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Question about Krell amp models

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Posted on January 22, 2015 at 10:23:41
George S. Roland
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I am interested in the Krell KSA 250 power amp, but I wonder what 250 W/ch amps followed it in Krell's lineup and whether or not they were considered an improvement.

Is there a website that lists the products Krell has produced, when they were introduced and discontinued, etc?

Thanks,

George Roland

 

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According to Krell model history..., posted on January 22, 2015 at 11:31:12
kootenay
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The KSA-250 came out in 1990 along with the KSA-150 model. FYI link below...



 

RE: Question about Krell amp models, posted on January 22, 2015 at 12:02:50
AbeCollins
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Be aware that some of those old Krell amps can sound wonderful but models like the KSA-250 run in true Class A bias.... all the time. In other words, this amp draws about 1300 watts from your AC outlet continuous whether it is playing music or not. 1300 watts might require it's own dedicated AC line and breaker and at 1300 watts, it is essentially an electric space heater

They're built like tanks but hot running amps of that age will likely need service, especially the capacitors.

I like the KSA-xxS models. The "S" suffix denotes Krell's "Sustained Plateau Biasing" scheme which keeps the amp in true Class A operation so long as the music is playing but throttles it back when the music stops.

I had the baby Krell KSA-50S and for a 'small' 50wpc amp, that box drew about 800-watts continuous as the music played loud and the heatsinks became very hot. When the music stopped, it would continue to draw 800-watts for many seconds until it throttled itself back down to a more reasonable levels.

I believe some of the newer Krell amps use a similar biasing scheme. It's a way to get true Class A operation but only while the music is present so as not to unnecessarily heat up the listening room if the amp is ON but sitting idle.

My KSA-50s to the left back around 2005



 

RE: Question about Krell amp models, posted on January 22, 2015 at 13:23:52
denden11
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I had a conversation with Dan D'Agostino a number of years ago and he claims that the KSA-250 is one of the best sounding amps. that he's ever built. I love mine and would never consider replacing it.

 

RE: Abe, You mean KSA200 , posted on January 22, 2015 at 13:34:08
A.Wayne
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I think you are talking about the KSA200, the KSA250 never had that high an idle current the KSA200 did pull 1300 watts idle..

 

RE: Question about Krell amp models, posted on January 22, 2015 at 14:50:09
Des
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I've owned the Little brothers KSA50/100--they were the best Amps Krell ever made

I'd say yours with the same genes would be in the same ballpark

Just get the caps checked and all should be kosher

Nothing K made after came close to that set in my opinion.

Des

 

No, I think the KSA-250 draws over 1300 watts, posted on January 22, 2015 at 15:48:03
AbeCollins
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Unless the article is incorrect.....

Within the body of the review Stereophile says the KSA-250 is fully Class A and draws 12-Amps. The specs page at the end of the reveiw also states 12-Amps continuous.

At 115 VAC nom. and 12A, that would be ~1380-watts.



 

RE: No, I think the KSA-250 draws over 1300 watts, posted on January 22, 2015 at 16:33:35
A.Wayne
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Krell Advertising talk Abe ....

" The Krell's rated output of 250W per channel into 8 ohms implies a standing bias current of 3.95A per channel—square root of 250W/(2 x 8 ohms)—if all this power is to be delivered with the output stage running in class-A. Assessing an amplifier's bias current is not straightforward, however, particularly when, like the Krell, it doesn't have a fuse in series with the rail voltages.

Looking inside the KSA-250, the emitters of the 12 pairs per channel of output-stage transistors appear to standing on series resistors of nominal 1 ohm value. The average voltage drop across these resistors was 110.5mV, implying a standing bias for each of 110.5mA; ie, a total of 1.33A. This will give a maximum power for true class-A operation into 8 ohms of 28.5W (14.5dBW) rather than 250W. " - John Atkinson

So
28.5 @ 8 ohm
14.25@ 4 ohm
7.12@ 2 Ohm

Total class-A Power .

Thats pretty close to what we measured when checking the idle on the 250, the 200 would pull 9.8-10.4 amps at idle, one of the mods we had for that one was to lower the bias after recap closer to 1 amp per side, we all felt it sounded better than the high bias and sure as hell didn't want to melt stuff .. :)

Regards

 

RE: No, I think the KSA-250 draws over 1300 watts, posted on January 22, 2015 at 17:06:08
A.Wayne
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I do remember some were measured at 8-9 amps draw over at DIY, some as low as 7 , my thoughts were those amps were in need of a re-bias. The KSa200/250 were power house amps thou..


