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?


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.
Call Pass Labs and discuss this with the service people for the real answers!
Hi Bryon,

Just a guess, but is it possible that when the cover is put in place and tightened down the physical alignment of the heat sinks and the output transistors changes slightly, with the result that the integrity of the contact between the two is degraded? If that seems like a possibility you might try replacing the cover but not tightening it down as much.

-- Al
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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
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


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."

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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....
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

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

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

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

Note the section on "Material Homogeneity" in particular.

-- Al
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.
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!


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.

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.

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?

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 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, 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.

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. 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?

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 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.
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.

Hi Bryon

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

I can not remove the lid or try a fabricated one like you designed with copper mesh and aluminium (very impressive by the way)... as I have roadworks near my house causing a lot of dust & dirt in the air.

*** So I tried ... elevating my amp. ***

Aim was to improve better/cooler airflow from under the amp, thus improving the chimney effect, hopefully to achieve a similar result to you.

My amp is similar to yours , though it is an integrated, the Pass Labs INT-30A. I was in the hardware store and found 3" high door stops made of stainless steel (also 3" diameter). I purchased 4 of them for a total of $12 as an experiment. Placed under the existing feet of my amp, it increase the space under the amp from 1" to 4".

I have also noticed that the heat sinks now run hotter and a improvement in sound (it is minor, though easily heard).

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.

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!