Krell FPB 600c 700cx 400cx Heatsink noises.

In 2001 I bought my complete new Krell cast music system: Krell FPB 600c amplifier, Krell KCT preamp and Krell KPS28c cd-player .Siltech cast interlinks. Speakers are JM Lab Utopia with Transparent ref.XL loudspeaker cables. However I certainly have the feeling that all the manufacturing dollars have been spent on sonically critical design elements,there is a mechanical problem,which is bothering me since the moment of purchase.Two or three times during a listening session the metal construction of the heated amplifier cabinet in operational mode makes a sound (tsssjakkk).Probably because the difference in heat of the heatsinks and/or heavy gauge alluminium chassisplates. When I bring the amp in stand by position after listening,this mechanical sound is there also ,several times. I phoned the Krell service department in 2002.Irving Gross told me that it was a diffraction problem.(traction and contraction of the heated amplifier cabinet.) The service department advised me to tighten all the bolts of the heatsinks. It did not help. During the upgrade from 600c to 700cx,end 2002, I told the German importer about the problem. He admitted that this diffraction problem was a tough one to solve.I had the hope that the problem would have been gone after the X-Upgrade but unfortunately it did not. In the english Hi-Fi News magazine of december 2005 (page 28/Howard's Way) I read an article about a former owner of the Krell 400cx."I sometimes got noise from the heatsinks through heating and cooling." He finally could not live with it and sold the amp. My question is ;does anyone has the same problem with this Krell FPB amplifier(600c,400cx or 700cx)? More important, which owner has a smart suggestion to eliminate this undesirable mechanical'vibration'. Otherwise...I figure I have to live with it, which I can because I simply love the sound of my complete Krell cast system.
I have a 400cx and the only time i hear the heat sinks is when there is no music playing. Like you I can live with it because of what the Amp does so well.
My 600c will make an occasional pop while heating up or cooling down but it is not excessive. It is such a great amp, I overlook it.
My friend leaves the krell amps on the entire day starting early in the morning, when we are having a listening section.

I have a 400cx and it does make that thermal expansion sound - almost like pinging the heat sink with a small mallet. I talked to Krell a service tech who told me essentially the same thing he told you, that’s "normal" operation. He also told me that they had a fix; send the amp back to them and they apply some kind of tape to the heat sinks. I don't know how effective this is. He did not mention tightening the heat sink bolts.

I decided to live with the occasional ping as opposed to shipping 110 pounds of amp across the country.

If the sound really bothers you could contact your importer or Krell directly to find out more about this fix.

Bob R.
A friend of mine has an FBP600 that has the same problem, but only a few minutes after being switched off, so it is not disturbing at all. Your (tsssjakkk) really describes the sound it emits!
Years ago I had a Jeff Rowland Model 8 that had the same problem - the solution was just the opposite - I fitted some nylon washers in the bolts and did not be over-tighten them. It solved the problem completely. But as the case structure of the Model 8 is quite different from the FPB one, perhaps the receipt is not adequate to your amplifier.
Anyway if you choose to tighten the bolts of your amplifier you should do it at the correct temperature and in a selective way - the noise means that some mechanical energy - tension - is released very fast - if the parts are allowed to move slowly there will be no noise. It is a question of knowledge or luck - but as most Krells do not have this problem yours should be solvable.
My 400cx does this but only once or twice during several hours of listening. Sometimes it annoys me a bit, but as Weiserb observes, leaving it on for extended periods minimizes the occurences. At this point it still isn't enough of a bother that I want to ditch the amp because everything it does so well balances this small annoyance out. But philosophically, Krell should provide a solution to the problem, simply because anything the equipment does to detract from musical pleasure should be addressed. Thanks for informing us as to the cause. Perhaps a layer of polymer between the output devices and the heatsinks would cure the problem. How have other manufacturers avoided this problem? I'll give Krell a call now that this has surfaced here.
My pinging stops once the amp is warmed up [300C]. It pings a couple of times upon warm-up and cool down though. It's no big deal to kind of lets me know when it is good to go...

Microstrip, out of curiousity, I assume you put the nylon washers between the bolt head and the fin and not fin and the structure of the amp. Did you notice whether the amp ran any hotter?

Pauline, I think we all would like to hear how you fare with this.

Good Luck.
Bob R.

I had an FPB600C and had the same problem. It did not disturb me. For the first one or two hours you hear some pop sounds for every 15-30 minutes. After that is was quiet but after switch off it started again when cooling down.

Now I bought a new FPB700CX and this one does not have that problem. So Krell did something with the heatsinks.

