Isolation transformer question

@ 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.

@lewm That is why I said, "well designed".  Sure, there is a lot of equipment that cut corners to keep the price down. However, I once thought the very basic bias supplies on our ESLs would benefit from refined power. I tried it and it made no difference. I moved the transformer to my line level equipment and it made no difference, so I sold it. The most sensitive piece of equipment, my phono stage, is powered by batteries and is isolated from the line during play. 

I found just messing around that feeding the ESL bias supplies from the lowest power PS Audio regenerator does make a small improvement in sonics. Years ago I tried 6 different power cords on my then SL M1 speakers (before I bought the present PX845s), and I heard differences with SQ having no relation to the cost of the power cords. In fact, the OEM SL power cords (basically 14ga hardware store types) were 2nd best of the 6. Go figure. Any noise inherent to the bias supplies can potentially move the diaphragms, much as spurious motion of a cantilever can also cause distortion.

@clustrocasual Isolation transformers are incapable of cleaning up AC power. They can provide isolation, which is useful for solving ground loop problems, but they can actually add to AC power distortion.

It is AC power distortion which causes some of the ills that people seek to solve with conditioners. In particular, the 5th harmonic of the AC line (so 300Hz in the US) causes power transformers to become noisy, power rectifiers to radiate noise and synchronous motors more likely to run in the wrong direction; all with greater heat.

If you run any transformer past 50% of its rating, the distortion will rise and things like the 5th harmonic show up. Here's a nice link to show you what goes on. https://en.wikipedia.org/wiki/Harmonics_(electrical_power)