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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
georgehifi's avatar
georgehifi
10,312 posts
 
Between 60hz and 180hz the Sophia 3’s are a hard load 3ohm with up to 50 degrees of negative phase angle (the dotted line is the -phase angel), it could make the 3ohms look like 1.5ohms to the amp. .

https://www.stereophile.com/images/211Wilfig01.jpg

Measure both heat sinks after a couple of hours playing normal level bass’y music. The heatsinks should never go over 60C. and that’s on the XA., the X I would expect to be less.

Cheers George
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pistha OP
31 posts
 
Called Pass and the engineer assured that this was just the heat sink making the noise from thermal expansion and contraction. Apparently the heat sink can be more tightly secured with a couple of bolts behind the faceplate. 

btw George - i measured the temperature on the fins and they were reading 100F & 110F on left and right sides. I mentioned this on the call with Pass and they didnt think it was a wild swing. 

Now i know this is a minor annoyance at best - am not going to do anything but enjoy music! thanks everyone.
georgehifi's avatar
georgehifi
10,312 posts
 
btw George - i measured the temperature on the fins and they were reading 100F & 110F on left and right sides. I mentioned this on the call with Pass and they didnt think it was a wild swing.
This is fine one side could be in a draft more than the other no biggie.

Just wanted to make sure the bias didn’t go out of whack on one channel. I’ve had much experience with amp heat, making quite a few massive (two man lift) 150w pure Class-A amps with self contained water cooling systems.

BTW: Also not having the bolts too tight can allow movement without creaking.

Cheers George
pistha OP
31 posts
 
This is fine one side could be in a draft more than the other no biggie.

I heard Pass mention the same thing about draft - silly me - i assumed he was talking about air draft from a/c vent! Now that you mention - what exactly is draft in the sense you mention above? 
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