60hz hum

[RestoredSparda]

Howdy. Checking in to see if this is an issue or not.

I've recently noticed a quiet 60hz hum through HD800 and HD650 on mainline. It may have always been there but I have just ignored it or thought it was an appliance.

I've removed all inputs, brought it to a different outlet with no source and the hum is still there.  Tightened all screws

It is pretty faint on low gain, but definitely noticeable on high gain. The volume level of hum doesn't change with the attenuator steps. It stays a constant volume. The volume of the hum increases and decreases if single ended or balanced is selected.

Also checking that volume levels are normal. With a 2vrms output from my dac the Mainline is VERY loud if course is on -27 and fine is on -7.5.

I can usually only listen with course on -36 and play around with fine.

Course at -18, -9, and 0 are unlistenable with both HD650 and HD800.

Thanks for any ideas!

[Paul Birkeland]

That seems like a very, very low level setting for the HD800 on the Mainline.  How is the wiring where you're using the Mainline?  Good earthing in the outlets? 

Can you listen to a 120Hz tone and a 60Hz tone to be sure it's 60Hz?

[RestoredSparda]

Hi Paul. Thanks for the help. Yes, wiring is all good in the house and no issues with any other electronics or amps.

Direct to the wall and through a balanced isolation transformer had the same results.

Just listened to both tones and it is for sure 60 Hz.

Any thoughts on what i may have screwed up for the gain to be so high?

[Doc B.]

Are your voltages all within spec?

[RestoredSparda]

Let me check again and report back.

[RestoredSparda]

All voltages checked out.

Kreg a is reading 11.11, and Kreg b is reading 10.7. Only differences I could find. However amp is still warming up and tubes making their way towards the set bias of 145

[Paul Birkeland]

Can you post some build photos?

Do you have access to a tablet or smart phone with a headphone jack? (and an 1/8" to RCA cable?)  My intuition is to start by verifying proper operation of the attenuators, which is easy to do with some AC signal.  I suppose if your DAC is hooked up to a streaming device you can just use it to stream a 60Hz tone too.

[RestoredSparda]

I'm able to stream or hookup to play any signal. Happy to try if you can explain the next steps.

[RestoredSparda]

Photos. I also checked all resistors and they are correct on the course and fine attenuator. They only change value when they are selected with the volume knob.

[Paul Birkeland]

I would add some more solder and turn the heat up on your iron all the way on the terminal strip joints.  Many of them look lightly soldered.  An iffy solder joint can definitely make 60Hz noise appear where it shouldn't be, and could also be a symptom of what I perceive to be a likely issue with the attenuator. 

I wouldn't reflow any of the joints on the attenuator switches themselves though. 

[RestoredSparda]

To be clear, the terminal strips near the attenuators?

[Paul Birkeland]

I would do all of the terminal strips. 

[Doc B.]

Yup, the logic here is if the terminals are questionable because the iron temp was too low that would be the case for all the terminals. I have a new video up on soldering that addresses iron tip temp, how much solder to apply, what to look for in a good joint, etc.

[RestoredSparda]

Reheated and added solder to all terminals. Reheated other questionable spots.   

No change in hum level, and overall volume level is still the same.

[RestoredSparda]

Tubes? On boot up with headphones in there is a much louder 60hz hum and then it goes to regular operation in a few seconds with very quiet 60hz tone.

I've tried a different 12au7 with no change. I don't have any of the rectifier tubes on hand to try.