoughts on the class issue, and the Lux amps in paticular;
From what I have experienced with these paticular amps over the 15 years or so of listening to them in various systems, here are some personal observations;
In class A mode these amps can be biased hot, [around 2 Amp] or quite cold, [around 900 MA] somewhere in the middle there is a sweet spot, [which will be dependant on the rest of the system, But personaly, I would say around 1.2 Amp] ] the following descriptions are somewhat extreme but hopefuly it will get my point accross; If the amp is biased too hot, it can sound slow,Fat, and ploddy, if its biased too cool, it will sound quicker, but lighter, and more forward sounding etc, to be honest,as a starting point; these amps sound very good in the A/B set up, [as stated in an earlier post, the class is determined by the Bias, and the selected transformer windings etc]I guess what I am saying is; Just make sure the amp is set up properly in its chosen class, before evaluating it properly.
Good listening.
A...

Thanks Anthony,

Currently, I don't know what the Lux is set up for, I'm presuming it will be as it was from the factory which should be A/B although the dealers did give the option to the customer.

When it gets here, I will obviously plug it in and give it a listen but I do have plans to get it to Phonomac and let him give it the once over and get it into Class A mode. I will pass on your thoughts on the bias and ask him to do his stuff with it. This is an area he knows well so I am lucky he's a good guy.

If it's ok, I may drop you a message or two over the course of action just to get your thoughts. I'm pleased someone with your knowledge has some exposure to this amp.



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[anthonyTD]

No worries!
Yes, fine.
A...

Thanks Anthony,

Currently, I don't know what the Lux is set up for, I'm presuming it will be as it was from the factory which should be A/B although the dealers did give the option to the customer.

When it gets here, I will obviously plug it in and give it a listen but I do have plans to get it to Phonomac and let him give it the once over and get it into Class A mode. I will pass on your thoughts on the bias and ask him to do his stuff with it. This is an area he knows well so I am lucky he's a good guy.

If it's ok, I may drop you a message or two over the course of action just to get your thoughts. I'm pleased someone with your knowledge has some exposure to this amp.



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[Bigman80]

No worries!
Yes, fine.
A...

Great stuff. Thank you.

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[DSJR]

Seriously, the whole class a or ab issue became pretty much a non event in the 80's when better matched and higher power transistors filtered into domestic availability I gather from a trusted engineer source who researched all this for his own successful designs beck then - the need for ungodly numbers of paralleled output transistors per channel significantly reduced, certainly in the mid 90's when new devices came on stream.

As for Krell's claims, my understanding was that say, if the original KSA 50 claimed 50WPC 'Class A' then this it would do (in the current definition of the term for a complimentary output stage) for the first 50 Watts (confirmed by Colloms reviews and heat generated ) BUT, the amp would then slide into AB for the remaining power available, as the '50' was in truth a 90WPC amp into 8 ohms according to tests I've read. All playing with terms and numbers I suspect and only of any importance to spec freaks.

I'd like to add that this whole situation in US based Top End was almost certainly the popularity of panel (various types) speakers in the 80's and early 90's having cruel loading and inefficiency, needing humongous amps with huge power supplies to drive them at all, let alone properly. Those days are gone now, but I've heard tales of older large Krells suffering thermal wear after thirty years - they're not as reliable as their exalted reputation would suggest and even the FPB models can suffer it seems now. Just don't pay silly money for them, that's all...

[Marco]

Hi Dave,

As for Krell's claims, my understanding was that say, if the original KSA 50 claimed 50WPC 'Class A' then this it would do (in the current definition of the term for a complimentary output stage) for the first 50 Watts (confirmed by Colloms reviews and heat generated )

Indeed. so in reality that's plenty enough power to produce a fantastic (way more than loud enough) sound, given the use of speakers that are efficient, and a room that's not too big. For me that's the real issue here, in terms of maximising the (undoubted) sonic advantages of Class A, done well.

