Is my Pass amp overheating?

And the winner is... Elizabeth!

Just talked with Kent at Pass Labs. Nice guy. Kent said that, by removing the lid, the temperature inside the chassis falls (that much is obvious). When the internal temperature sensors detect falling temperature, the amp increases the bias. That's why the bias needle went from 11 o'clock to 1 o'clock. And when the bias goes up, the cooling fins get hotter. So...

lower internal temp -> increases the bias -> hotter cooling fins

...and...

higher internal temp -> decreases the bias -> cooler cooling fins

Makes total sense.

Here's the most interesting part: Increasing the bias also improves the sound quality. A LOT. Kent didn't seem surprised by that. He said that the bias level is determined by weighing various considerations, including not only sound quality but also heat output in the average domestic environment and safety issues associated with very hot cooling fins.

I mentioned to Kent my intention to leave the lid of the amp off. He wasn't too supportive of the idea. Something about lethal voltages. As far as the amp's lifespan, Kent said that running the bias that high isn't bad for the amp, since the parts have considerable temperature tolerances. In other words, I shouldn't worry too much about shortening the lifespan of the amp. And I'm not.

The issue of death by electrocution is a bit more serious. I'll have to weigh the value of better sound quality against the value of living. The amp is located high in a closet, so it would be a very bizarre set of circumstances that would lead me to reach up over my head and stick my hand in the amp while it was on. But I suppose anything is possible. Good judgment tells me to put the lid back on. Bad judgment tells me to enjoy this unexpected "upgrade."

Bryon

Great call, Elizabeth. Way to go!

Best regards,
-- Al

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....

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