Regards..

 

RE: Question about Krell amp models, posted on January 22, 2015 at 18:14:06
fantja
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As always, beautiful pic! Abe.

 

Thanks a lot! NT, posted on January 22, 2015 at 20:16:27
George S. Roland
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Thanks a lot! NT

 

RE: No, I think the KSA-250 draws over 1300 watts, posted on January 22, 2015 at 21:30:59
AbeCollins
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I've never owned one or measured one. I only read what was published in the Stereophile review.

Your measured 7A - 9A @ 115VAC nom. is still a hefty 800 - 1000 watts of continuous power draw and heat dissipation indicating very much a Class A biased amp.



 

The KSA50 or 100 are on my "Amplifier Eleanors" short list... nt, posted on January 23, 2015 at 05:48:38
Dman
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nt

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RE: No, I think the KSA-250 draws...1440 watts, posted on January 23, 2015 at 06:03:08
cdb
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"115VAC nom." is long gone. It's now 120VAC nominal per ANSI C84.1 Assuming your utility company adheres to this standard, they are allowed a +/- 5% swing in this 120V number for normal operations, as delivered to your meter lugs; 114-126VAC.

What happens downstream from there is your problem. A utility has no control over interior wiring quality in a customers location.

Per NEC, a branch circuit load in operation for 3 hours or longer is limited to 80% of it's nominal rating. Thus a typical 15A residential branch circuit is limited to 12A continuous.

Krells' power consumption rating of a nice round number (12A) seems a bit convenient and possibly due to the circuit restrictions of the NEC.

Otherwise you are correct. The Krell has an electrical 'mileage rating' along the lines of an Abrams tank.

For a teardown vid of a KSA-250, see:
https://www.youtube.com/watch?v=Ng3RlJb1SsU

Not much chance of air circulation inside that chassis, IMO. Especially considering the power consumption rating.

 

RE: Question about Krell amp models, posted on January 23, 2015 at 06:09:31
Mr Peabody
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The KSA are definitely Class A and put out a bit of heat. I do agree, IMO, they were probably the best sounding amps Krell put out. If wanting a bit more modern Class A you might consider looking at the Pass Labs.

 

RE: No, I think the KSA-250 draws...1440 watts, posted on January 23, 2015 at 07:12:53
AbeCollins
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I don't have a problem downstream. I don't own the Krell KSA-250. My only point is that it is a true Class A biased amplifier that draws a huge amount of continuous power, likely well over 1000 watts. It runs hot and anyone contemplating buying this amp should be aware that such an old amp that runs so hot might need servicing. If the amp hasn't been serviced by a previous owner, it will likely need servicing now or very soon.

I don't think +/- 5VAC nom. on the AC mains is going to alter the fact that this amp is a space heater but thanks for the clarification on the current 120VAC standard.




 

RE: Question about Krell amp models, posted on January 23, 2015 at 07:24:48
AbeCollins
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I'd be curious to know how much power that KSA-250 truly draws from the AC mains while sitting there idle. Have you ever measured it with one of those inexpensive Watt meters? I would love to try one of the old Krells again someday but probably not in the summertime. ;-)

I thought my baby Krell KSA-50S was an incredible sounding SS amp. The only other Krell amp that I've owned since then was the KAV-2250 which was painfully dry and harsh. It had kind of a gray and sterile sound to it, lacking any soul and killing all musical enjoyment, almost like my 7BSST2 Brystons I had but these newer Brystons were at least smooth and not grainy like the KAV-2250.

Kill A Watt power meter. $20 USD
There are various models of this meter with additional features but the basic model is more than sufficient to measure the power consumption (in Watts) for electronics and appliances around the home.





 

RE: No, I think the KSA-250 draws over 1300 watts, posted on January 23, 2015 at 08:07:11
A.Wayne
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Yes, but i said those amps possibly were in-need of re-biasing ( bias was all over from as low as 5-9 ) notice what Atkinson actually measured it pulls no where near 7 amps at idle ...


" The Krell's rated output of 250W per channel into 8 ohms implies a standing bias current of 3.95A per channel—square root of 250W/(2 x 8 ohms)—if all this power is to be delivered with the output stage running in class-A. Assessing an amplifier's bias current is not straightforward, however, particularly when, like the Krell, it doesn't have a fuse in series with the rail voltages.

