Is my Pass amp overheating?

As an after thought, I wonder what Nelson would have to say about this whole thing. You should ask him your self. He has his own dedicated site on DIY.com. It would be a very enlightening experience to say the least. I'd love to be a fly on the wall observing how he tweaks his personal gear.

I talked with Kent at Pass Labs about the amp's autobias. He was the one who confirmed that removing the lid increases the amp's bias because it lowers the internal temperature in the chassis. That triggers the amp to turn up the bias until it reaches the internal temperature it expects. I didn't ask him whether the amp has adjustment pots, but from his other comments, I would say it probably doesn't.

Kent did discourage me from modifying the amp in any way, because of the risk of electrocution. I suppose it's good practice for a manufacturer to say that. I didn't fully heed his words of caution, but I don't think my custom lid is a big risk.

As far as asking Nelson about this over at DIY.com, I'm a little reluctant. Although he has a reputation for being a really nice guy, I get the impression from his posts that he doesn't appreciate people tampering with his amps, probably because "it ain't broke, so don't fix it."

But maybe I can ask him some general questions about how he determines the optimal bias level for his amps. From what I learned from Kent, turning up the amp's bias isn't a danger to the amp itself, because its parts are all spec'd to operate under significantly higher temperatures than the factory bias setting generates. Because of that, he also didn't seem concerned that turning the bias up would shorten the lifespan of the amp.

Bryon

We're talking apples and oranges to an extent here. I made a comment on Tech Talk under "tubes versus solid state audio amps-the last word". Out side of cranking up the bias optimally, you have the issue of both channels being equally biased. Herein lives everything you're going to love or hate about a particular amp. Without it you have no channel balance, stereo image, frequency extension, or the ability to determine low level resolution, speed, or any other sonic parameter. This is all mind you, relative to how accurate the amp is set up. But my contention is that there is virtually no tolerance available, or range within which balance can be achieved in an ss amp. It's all or nothing when it comes to realizing full potential. It has to be dead on, period. Built in tolerance, aka, auto bias, defeats this possibility and ends up being a compromise. Same thing with servo loop offset.
The reason I believe user adjustments are not incorporated is to safeguard the outputs from very quickly over heating in the hands of incapable owners, requiring additional protection circuitry which in turn costs money to implement. It seems simple breakers installed near the outputs connected to ac would do the trick along with a single meter to visually keep current centered within a pre-determined ball park.

...you have the issue of both channels being equally biased. Herein lives everything you're going to love or hate about a particular amp. Without it you have no channel balance, stereo image, frequency extension, or the ability to determine low level resolution, speed, or any other sonic parameter.

Can you say more about this? I'm not challenging you, I just don't quite understand how it works. How do differences in L/R bias level result in, e.g., differences in low level resolution?

bc

I can't explain technically why this is. Others in the know could weigh in here and describe how electrical behavior equates to sonic outcome in this matter. But I learned early on in this hobby that channel balance is absolutely essential to realizing all of the performance attributes a given amp boasts on it's spec sheet as they relate to your ears. And I'm not referring to the balance knob on your pre. Perceived resolution diminishes simultaneously with the degradation of stereo image. The quality of the stereo image is dependent on how accurately one channel is biased with the other. Remember the definition of "stereo". The accurate placement of sound sources in space. When you don't have this, resolution can't help but suffer since amplitude is not identical on both sides. Probably because quiescent current is lower in one channel and therefore those power transistors are operating with less output. If you are not experiencing "pin point" imaging, regardless of room acoustics, then the bias is off. Any basic amp is capable of this when functioning properly. Not only properly, but potentially better than spec. By the time an amp has been transported to the show room floor, enough tossing around has taken place to knock it off kilter if it ever was right on. You can fool yourself into thinking you're listening to stereo when all you really have are right and left sources. Frequency extension is affected by accurate channel balance in the same way. Especially the bottom end. It is a very good indicator of whether you're in the ball park.