Isolation Transformers Power Requirements

[Joshua_G]

I'm going to order from a local factory 3 Isolation Transformers with floating secondary windings:
One for all the digital sources, one for all the analogue sources (TT, phono stage and pre-amp) and one for the power amp.

I know that the isolation transformers power ratings should be way above the actual power consumption of the gear they are feeding, I only don't know by how much. Should the isolation transformers' power rating be 2x the power consumption of the gear, 3x, 4x, or what?

My stereo setup is expansive High-End one.

[bob91343]

No, a good quality isolation transformer will have built in safety factor so you don't need to get them rated higher than your actual load. Of course, smaller transformers will get hotter so if that's a concern, get bigger ones. You will spend more money, and the wasted power may be more or less depending on the specific design parameters.

So I wouldn't go beyond double, and in fact would like to run near 100% of rating, especially for the power amplifier which will draw different power depending on the signal and probably will need even less safety factor.

[Speedskater]

Originally Posted by bob91343

So I wouldn't go beyond double, and in fact would like to run near 100% of rating, especially for the power amplifier which will draw different power depending on the signal and probably will need even less safety factor.

While it's very true that the power amp's continuous current is low compared to it's max. power. But a power amp will draw a large amount of current for a small fraction of each power line cycle. Some say that these pulses may be 5, 10 or even 20 times it's rated current. So I would oversize the power amp's isolation transformer to provide a low source impedance for these current pulses.

[Joshua_G]

Originally Posted by Speedskater

While it's very true that the power amp's continuous current is low compared to it's max. power. But a power amp will draw a large amount of current for a small fraction of each power line cycle. Some say that these pulses may be 5, 10 or even 20 times it's rated current. So I would oversize the power amp's isolation transformer to provide a low source impedance for these current pulses.

This is more in accord with many reports I heard from fellow audiophiles that often isolation transformers, or mains step-up/step-down transformers decrease the dynamics of amps. This is more so on power amps than on pre-amps, which mostly work on class A, so there aren't large swings in their power consumption.

So, does anyone has any data, preferably based on experience, about the recommended power overhead of isolation transformers?

[johansen]

The question is not can you run it at full ratings, but the temperature rise.

Modern toroidal transformers have almost zero core loss (5 watts for a 1Kva unit), but temperature rise is still typically 60C at "rated" output. However, as it is 80-90% copper losses, running at 70-80% rated load means its actually affordable to get that 30-40C rise.

Poor power factor from a rectifier load may require significantly larger transformers, and this de-rating is easily estimated from calculating the rms current feeding the rectifier. there's no extra losses, its all I^2R.
(This is where a choke input filter can really help out.)

Is there any reason you can't use one transformer with three secondaries?
It would save weight, and if you are building more than a few it would cost less.

Most of the low to mid range audio equipment I fix has an E-I transformer powering it, and its crap. The cores are run partly saturating, and this is why they have a 30C temp rise @ no load.
in many cases, the 105C thermal fuse is the only failed part.

Any difference in sound caused by running an amp/preamp off an isolation transformer is caused by the fact that its no longer grounded.

Electrically there is almost no difference between the wall outlet and the isolation tx. if you were to stick a resistor to drop 3 volts and an inductor to drop 3 vac from the line at full load, this would simulate 90% of all iso tx's out there. (aside from the floating ground)

the next step in that debate would be to run your amp off a stack of batteries.. would zero ripple change anything?

[Tony]

Modern toroidal transformers have almost zero core loss (5 watts for a 1Kva unit), but temperature rise is still typically 60C at "rated" output. However, as it is 80-90% copper losses, running at 70-80% rated load means its actually affordable to get that 30-40C rise.

heat is inevitable in any transformer because of its weight and chemical composition, choice of flux densities also have a lot to do with this...

Electrically there is almost no difference between the wall outlet and the isolation tx.

maybe, if your isolation transformer has the same kva rating as the pole transformer where your power was derived from....otherwise, comparing a 25kva transformer to a 2.5kva isolation transformer say, which do you think will have lower impedance?

why not request your utility company to provide you with a balanced service? the type where L1 an L2 are neither line and neutral derived and isolated from the pole to your residence....

[johansen]

the core loss i mentioned is a legit value from an american company.

well, of course the grid is going to have lower impedance.. but we're talking about .5 ohms vs .1 ohms... (the 1 gauge AL wires feeding your house are not negligible in this case)
but that's not going to make a difference anyway.

if you can hear the difference between a 1 ohm and a .1 ohm resistor in the ac line... (and a 1% and 5% choke for good measure) then the power supply needs to be scrapped.

Also, you might be surprised at just how bad the grid is in parts of the country.. lots of third harmonics in there. in some cases you can actually see the distortion with just an oscope, no sa required

[Tony]

Also, you might be surprised at just how bad the grid is in parts of the country.. lots of third harmonics in there. in some cases you can actually see the distortion with just an oscope, no sa required

something an isolation transformer will not cure.....

[Speedskater]

Originally Posted by johansen

Any difference in sound caused by running an amp/preamp off an isolation transformer is caused by the fact that its no longer grounded.

An isolation transformer does not eliminate the safety ground (EGC). The isolation transformer uses it's own separate EGC back to the service entrance.

[Joshua_G]

Originally Posted by Speedskater

An isolation transformer does not eliminate the safety ground (EGC). The isolation transformer uses it's own separate EGC back to the service entrance.

Indeed, however the 2 wires supplying the AC are floating to the safety ground.