I need a better headphone amp

[mahatmakanejeeves]

11. Bad ground.

You have a high-impedance input. The manufacturer specifies 100k, but 20k is probably more like it. The manufacturer also says it has a 100k output impedance, which wouldn't work too well for a headphone amp.

Anyway, it's like a guitar amp with a cord that's not plugged into the guitar. The amp is picking up the hum. Try pressing on parts of the solder traces with some non-conducting device - a piece of plastic. Not a jeweler's screwdriver. See if you can find someplace where you press that makes the hum goes away. Then you'll have to solder that bad part back to what it's supposed to be attached to.

That's a bad description, I know.

Chinese devices are not, IMHO, known for the integrity of their hardware. Specs are okay, but the hardware gives out easily. If a tube socket has deteriorated, there could be a bad connection there causing the hum.

PS: take off the ring before working on electrical stuff. Even 12 volts. Especially 12 volts, if a car battery is involved. I can dig up an OSHA report to show why.

Even an el cheapo voltmeter - the free one from Harbor Freight Tools - will work. With a voltmeter, you could see where the hum (AC voltage) was. An oscilloscope would work too.

I'm about to log off for the day.

Best wishes.