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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
 
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
pistha OP
31 posts
 
George, I dont drive them hard - I dont think i ever exceed low 80db spl. I also notice that the bias meter doesnt ever leave class A for my listening level. I drive a pair of wilson sophia 3s. I dont think the sophias swing wildly in impedance either (per wilson). I've gota be honest - dont know what a high phase angle speaker is! I'll give the temperature measurements a shot - thanks for the suggestion!

Ragav 
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
Post removed 
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.
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