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Krell FPB - one channel hot; the other barely warm

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Posted on April 9, 2010 at 15:07:58
JerryS
Reviewer

Posts: 2026
Joined: February 24, 2001
 I just received a used FPB 300cx stereo amp. It has been making nice background music for a few hours. The left channel's heat sinks are hot - too hot to touch for more than a moment. The right channel's heatsinks are just warm, and I can comfortably rest my hand on the top of them indefinitely.

Is this normal?

Thanks,
JerryS

 

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RE: Krell FPB - one channel hot; the other barely warm, posted on April 9, 2010 at 15:29:48
varkdriver
Audiophile

Posts: 340
Joined: July 28, 2002
 This would generally indicate a bias issue. The FPB has autobias, so I would expect a control chip or other component has failed. I would talk to Krell.

 

RE: Krell FPB - one channel hot; the other barely warm, posted on April 9, 2010 at 15:51:44
AbeCollins
Audiophile

Posts: 46302
Location: USA
Joined: June 22, 2001
Contributor
  Since: February 2, 2002
 I'm not familiar with the FPB series but on my old KSA-50S that I once owned there were LED indicators for each channel. Both sets of LEDs (Left and Right) would light up simultaneously showing the bias level of each channel. They were always in sync, i.e. same number of LEDs lit up per side.

I'm not sure if the FPB series has the LED indicators but if one channel is significantly hotter than the other for no good reason, that doesn't seem right.


 

Had the same problem (among others) with my KSA-250, ..., posted on April 9, 2010 at 16:24:07
Neil49
Audiophile

Posts: 8850
Location: Northern Virginia
Joined: June 13, 2003
 and after returning the amp to the factory for warranty service, the unit came back without any description of what was done, and the same temperature imbalance. Incidentally, the tweeter on the channel corresponding to the hotter side of the amp had burned out--never ascertained whether the failure was related to the temperature (bias?) issue, but I lost confidence in the amp and its manufacturer after this episode and previous, more serious problems.

 

RE: Krell FPB - one channel hot; the other barely warm, posted on April 10, 2010 at 10:20:47
JerryS
Reviewer

Posts: 2026
Joined: February 24, 2001
 There are 2 trimpots: R236 and R179. Does anyone know if one of these adjusts bias, and if so, which test points are used and the nominal reading?

Update on my original post: The difference between the temperature of the heatsinks is much less (both channel are hot) after an hour or two of listening at fairly high levels. However, the left channel is still hotter than the right channel.

This is one mighty fine sounding amplifier!

Thanks,
JerryS

 

RE: Krell FPB - one channel hot; the other barely warm, posted on September 23, 2010 at 14:27:03
Charles Peterson
Audiophile

Posts: 44
Joined: September 6, 2001
 I just observed large imbalance in heat in my Krell FPB 300 that had been serviced last year (2009).

In fact, I get large imbalance just letting amp idle. I left amp idle for days, left channel got extremely hot (touchable for 1.5 seconds max) right channel just nicely warm. I've decided never to let this amp idle, it wastes loads of power and may not be good for the amp either. There's never a problem with cold sound, the autobias cranks up the moment you press the start button.

Now I have wattmeter on AC power, it seems Krell draws 480 watts instead of 350 at idle. Just playing FM at normal levels on my Acoustats I was chewing up 1000-2000W (!) Goodness knows when I'm really rocking. Even drawing 2000W it takes awhile to heat up (huge thermal mass) but once it does heat up it doesn't seem heatsinks dissipate enough heat for longevity.

In my case, and also original poster, there may be HF problem (evidence: burned tweeter for poster; in my case I am working with modified Sony XDR-F1HD a notorious RFI generator.

Krell has bandwidth to 200Khz or beyond, may be amplifying stuff you can't hear.


 

Could be an RF or oscillation problem. , posted on September 23, 2010 at 15:38:49
jihad
Audiophile

Posts: 2342
Location: chain o' lakes
Joined: February 20, 2003
 I'd have it checked out if you plan on keeping the Krell.

It doesn't necessarily have to be a problem with the amp, could be an IC or source.



Never attribute to malice that which can be adequately explained by stupidity.

 

RE: Krell FPB - one channel hot; the other barely warm, posted on September 25, 2010 at 07:39:18
Nglazer
Audiophile

Posts: 365
Location: New york
Joined: June 22, 2003
 I have been running an FPB 300cx for some 8 completely trouble-free years. What you are experiencing is abnormal and you should have Krell look at it. And it IS one sweet-sounding amp. Never heard any benefit in moving to more current versions of Krell amps (no pun intended). The FPB series may well have been the nest Krell has ever made.

Neal

 

RE: Krell FPB - one channel hot; the other barely warm, posted on May 8, 2011 at 02:42:15
Perigo
Audiophile

Posts: 87
Location: Italy
Joined: September 20, 2004
 The Krell FPB series has two microprocessors on bias cards. One per side. When service operates to change something inside (bias or siderail cards, above all) these microprocessors needs to be resetted. The "reset procedure" is very simple and in 1998 it was operated on my Krell FPB300, but I don't remember exactly how. It would be interesting if in these thread someone from Krell service or other technician that knows it, posts here for all.
Thanks.

 

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