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Krell KSA50s class A or A/B ?


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Disclaimer- I don’t have a lot of “technical” knowledge. This is more of a curiosity thing for me.

 

I have always thought that the Krell KSA50s was a “class A/B” amp ,not sure why but probably from the Stereophile review. Then the other day I was looking through the amps manual the other day while listening to some music and what I can tell it says that it is “class A”

I have taken a screenshot of the relevant part.

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Now I’m not sure if Krell is just using flowery language with “plateau biasing” but with my limited technical knowledge it sounds like the amp is all “class A” ?

 

here is an excerpt from the “Stereophile” review 

 

The "Sustained Plateau Biasing" concept is a patent-pending way for Krell to have its class-A output-stage cake and eat it too, without paying the penalty of wasting large amounts of wall power as heat. A fast "anticipator" circuit, one for each channel, looks at the incoming signal and increases the output stage's bias current if the level would be likely to turn off one half of the transistor array on half-cycle peaks. In this manner, when there's little or no signal, the amplifier is not being asked to pass a large standing bias current with the concomitant radiation of heat. If the signal level allows it, the amplifier's output bias progressively drops back to the lower levels after 20 seconds or so. If continuous high powers are required that would lead to overheating, the amplifier reverts to classic class-AB operation.

 

So basically I am just wondering if one of them is wrong or if they are both right but using different terms for the same thing ?

 

I know this is just a technicality in the end but I am curious just the same.

 Thanks 

Pete


 

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  • 2 months later...

Pete,

they are both right but the Krell explanation is the truly correct one as it explains the condition more accurately. When the amp is driven hard if the heatsink temperature reaches 80 deg C, then the plateau bias drops down one position so that the amp does not overheat, thus reducing the amount of class A power available. So it might be in your particular case it will not ever get that hot unless you push it hard, in which case it will always operate in class A mode. Mind you, many class A amplifiers don’t maintain full class A when driven hard, especially into low impedance loads, irrespective of whether they have sliding bias, plateau bias or fixed bias. 

   I’ve never measured the class A power capability of this particular model so can’t say how it stacks up, but in practice there will at least be a significant amount of class A power likely rendering any loss of sonics caused through the class B component unlikely. I guess you have to trust the specs Krell provide considering their many years of experience in design and manufacture of class A amplifiers.

   Hope that makes some sense

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Thanks mate for the explanation, it makes a bit of sense now. I like it loud quite a bit but I have fairly efficient speakers so it rarely changes bias but even when it does I can’t hear any difference. I only know because the lights on the amp change, maybe this would change if I were to try some harder to drive speakers. 

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No worries :)

When the bias level changes it still stays in class A operation, except if the maximum or top level of bias level drops down by one level if the heatsink temperature exceeds the 80 degree C threshold, then it will operate in AB mode.

Of course, dropping down from the top level of bias by one level normally it will still remain in class A mode if that thermal threshold was not reached. And as you said, if your speakers are efficient and relatively easier to drive, it’s far more likely to stay in class A operation.

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