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In Reply to: RE: No, I think the KSA-250 draws over 1300 watts posted by AbeCollins on January 22, 2015 at 15:48:03
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 01/22/15Follow Ups:
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
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