Collecting your answers: why do the isolation transformer work on the audio equipment

[auchain]

Someone said that the video and audio gear would perform differently when using different isolation transformers, such as toroidal transformer, R-core transformer and laminated EI transformer.
But why are there such differences? And what are the differences?

[celeste]

I have tried to answer this 3 times before, but I get signed out and my words get eaten. one more try.

I am not sure I can come up with all the differences, but here is a list of at least a few. Bandwidth, impedance, efficiency, noise radiation and pick up and excitement current.

Low bandwidth is a good thing in a mains transformer, you only want the fundamental getting through. Toroids are naturally wide bandwidth. EI's can be made to be quite low with electrostatic shield or split bobbin.

Because the windings couple so tightly on a toroid, they usually have fewer windings for a given voltage, so DCR is lower, making source impedance lower.

Toroids are more efficient, more of the input comes out as output and less as heat and noise.

Because a toroid has a more tightly controlled magnetic circuit, it radiates less and is less effected by outside forces.

Excitement current is not usually much of a problem till a transformer gets big, but with toroids over about 500VA, inrush limiting or zero crossing switching should be thought about to prevent turn on fuse blowing. The low DCR in the windings do not limit current till the magnetic forces build up.

[keithcc]

Originally Posted by celeste

I have tried to answer this 3 times before, but I get signed out and my words get eaten. one more try.

I am not sure I can come up with all the differences, but here is a list of at least a few. Bandwidth, impedance, efficiency, noise radiation and pick up and excitement current.

Low bandwidth is a good thing in a mains transformer, you only want the fundamental getting through. Toroids are naturally wide bandwidth. EI's can be made to be quite low with electrostatic shield or split bobbin.

Because the windings couple so tightly on a toroid, they usually have fewer windings for a given voltage, so DCR is lower, making source impedance lower.

Toroids are more efficient, more of the input comes out as output and less as heat and noise.

Because a toroid has a more tightly controlled magnetic circuit, it radiates less and is less effected by outside forces.

Excitement current is not usually much of a problem till a transformer gets big, but with toroids over about 500VA, inrush limiting or zero crossing switching should be thought about to prevent turn on fuse blowing. The low DCR in the windings do not limit current till the magnetic forces build up.

Toroids behave badly when there is any DC offset on the line. Thanks to all the switchmode equipment out there and the barely adequate grounding specified, from an audio standpoint, in the NEC that happens a lot now. The biggest single offender, besides welders, I am told, is the Maytag Neptune washing machine.

Ten or fifteen years ago a lot of high end audio equipment was using toroid PTs. They are less common today. There is a fix, which is to slit the core, but then you need a bigger heavier core.

As far as isolation transfoemers go, any adequately rated one should do provided you don't run into problems like these.