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

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

Sorry, my last post got all scrambled. I was trying to link to a website that sells copper mesh like this.

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

Update...

I built a new lid for my amp out of aluminum and copper mesh. You can see a picture of the lid here. You can see it installed here.

The copper mesh is electrically continuous both with the lid and the chassis of the amp, which improves the mesh's RFI rejection, according to the folks who sold it to me.

With the new lid installed, the amp's bias needle is at 12 o'clock at idle. That is down from where it was when the lid was off, but up from where it was when the stock lid was on. So...

-With stock lid: 11 o'clock.
-No lid: 1 o'clock
-My new lid: 12 o'clock

This makes sense, since the copper mesh is only 70% open, and the lid's aluminum frame narrows the opening somewhat. So it's a little hotter inside than it was without the lid on.

When I get a chance to sit down and listen, I will report back the results.

Bryon

Just realized that I never reported back the listening results after installing the aluminum and copper mesh lid that I installed on my Pass amp.

As I mentioned in my last post, the mesh lid DECREASED the amount of heat inside the chassis relative to the stock lid but INCREASED the amount of heat relative to no lid. As a result the bias of the amp is highest with no lid, lowest with the stock lid, and half way between the two with the mesh lid.

The listening results are these... IMO, the amp sounds best with no lid, i.e. at the highest bias setting. At that setting, the instrument timbres are the most realistic and the overall presentation is the most musical. Next best is the mesh lid, i.e. at the middle bias setting. Then the stock lid, i.e. at the lowest bias setting.

Although no lid sounds slightly better than the mesh lid, it makes me a little nervous leaving the lid off permanently, even though the amp is in an equipment closet.

On a slightly different subject, it's worth pointing out that the amp must be fairly impervious to RFI, since the addition of the copper mesh, which reduces RFI significantly, was less beneficial to sound quality than the difference between the no-lid bias setting and the mesh-lid bias setting. I suppose that's because of Pass's "Super Symmetry" design? Or maybe that's just marketing? I don't know.

In any case, I'm still pondering whether to go with the custom lid or sans lid. I wish there were a way to turn up the bias manually.

Bryon

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 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

...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

07-07-12: Btselect
I think you are making a mountain out of a mole hill, I would just get a fan or two, place one under the amp and or one on top of the amp sucking out. I recommend using the Noctua NF-P14FLX fan.

I happen to own two 120mm Noctua fans already, which were formerly used to cool a Class A amp that I no longer own. So the idea of using fans has not failed to occur to me. As far as making mountains out of molehills...

Using the fans to cool the amp is NOT sufficient to change the amp's bias significantly. I've tried it. The amp's thermal vents are both narrow and few. The lid must be either removed or replaced. If the lid is removed, the amp is exposed to a significant amount of RFI, because there are 3 switching mode power supplies within 3 feet.

I've taken extensive steps to reduce the effects of RFI in my system, which were discussed at length in another thread. Removing the lid permanently has the potential to undo many of my efforts on that front. Not to mention, the fans themselves produce a considerable amount of RFI, so the idea of sticking two additional RFI sources directly above and below the amp isn't that appealing to me. Hence my decision to fabricate a mesh lid, which allows for much greater circulation than the stock lid (and therefore higher bias setting), while still protecting the amp from RFI.

So I hope you can see that the situation is neither a mountain nor a molehill, but something between.

Bryon

12-22-12: Duke40
Thank you for posting this thread, it lead me to try something a bit different to try to achieve a similar result.

You are welcome, John. Glad you figured out a solution that worked for you.

Also, thank you for posting your system. I have Focal Micro Utopia Be and reading your system was one of the things that gave me confidence to try and buy a Pass Labs 30wpc Class A integrated amp a few months ago.

Again, you are very welcome.

Happy listening!

Bryon