However it was never a problem to me. You hear when the amp is getting warm and when the pop sound stops you know it is warmed up completely. Of course leaving it on all the time will solve this problem but costs a lot of energy.

Off topic, but this seems to be a good captive audience:

Are any of you guys using a tube pre-amp? I have been kicking around the idea of the Aesthetix Calypso or an Audio Research SP-25 MkII or the newer SP-26.

I read in the manual about some caps, or something, that need to be put in if a tube amp is used...
Dbld, I have never had a problem running any Krell amp with a tube preamp. The only time there is a problem is if the preamp leaks DC. The older BAT pres did cause the protection circuitry to shut down many amps that had no capacitors in the signal path.
Thanks - I am completely naive about tube preamps. I have done a lot of thread searching but I still feel like I am flying blind.
Dbld, If I recall, use of a tube pre-amp without the changes to the amp could affect your warrantee on the amp. There was another thread on this subject last summer but it seems to have been deleted. Lots of folks reported using tube pre-amps with Krell amps without any ill-effects. I use a KCT with a CAST connection, so it's not really an issue for me. I think your best bet is call or email Krell technical support.

Bob R.
I use an Audio Research Ref 3 tube preamp together with a Krell FPB700CX. No problems at all. The new tube preamps all have a DC offset which is only microvolts.

The Krell amps are DC coupled and if your tube amp has a DC offset if will pass through the Krell directly to the speakers. Putting a cap will prevent DC to pass. This is the reason that the caps are introduced to prevent DC to pass.

If you have a good voltmeter you can measure the DC offset of your tube preamp. But is should be close to 0 so I gues with the modern tube preamps there is no problem for this.

One of the main purposes of coupling capacitors at the output of tube preamps is to block DC. A properly designed tube preamp running at spec should be putting out very little DC. I measured mine and it is less than 5 millivolts when fully warmed up. Best advice is to let the tube preamp warm up 1/2 hour before taking it out of mute. When cold, the DC voltage is around 25 millivolts. So, do measure the voltage at the RCA jacks on whatever model you wind up getting.
Sprbggr, I have a 700cx and have been entertaining the thought of getting one of the new EVO preamps. This way I would be able to use CAST. I have also wondered how good the REF3 or LS-26 would sound with the Krell amp. Did you make any comparisons before you decided on the Ref3?

Surely,I put the washers between the bolt head and the plates!
The temperature did not change.

Just read from my new ASR Emitter II exclusive manual:

Noise from the main unit while warming up/cooling down - remedy loosen the screws located in the heatsinks of the front-and/r backplate of the ASR.

But happily it was not needed -my unit is completely silent!
Thanks Bob R. I remember that thread but cannot find it anymore.

Peter, you are using the Ref. 3 and I have been considering the LS25 MkII. Did you have to have anything done to your 700CX when you got the Ref 3? Do you know if the LS-25 MkII would be OK without changing the inputs on the Krell?

Sorry everyone about the highjack...I always worry about the "anti-krell" crowd when posting anything referring to "Krell" in the title of a thread. Sad but true.
Hi I did do nothing to my FPB700CX. It just works fine. The LS-25 or 26 will also work without problems. The Krell will for sure not go broken.

Well I know there is a lot of anti Krell crowd but their power amps are world class. I had Classe, Levinson, Pass, Chord ad many more. Nothing even comes close to the FPB700CX. The new Krells sound very sweet in the midrange and have a very controlled bass. Also it is just very enjoyable to listen. Also their CD players and preamps are very very good. I had KCT, KPS28C, FPB700CX together and the sound is close to perfect. But the Wadia 581 + Audio research Ref3 is still a little better tha KCT + KPS28C.

Other amplifiers are good as well but to my taste I prefer Krell.

There is a super easy fix for this! Loosen the heat sink bolts just a slight amount each and the problem will be gone. I specialize in thermal engineering so I know all about it. It won't hardly change your thermal impedance so don't worry about that. Mechanical resonance is a lot more sensitive to contact pressure than thermal resistance is.

I am surprised Krell told you to tighten the bolts. That is totally against the laws of physics and will never solve the issue (as was found out). The ASR guys however, they did their homework. Hard to beat those Germans.

Aball, that makes sense. I am going to do that very thing this weekend and will report back. NOw if I can just find my torx drivers...

