|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
50.183.218.111
In Reply to: RE: Question about Krell amp models posted by George S. Roland on January 22, 2015 at 10:23:41
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 heaterThey'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
Edits: 01/22/15 01/22/15Follow Ups:
As always, beautiful pic! Abe.
I think you are talking about the KSA200, the KSA250 never had that high an idle current the KSA200 did pull 1300 watts idle..
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.
Edits: 01/22/15 01/22/15
"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.
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.
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 OhmTotal 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
Edits: 01/22/15 01/22/15
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.
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
Edits: 01/26/15
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..
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.
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
Edits: 01/23/15 01/23/15 01/23/15
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: