Very low level hum and how to get rid of it

[ilya]

That looks like dropping out of regulation. Consider making reservoir cap larger to reduce ripple voltage as easier and cheaper fix than buying bigger transformer.

or not...

JR

JR, I've just checked this and you're correct. I've simulated mains voltage drop with my auto transformer and there it was.
I think I'll add some wire to the transformer, because I can't go any larger with the caps because of the room restriction. Hopefully, a couple more volts won't take that much to wind

Curiously, only positive rail goes out. The negative one still regulates. I wonder what's going on there...

[JR.]

Different Voltage regulators have different drop out voltages (look at data sheet). Also the current draw will affect when they fall out of regulation.

They made LDO, "low drop out" regulators that require less unregulated headroom.

If you are doing a product design for mass production it is good practice to anticipate possible low line and high line mains voltage variations. This is not as bad today as it was in the past and some countries are worse than others. +/-10% min, I've seen as much as +/-20% used.

JR

[mediatechnology]

+1 what JR said.

And once you do think you have the bulk filter cap large enough, put the unit on a Variac and lower the AC input line voltage to the lowest operating voltage.

If it continues to drop out, the filter cap still isn't big enough or the transformer has too low a secondary voltage.