Isolation transformer question

There are plenty of good reasons to use an isolation transformer including blocking DC, preventing ground loops and getting a quiet neutral. Of course the biggest variable is the quality of your AC in the first place, which is my case is just dreadful. (The corporate parent of my electric utility has paid tens of millions of dollars in penalties for its bribery and fraud schemes, which only drains money that could be spent to improve local infrastructure.)

You need to know that isolation transformers hum. I threw out a 2kv Soundtrapper isolation transformer because the noise of the hum more than counter-acted any perceived benefit to the electronics’ noise floor

lloydc

You need to know that isolation transformers hum.

Mine don’t.

If there’s a hum, that most likely indicates that there’s a gremlin somewhere else in the circuitry. Possibly a grounding issue.   Yet, poorly made transformers can hum - if the windings are uneven or if the lamination deteriorates.

@ steakster, you are fortunate.  It is my understanding that the larger the iso transformer, the more likely it is to hum.  IME, there is always some hum on very large whole-house transformers, but they are located outside or in their own room, far from a listening room.  They could be very useful (I think @whart and M Fremer had some installed). I had two "audiophile" transformers, and kept the smaller one (which I use on front-end components with no problems).

A cursory search on Google confirms that transformers hum by nature, e.g: 

"There are actually multiple causes of transformer noise. The main one is the Magnetostriction Effect. This is when the current that flows through the transformer’s coils creates a magnetic field. The magnetic field then changes the dimensions of the transformer’s iron core. The core expands and contracts with the alternating current, which causes a humming sound."  Landmark Electric Inc.

Perhaps one based on a toroid would hum less?

There’s no attempt in this discussion to distinguish among isolation transformers vs isolation transformers plus regulation and/or additional filtering using caps and inductors vs just a box of HD caps and inductors that filter only vs AC regenerators, which are actually amplifiers designed to take in wall AC, convert it to DC, then output it as 60 Hz at 120V (or variants thereof). These are 4 kinds of very different devices. 
 

I use an AC regenerator on my basement Beveridge speaker system because the AC down there is contaminated by utilities, and it’s made a difference. On my Sound Lab system I use a small PS Audio regenerator just to feed the ESL bias supplies. That was a serendipitous discovery. Anyone who thinks the linear PSs built into our gear are universally well designed and deliver perfect DC ought to invest in an oscilloscope.