The problem is these days there's a dearth of really good high-efficiency speakers, and the best ones are BIG, so most folks aren't willing (or able) to house them.

The fact is, however, that in most domestic situations, given the use of large, quality high-efficiency loudspeakers (say 92db and above), if you are lucky enough to own such [count me in with the Tannoys. which are 95db], 50W of Class A is more than enough to take the roof off your house, never mind blow your ears away!

So ultimately, in terms of the above Krell, it's the first 50W that matters!! Not the rated full output of the amplifier. Jeez, with large speakers of 92db efficiency and more, HALF of that power, in an average sized room, would still be deafening.

Therefore, all this pish about which type of amplifier is truly representative of being a Class A design, is moot, and in the real world (the one you listen in) completely irrelevant.

The fact is, as long as the amp concerned is used well within its Class A power envelope, which in the above scenario would almost certainly be the case, then you'll be hearing (and enjoying) the subsequent sound, delivered in Class A. Simples

Marco.

[Macca]

Fact is a modern class B amp will have crossover distortion that is way below audible levels so the advantage of class A operation isn't what it used to be by comparison.

I think it's more important that the amp has the ability to drive low impedance loads without flaking out. That's the big advantage of amps like the classic Krells. Assuming you don't just listen at low volumes and to simple acoustic music of course.


The 50watt KSA50s had no problem filling a church hall with sound with absolutely no distortion evident and it didn't even get to the third level of plateau biasing. Speakers were 90dB efficiency which isn't that high by modern standards.


It's all very well to say get more efficient speakers but the iron rule is you can only ever have two of these three: Deep bass - high efficiency - small cabs. So to get decent bass response from a high sensitivity speaker it needs to be big: fridge-freezer big. Wardrobe big. Yes you could use a sub but we've all been there and know how that works out. Even best case it's never quite as good. And before someone says DSP, well no. You can iron out peaks with it but using it to bring up the level of the low bass requires a ton of power - thousands of watts. So your super-efficient speakers need to be able to handle that. Good luck!

[anthonyTD]

The class of any amplifier is only relevant if you like what the whole package does, whether it be class A, AB, or D etc, if it doesent connect with you on a musical level, then it matters not.
There are some very good examples of amplifiers from all topologies, and there are equaly as many bad examples etc, its the design over-all that makes for a Great, or not so Great product.
A...

[Marco]

Fact is a modern class B amp will have crossover distortion that is way below audible levels so the advantage of class A operation isn't what it used to be by comparison.

Yup, but in my experience, when genuine Class A's done well, in the right amplifier, sonically it takes some beating.

However, as you say, Class A alone, is no magic bullet, just the same as anything else in audio isn't. Therefore, an amplifier will always be more about the SUM of its parts, than the influence of any single aspect of its design, and ultimately *that* is what it'll be judged on

The 50watt KSA50s had no problem filling a church hall with sound with absolutely no distortion evident and it didn't even get to the third level of plateau biasing. Speakers were 90dB efficiency which isn't that high by modern standards.

Which system and/or instance are you referring to above? Sorry, but it's not clear.

It's all very well to say get more efficient speakers but the iron rule is you can only ever have two of these three: Deep bass - high efficiency - small cabs. So to get decent bass response from a high sensitivity speaker it needs to be big: fridge-freezer big. Wardrobe big.

Yup, like my Lockies... Best (220 litre) 'wardrobes' I've ever bought! Shown below at Scalford Hall, as part of an AoS system we demonstrated, along with my TD Copper amp and various other bits of kit (note how the speakers are wider than the door - no weeny-boy WAF pish there!):






As you know, the combination of the above 95db efficient, 15" Tannoy DC-equipped 'wardrobes', and the Copper amp, with either 30W of Class A, fitted with KT88s (or 50W Class A, fitted with KT120s), when either is operated within its Class A power envelope, is quite something.

I just hope, mate, that you can get your own Tannoy DCs built before Christmas!!

Marco.