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Is my Pass amp overheating?

The amp is an XA30.5.

Yesterday I was doing some tinkering and the amp was turned off for a couple hours (unusual for me). When I was done, I sat down to listen while the amp was still cold. To my surprise, it sounded BETTER than I'd ever heard it.

The amp is Class A, so according to conventional wisdom, it needs to warm up before sounding its best. Yet it definitely sounded better BEFORE it warmed up. So I'm wondering if the amp is overheating. To test this, I took the lid off the amp. Three things happened:

1. At idle, the bias needle moved from 11 o'clock to 1 o'clock (unprecedented).

2. The cooling fins became much HOTTER (not cooler, as I would have expected).

3. The sound quality of the amp remained "improved" even after several hours.

Can someone please explain what's going on?

Thanks,
Bryon

P.S. The amp is in a closet. But I don't think that's the problem, for the following reasons: The amp is on the top shelf of an equipment rack, so there is two feet of open air above it. There is a large fan in the ceiling of the closet that sucks air and sends it through a duct to the outside of the house. So the closet stays very close to the temperature of the rest of the house. Also, I can reproduce all the effects described above with the closet door open.
bryoncunningham
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almarg
9,670 posts
 
Great call, Elizabeth. Way to go!

Best regards,
-- Al
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bombaywalla
1,958 posts
 
Here's the most interesting part: Increasing the bias also improves the sound quality. A LOT.

good call Elizabeth! right on the money.....

Yes, increase in bias for a class-A will improve sound quality. This is a well-known fact. It is not used by most people at home 'cuz it means sticking your hand inside your amp & the amp manuf always assumes that the fellow @ home is a do-do & will electrocute himself. Not a bad assumption given that the insurance liability is very high.
Inside my amps there is a potentiometer that can be used to set the bias by turning the screw with a screwdriver & using voltage meter to measure the voltage across the emitter resistor. The best 'compromise' setting comes from the factory - good sound & not excessive heat BUT the many of us (after speaking with the factory) turn up the bias by as much as 50% for much improved sound. We have to be careful as we can take the output transistor out of its SOA very quickly. We use a factory provided jacked-up bias number to be sure.
Do you have the means to adjust your bias? If yes, you could do this one time & live w/ the increased heat. Might not be an issue if you have forced cooling?
I've heard that Pass/Threshold amps at idle had their heatsinks at 54C. This was normal. If you are in this range with the increased bias you might be fine long-term....
arh
366 posts
 
When you take the top off the amp you are losing the chimny effect that is used to cool the output devices. As Liz says you are hearing the different bias effects on sound. When we hear a difference in sound we tend to think it is initially better. This may not be the case in the long run with a lot of different music. Put the cover back on. Call Pass to verify everything is ok
Alan
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tpreaves
2,686 posts
 
Damn Liz, you are one sharp tack!!!!
bryoncunningham OP
890 posts
 
Using risers for the lid is a good idea. I was thinking about using some
copper mesh. It would provide ventilation as well as some RFI
protection.

There are lots of different mesh options in terms of perforations per inch.
Anyone know how small the holes have to be in copper mesh to provide RFI
protection?

Bryon
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