Headphone amp extremely low noise floor turns to distracting hum after 3 days of use

[ElArte]

-Tried different tubes
- Hum is audible even when the volume knob is at minimum
- From a cold start, hum is very low and then appears to increase as tubes warm-up

Here the board in question:

My electronics days date back 30 years, so for sure, I cannot say I understand this circuit.
I am re-learning some, but not there yet.

For some mysterious reason, I suspect those gray caps between the 2 tube sockets.

Anyone can chance another guess?

 

[rayma]

Start with solder joints, component polarity, and scope the power supply and the filament ripple.
Is the pot's body (functions as a shield) grounded? If not, solder a wire to it and connect it to ground.
The coupling capacitors are probably the last thing to suspect, though.

 

[ElArte]

Thanks!

The thing is it was dead quiet for 3 days straight. It’s why I wasn’t going to suspect grounding issues.

 

[rayma]

Sneaky things, these circuits.

 

[ElArte]

Something was definitely off with the external switching power supply.

After a while, I noticed a buzzing sound coming out of it; I went to pick it up and it was literally burning hot.

Ordered a new one. We will see.

 

[rayma]

That doesn't sound right, those should run cool. Make sure the current draw is within spec,
and there is no short on the output.

 

[ElArte]

The new AC adapter came in today. I got a Meanwell with similar specs.

Everything is now back to normal. Super low noise floor again.

Thanks again for your help!