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Old 20th May 2004, 10:35 PM   #1
K-amps is offline K-amps  United States
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Default Krell KSA-250 not true class-A?

I just got a Krell KSA-250 (reputedly full power 250 watts pure class-A per channel).

However as I powered it up and played it, initially I perceived less heat radiated than my Krell KSA-100 (100w class-A). Upon some investigation, I find the each of the 24 OP devices to be biased at 111mV or 0.111 amps into the 1 ohm emitter resistor.

Figure in some calculations, and I get a class-A OP of 28.6 watts RMS. About 200 watts dissipated as heat (rails are +/- 77dc).

Not exactly "pure" class-A is it? more like high biased AB or Whatchamacallit A/B?

I then read an article by John Atkinson (confirming the same)saying that Dan D'Agostino decided at the last minute to change the bias from 4 amps to 1.35 amps or so...

Does that qualify as outright misrepresentation? I mean all the hype / advertizing/ user manuals were written around it being a 250w pure class-A amp but it is not.

Regardless to say it sounds good and the 4.5kVA toroid is to die for!

K-
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Old 21st May 2004, 11:41 AM   #2
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I owned one of these a while back.

Definitely NOT pure class A. Otherwise it would dissapate
500 watts of heat which it clearly does not.

Call it high bias class AB.

No plateau bias or anything like that either.

Still a very nice amplifier.
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Old 21st May 2004, 12:29 PM   #3
K-amps is offline K-amps  United States
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So what do you use now and how does it compare to the KSA?
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Old 21st May 2004, 10:35 PM   #4
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krell fbp600

Much better than the ksa250 in every way except for
amount of heat generated.
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Old 25th May 2004, 06:19 PM   #5
K-amps is offline K-amps  United States
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I talked to a tchchie at Krell. He says the KSa-150/ 250 used auto-biasing and thus it was class-A all the way to 250 watts.

It then struck me that he was talking about the sustained plateu bias used in the "S" seires, he confirmed that the "S" series used the "sustained" version of the auto-biasing but the non-sustained version was implemented in the KSA-250...

I guess I believe him....
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Old 25th May 2004, 09:12 PM   #6
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Quote:
Originally posted by K-amps
I guess I believe him....
In that case you will thrilled by the "anticipator circuit" Krell
used to ascertain what the bias should be in advance.
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Old 25th May 2004, 09:20 PM   #7
K-amps is offline K-amps  United States
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Quote:
Originally posted by Nelson Pass


In that case you will thrilled by the "anticipator circuit" Krell
used to ascertain what the bias should be in advance.
Good to have you back and active on the solid state area Nelson !

Dare I say, do you imply that the anticipator circuit used some sort of "delay" for the main signal? And if so, would the induction of a KSA-250 in a HT system cause surround decoding anomalies that are time dependant?

If not please share your thoughts on the anticipator circuit,

best!

K-

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Old 26th May 2004, 08:57 PM   #8
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A high fixed bias "anticipates" the needs of the signal...or you could simply call it "sustained" in order to give it some marketing pizazz.

Grey
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Old 26th May 2004, 08:59 PM   #9
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Quote:
Originally posted by K-amps
please share your thoughts on the anticipator circuit
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Old 26th May 2004, 11:20 PM   #10
Gofer is offline Gofer  Netherlands
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Default outputvoltage independent biassing

Some time ( 1 week) ago I requested the fine members of this forum a schematic about a Yamaha MX-630 power amplifier. Peter Daniel was so kind to dish it up for me.
I am still puzzling how it works, but it looks like it has a biassing program of the ,,sustained'' kind.
The VAS has it's own (from the rest decoupled) powersupply that is riding with the output.
To me it looks like it is a outputvoltage independed biassing cirquit, in order to let the other halve of the output transistors conduct all the time at the preset level.
Maybe it's worth to take a look at it.
It would be interresting to try this with lateral mosfets because the working voltage of the VAS can be higher than the supply rails.This in order to have higher output voltage swings with the advantage of no crossover distortion.(LatMos don't have this to much compared to BJT's anyway).

I will never stop learning,

Greatings to all,

Govert-Jan
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