Need some help fixing a faint hum

[MikeSattler]

Yep, looks like all the locations are getting good continuity.

[Paul Birkeland]

What are your IA, OA, OB, and Kreg voltages on each side?  Just out of curiosity, how long did this kit hang around before being built?  Sometimes in a humid environment, the LEDs can absorb moisture over time and then experience damage whilst being soldered.  Generally this takes 6+ months of exposure to a humid environment. This issue will manifest as a problematic voltage.

[MikeSattler]

Well, something is definitely up because a few of these readings are higher than when I ran the checks a few days ago. Listed all the voltages below from the manual plus the few extras you asked for. This build did sit for quite a long time before being finished. I started it a few years ago and had some big life events and it just fell off my radar for awhile. It's been sitting in my basement in a room with a dehumidifier that keeps it around 55 humidity so hopefully nothing got damaged.

Power supply
* 284v
* 6.3v

C4S Side A
* IA - 283.5v
* OA - 220.6v
* OB - 144.3v

C4S Side C
* IA - 283.5v
* OA - 219.8v
* OB - 143.3v

Reg board side A
* Kreg - 10v
* Breg - 219.8v
* -Reg - 0v

Reg board side B
* Kreg - 10.1v
* Breg - 220.6v
* -Reg - 0v

Terminal 20 bias reading - 144.4
Terminal 30 bias reading - 143.5

The power supply and IA readings were both really close to 275v when I first ran the checks. I also set the bias a few days ago right at 145v and those have dropped as well.

[Paul Birkeland]

OK, so your voltages are doing what they are supposed to, which is helpful. Can you procure a three light outlet tester and plug it into your outlet?  An open ground may also create the issue you're experiencing (as could other powerline issues).

[MikeSattler]

Okay, shoot I was hoping the higher voltage on the +275 terminal was out of spec rather than trying to hunt down a phantom grounding or powerline issue.

My outlet tester is showing a ground so I don't think thats the issue either.

Just out of curiosity, is it possible I'm just being too picky? I mean, the hum is very very faint, enough so that it's hard to hear with any other noise in the room. I just assumed something was up because I never had that issue with my Crack or some of the other amps i've used. Is that just normal operation for the amp?

[Paul Birkeland]

Does it change with the high/low impedance switch?  What headphones are you using to pick this up on?

[MikeSattler]

I first noticed it with my HD800's. Then I tried a few lower impedance headphones (portapros and IE600's) and it's more noticeable with those headphones.

When I first posted it didn't seem like the impedance or balanced switches had any effect, but now after resoldering a bunch of joints and sanding those ground points I am noticing a bit of a difference. It gets slightly louder when switching from low to high impedance and it almost doubles in volume (still faint though) if I switch from balanced to unbalanced while using the TRS input. Hum stays about the same volume when using the XLR output and changing from bal to unbalanced.

[Paul Birkeland]

I would try taking the amp to a different location (work is usually easiest) just to gather a datapoint as to whether it's environmental or not.  Stuff like this can creep through your powerline and get into the signal path, and we can certainly help with ways to combat that if it ends up being the issue. 

[MikeSattler]

Okay sounds good, i'll take it to a friends house sometime this week and check back. Thank you for all the help so far!

[MikeSattler]

Unfortunately no luck on moving the amp to a different house, noise is still there.

However I may have somewhat narrowed down what's going on by accident. I still had my portapros connected to the headphone jack while the amp was flipped upside down and I was about to desolder the regulator board to reflow those joints. I heard a loud hum out of the portapros when I moved the soldering iron close to the amp (even though it was unplugged and powered off obviously). Put the portapros on to hear the hum while moving the soldering iron over the internals of the amp and its the exact hum I'm hearing when the amp is on. No clue what that means but the hum seems to be the loudest when my soldering iron is close to the output transformers?

[Doc B.]

Your soldering iron puts out a strong AC field. That radiated field what is getting into the transformers. You might want to double check that the drain wires that connect from each transformer to the safety ground have been well connected to the safety ground screw, and that the insulators under the all transformers are properly installed to insulate the transformer from the chassis. If they are not there is the possibility that the radiated field from the power transformer (which is, by design, a very small magnitude for a transformer of this size) is getting into the output transformers via eddy currents generated in the chassis panel and up thru any mounting screws that are not isolated from the chassis.

[Paul Birkeland]

I would also double check the DC resistance between the earth lug at the back of the chassis and all the headphone jack terminals with black wires on them (set to unbalanced).

[MikeSattler]

Thanks Doc, I'll check the transformer mounts tonight an make sure everything is properly insulated.

I think the ground lugs on the output transformers are working because I'm getting continuity from the earth lug to the metal housing of the transformers.

Thanks as well PB, measuring the headphone output ground wires with the switch on unbalanced I'm getting .1-.2 ohms on all 3 headphone jack ground connections (which is what I get measuring any of the ground lugs).

[MikeSattler]

Not sure if it matters or not but I just noticed that I get a little extra resistance measuring ground on the main transformer. If I clip one lead to E and then measure the top bell screws or the side of the transformer where it's not coated I get like 2-4 ohms instead of the usual .1-.2 for ground connections.

[Doc B.]

2-4 ohms should still be fine. You might just try tightening the mounting screws a little. It's difficult to know whether all the connections are as they should be from an ohm meter reading since they should read close to 0 ohms from the transformer to the safety ground if properly isolated and connected to the safety ground, and would also read close to 0 ohms if they happen to not be isolated and are touching the chassis right at the transformer. Because of this a visual inspection is probably best to make sure that all the transformer mounting washer hardware is installed in the right order so the transformers "float" where they are mounted and only connect to the chassis electrically at the safety ground.