Is my Pass amp overheating?

Hi Csontos - I just read your thread on DC offset and bias. Very interesting.

You just hit the nail on the head Bryon.

Are you saying that there's a way to manually adjust the bias on my XA30.5? According to Pass, the autobias on the amp is regulated by internal chassis temperature, which is consistent with my findings comparing the stock lid, the mesh lid, and no lid. In light of the amp's autobias, is it unlikely that the amp also has adjustment pots to manually set bias?

Bryon

I'm not familiar with your particular amp. What I don't understand is why manufacturers don't incorporate user adjustable bias controls in ss amps the way they do with tubes. It's especially relevant in your case where the tolerance in the auto-bias prevents the amp from ever achieving pin-point accuracy. You've just verified where the ultimate potential of your amp lives, as with any ss amp. At least with a fixed bias you can fiddle with it till you find it, not so with yours. If you could, this thing would knock your socks off!

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.