Looking inside the KSA-250, the emitters of the 12 pairs per channel of output-stage transistors appear to standing on series resistors of nominal 1 ohm value. The average voltage drop across these resistors was 110.5mV, implying a standing bias for each of 110.5mA; ie, a total of 1.33A. This will give a maximum power for true class-A operation into 8 ohms of 28.5W (14.5dBW) rather than 250W. " - John Atkinson


A total of 28.5 watts per channel class-a power ......!!!! there is no way it's pulling 1300 watts at idle..


Krell manual:

B.
POWER CONSIDERATIONS
The KSA 150 is rated as delivering 150 watts per channel into 8 ohms. The
amount of Class A power is adjusted for the finest sonic performance with a minimum of generated heat and energy consumption. The amplifier’s maximum power into 8 ohms is 220 watts per channel. The KSA 250 is rated as delivering 250 watts per channel into 8 ohms.

This bias scheme was developed due to heating issues in the KSA200, which was biased at 11-12 amps at idle, the KSA250 bias scheme was to eliminate the high class-a bias and heat and puts out 28.5watts class-a @8ohm as measured by JA, Instead of the 200watts class-a power as put out by the KSA200. KSA250 pulling 9 amps is out of bias and needs to be checked, as i stated earlier...


Regards..

http://krellonline.com/assets/support/KSA150250_MAN.pdf

 

Does anyone know what this means? NT, posted on January 23, 2015 at 09:08:18
George S. Roland
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NT

 

RE: Question about Krell amp models, posted on January 23, 2015 at 10:18:23
denden11
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Hello Abe. I have measured mine many times over the years and it consistently pulls 12 amps at idle. It is on a dedicated 20 amp circuit. There is nothing else on that circuit. It does a pretty good job of heating the room (26'x17'x8'). In case anyone doubts the validity of your numbers and those advertised, the KSA 250 went thru a change during its life. People complained about the heat and Krell reacted by lowering the bias. My amp is the high bias. You can tell by looking at the back of the amp. The location of the binding posts are different. Everything else looks the same and there was no model identification change.

 

Eleanor was "THE" car in a movie. the ultimate Mustang to steal in 'Gone in 60 Seconds' I think, posted on January 23, 2015 at 10:54:59
So the Eleanor list would be the most wanted to beg for borrow, orsteal.
The ones you would do almost anything to own.

 

RE: Question about Krell amp models, posted on January 23, 2015 at 11:15:27
A.Wayne
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What year was the change , i had two and the only one had that high a bias was the ksa200, Stereophile's test amp was also low bias (28 watts class-a) ...

 

RE: Question about Krell amp models, posted on January 23, 2015 at 13:28:25
denden11
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My purchase order was dated 8-16-1990. SN# 6735. Bias was cut back sometime after that date. If you need more specific information, I'd suggest giving Krell a call. If you're in Michigan, you are welcome to come over with your VOM.

 

So, do you want VERY early "fan amps," or the slightly later versions?, posted on January 23, 2015 at 15:39:29
David S.
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Me? I'd be on the fence.

For a very small system, I'd probably want a late model KSA50 with the external heatsinks.

In a slightly bigger room? I might go for a pair of KMA100 or a solo KMA200 amp with the fans.

People who think the old Krell were nothing but brutes that focused on bass slam probably never heard them in the right systems.

 

RE: Question about Krell amp models, posted on January 23, 2015 at 18:50:17
The new Vanguard integrated is one of the best amps they have made...shockingly good !

 

BINGO!!! Thanks for posting that... nt, posted on January 23, 2015 at 19:04:46
Dman
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nt

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I forgot about that. Thanks for the post! - nt, posted on January 23, 2015 at 19:28:16
AbeCollins
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.

 

RE: Question about Krell amp models, posted on January 26, 2015 at 10:01:17
A.Wayne
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More clarification :


[11 amps is much too high at idle. It should be closer to 2 or 3 amps and will then proceed upwards as the volume of playback increases.

Patrick Bresnahan
Sales, Technical Support, Service
Krell Industries LLC
45 Connair Road, Orange, CT 06477 - USA
Pbresnahan@krellonline.com
P - 203-298-4017 | F - 203-891-2028 ]


As confirmed and measured by Stereophile, Did not confirm if there was a model split....


