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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's avatar
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almarg
9,670 posts
 
Hi Bryon,

Elizabeth's answer strikes me as reasonable. For additional information see this paper.

Note the section on "Material Homogeneity" in particular.

Best,
-- Al
arh
366 posts
 
The copper mesh with any size holes will not provide RFI protection
Alan
bryoncunningham OP
890 posts
 
Hi Alan. Here's an except from the article that Al linked...
In general, apertures should be smaller than L = l/50 [where L=length of slot (in meters) and L ³=w and L >> t and l = wavelength in meters]. To achieve acceptable attenuation values at a frequency of 1,000 MHz (not unusual for high-speed digital devices) apertures should not exceed 6 mm.

Actually, it's quite remarkable that mesh with apertures up to 6mm provides effective RFI protection, since 6mm apertures look like this. That mesh is 77% open and would provide more than enough ventilation.

In fact, I was thinking about taking Elizabeth and Al's advice and choosing something more like this, which has 1.3mm apertures and is 67% open.

Thanks Al and Elizabeth for your help. Very appreciated!

Bryon
koestner
670 posts
 
I would think small holes would provide protection as that is how a microwave oven glass door works. We see through the small holes and the waves can't penetrate the screen.
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