Hopefully it will be obvious as to where the bolts are located and not require further diassembly.
Almost everthing audiophile goes against the laws of physics Aball.
I'm no technician but I would think that placing thin teflon washers where the heatsink is secured to the body of the amp might help. One thing for know that the heat sink is doing its job... disipating heat!
Rwwear - hahaha. Well it sure does seem that way, doesn't it! But thinking that is missing The point. Your ears aren't making it up - or at least I hope not. :)

Loosening all the bolts and all 96 heatsinkbolts, just a slight amount each... and the annoying mechanical noise is completely gone! My Krell FPB 700cx amplifier is now not only sonicly but also mechanically drop dead quiet. Thank you all for your kind mail and smart suggestions, specially Microstrip, Stevecham and ABALL. It took some time before I could inform you about my experiences. Because the final solution is just the opposite as the Krell Service Department advised me to do in 2002 (tightening instead of loosening); most of the bolts were over-tightened and very difficult to loosen. About 12 heatsinkbolts were slightly damaged (they are easy to strip). I had to order new heatsinkbolts before I made an attempt. It was a teachable experience with satisfying results.
Great! Glad to be of help. Enjoy the music (and the silence).

I have this amp, and I get a pop here and there on warm up and cool down, but once it is fully heated up I don't hear a thing.

Anyway, music is usually too loud anyway. : )
I'm glad to see this discussion. My Krell FPB 300 (not c or cx) was quite noisy. Whenever the music got softer for a bit, I'd hear the ping (or as I called it, klink). I figured that loosening all the bolts would help and was surprised when when a former Krell technician didn't agree and suggested thermal tape instead. I virtually fixed the problem by removing the top cover completely. Many of the top cover bolts were extremely tight, overtightened for sure, and I wouldn't have been able to loosen them without a Wiha handlebar style Torx driver. However it looks like amp should not be moved without replacing top cover since it is necessary for mechanical integrity. Now that I've discovered this thread, I may try loosening all the bolts, though I was afraid of doing anything that might affect output transistors. But right now, I've had one channel go bad anyway, might try that before shipping to Krell for repair or upgrade.
My Classé amp once started making a single, loud metallic ping sound whenever it was powered up. It turned out I had torqued the screws holding the top cover too tight after giving the amp a checkup. Loosening them a tad bit solved that issue.
I bought 2 300Cx back in 2003 when I was on assignment in Singapore to drive the Infinity Beta. One of the amp had this noise about an hour into listening. I called the Krell dealer in Singapore Ultralinear about the problem and they did nothing to solve it. They told me it was caused by separate construction of the amp main body and the heatsink. When the amp heated up the heat caused the metal parts to expand. I had been a loyal Brell since the 80s. I gave up completely on Krell after the 300Cx.
I owned the 400cx and now own the EVO 402 and have never heard a peep out of either amp. I listen to all kinds of music, leave the amp on all the time, listen loud, listen soft, listen to the TV/dvd (sometimes, opera mostly).
Those amps have never done anything but make music. My listening room can get warm or be cool so I am surprised that my experience is different from many on the list.
The noise is more common to the earlier runs of the FPB series Hunter. The later issues had sound deadening washers added.
I'm back trying to get the noise out of my fpb 300 again. Loosening screws often seems to help. But which, if any, of the outside screws on the left and right side heatsinks actually attach semiconductor devices? I'm not sure I'd want to mess with any of those.
8 years with a 300cx and never heard any type of noise. Amp performs flawlessly and has been problem-free.

I have just now tried the solution being described here, loosening all the external bolts on my Krell FPB 300. All by hand (no torque wrench) I set them all to a snug but not hard torque. If anything, the noises have gotten worse.

It's a pity there is so little information about FPB construction online. I would like to know if the heatsinks, which are apparently attached to 3 rails along the side of the amplifier, can be completely removed. My initial fear that simply loosening the bolts could cause an electronic failure were apparently unfounded. But taking this one step further and actually removing each heatsink to check and clean the rails behind it, I'm not sure if that can be done safely. I'm pretty sure most noises are caused by small movements of the bolts, heatsinks, and rails under varying stresses caused by differential thermal expansions.

I'm going to continue trying different methods of torquing the bolts for now. I'm getting a wrench to set the torque consistently to some level between 10 lbin to 50 lbin. I can try low and I can try high. Also there's a question of HOW you torque the bolts. For example, should you do it when the amp is cold, when it's idling, or when it's going full blast? It's probably safest to do it wihen the amp is cold, but probably more useful to do it when the amp is idling or hot.

The factory and one other service guy has suggested putting heat conducting tape on the rails. At first, I didn't like that idea at all. But I may ultimately have to get that done. Also I wonder about the washers that were allegedly used on later runs of the FPB series, I wonder if I can add those washers to my unit.