Regards ..

 

RE: No, I think the KSA-250 draws over 1300 watts, posted on January 26, 2015 at 12:21:22
samman11
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My Krell KSA 100 was recently refurbished by Krell and they DID turn down the bias a lot. I believe they found the lowest distortion with the new transistors in place to be about 85mV for idle bias. The amp runs quite cool but the sound is to die for. No more heat.

 

RE: No, I think the KSA-250 draws over 1300 watts, posted on January 26, 2015 at 13:33:25
A.Wayne
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When we re-capped the ksa200, the bias was lowered alot from 11 amps to 2 amps and the sound was better IMO , the amp ran warm to the touch at 1 ohm.

When purchasing the KSA250 back then, i was told it was much lower biased than the ksa200, for safety and temp reasons , they were using a new sliding bias scheme ...


Regards

 

RE: Question about Krell amp models, posted on January 26, 2015 at 17:10:41
denden11
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Boy---this is really bugging you isn't it. Are you anal or just want to be King ? I happen to know Patrick very well but Dan is the guy that built my amp.

 

RE: Question about Krell amp models, posted on January 26, 2015 at 18:21:34
A.Wayne
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Projecting ?

What information hurts, I also know the parties involved and really, so what, others should go by you only, wake up lunchbox, it's not a cancer cure, so save it, your beef is with Krell and stereophile, but go ahead, make it personal while sprouting off more dis-information ...

 

RE: Question about Krell amp models, posted on January 27, 2015 at 05:37:06
denden11
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I don't have a beef. I was talking about my personal amp. which I've owned for the past 25 years (unlike you). You were challenging the validity of the specs on "my" amp.

 

RE: Question about Krell amp models, posted on January 27, 2015 at 08:31:24
A.Wayne
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I wasnt challenging anything or anyone, you were the one who decided to make it personal, my only intent was providing data I'm aware of after owning and using Krells over the years and pay close attn, Krell confirmed my assertions, so did JA in his stereophile test, just because you have one that doesn't fit, go take it up with Krell and i never disputed your take, i said i was not able to confirm nor deny, with basic english comprehension that means it stands, that early versions may be biased like the older KSA200 and later versions not , as tested by Stereophile , i dont know, i can tell you its more likely that one is out of whack if it measures 11 amps than not, maybe Dan biased yours that high , i dont know and frankly dont care.

As to my experience with Krells over the years ,

KSA100
KSA300
KSA200
KSA250
FPB300
FPB300cx
FPB600cx

So your 25yrs with one amp means nothing to me, I was only providing information that may or maynot benefit others, not some head in the sand bunk.

Now go call Krell up and straighten them out, apparently they dont know what they are talking about, email Atkinson too .....

Regards.


PS: you should try lowering the bias on yours, you might be in for a suprise..

 

RE: Question about Krell amp models, posted on January 27, 2015 at 10:24:04
denden11
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Experience?

KSA 50
KSA 80
KSA100
KSA150
KSA250
PAM 5
KSP 7B
KRC
KRC HR
Stealth

 

Not to quibble, but Krell never made a KSA300, posted on January 27, 2015 at 10:51:35
David S.
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Sorry, not my fight, but if you both want to get all hot & bothered over details, I'd just point that one out.

Krell DID make an MDA 300 (mono version of the KSA 150, which was a great amp.)

Krell also made a KSA 300S - one of their "Sustained Plateau Biassing" series of amps. This was also a pretty nice amp.

BUT, they have never made a KSA300 or KSA 300 amplifier.

 

RE: Not to quibble, but Krell never made a KSA300, posted on January 27, 2015 at 11:43:27
A.Wayne
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Yes, KSA300S, Typo, semantics really , left off the S designation, kinda like you saying KSA300 twice ... :)

Regards..

 

RE: Question about Krell amp models, posted on January 27, 2015 at 11:52:33
denden11
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A.WAYNE----You know what----I apologize. I'm an old man and the older I get the more I let things get under my skin. Don't know why. I'll try to do better.

 

RE: Question about Krell amp models, posted on January 27, 2015 at 12:35:57
A.Wayne
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Denden11,

No apology necessary , these things sometimes get carried away , in my laziness in typing i may have not expressed correctly what was discovered and may have given you the impression i was discarding your input , that was not my intent.


Regards

 

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