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Pass labs X250.5 cracking noise

Hi all, recently bought a used X250.5 from a dealer on audiogon. Really happy with the SQ. The amp makes a crackling noise after powering down. Its a few cracks/pops - very intermittent and then it subsides. Spoke with Pass engineer and he assured me that the noise was coming from thermal contraction of heat sinks etc after normal operation. This weekend I noticed that the amp made cracking noise during warm up too. Again this is one or two and then after the amp warms up - i dont hear any more pops. Or i guess the sound from speaker masks it perhaps? I did pause music and noticed the amp for about 30 minutes and no cracking/popping noise during that period. Anything I should be worried about? I'll call Pass again this week - but wanted to see if there were any thoughts?
pistha
jsd52756's avatar
jsd52756
437 posts
 
From my Krell owners manual: "When powering on any system, turn on amplifiers last. Turn off amplifiers first. "
I made the mistake of turning off the pre-amp first once & the sound that came out of the speakers was not pretty.  Scared the heck outa me. 
Perhaps that's the issue.  Good luck, John
pistha OP
31 posts
 
Hi John, Thanks - I pretty much do the same thing - Amp powered on last and powered off first. The noise I refer to is a mechanical popping noise (like a single popcorn bursting) from the amp - not the speakers. 
georgehifi's avatar
georgehifi
10,312 posts
 
If it were an XA- model then yes having far more class-A bias on them you would expect a certain amount of expansion and contraction with warm up and cool down noises.

But your’s is an X which is lower in bias than the XA, and should be getting that warm, unless you are driving it hard into difficult to drive low impedance/high -phase angle speakers.
What are your speakers and do you push the amp loud?

Also are the heatsink/s hot to touch after a couple of hours?
Be good if you could measure the surface temp in the middle of the heatsink on both sides as they should be equal.

Cheers George
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