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]

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]

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?

 

[TonyTecson]

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

 

[TonyTecson]

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]

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]

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.

 

[Joshua_G]

As for toroidal transformers which were mentioned earlier – E-I transformer transfer less of the RFI existing on the mains power lines. I prefer better RFI isolation over better efficiency.

As for having 3 trafos:
It is important to have a different trafos for the analogue and digital sources, since digital sources transmit high frequency interferences into the mains.
It is important to have different trafos for the power amp and all other sources, including the pre-amp, since the power amp may cause fluctuation of the supply AC voltage feeding it. Should there be 1 trafo with 2 secondary windings, the voltage feeding the analogue sources and the pre-amp may also fluctuate.

I'm aware of all the strict engineering considerations.
However, when it comes to audiophile-level stereo sets, experiences shows that sometimes strict engineering considerations aren't enough. For instance, as far as I know, strict engineering doesn't reveal the huge audible differences between different interconnect cables. However, audiophile experience shows that such differences do exist.

 

[TonyTecson]

one big isolation transformer with 3 secondaries will be better than 3 smaller one's.....but of course it's your money, go for it.....

 

[Joshua_G]

one big isolation transformer with 3 secondaries will be better than 3 smaller one's.....but of course it's your money, go for it.....

Better in what way, other than more economic?
Do you have experience with such isolation transformer?

 

[TonyTecson]

Better in what way, other than more economic?

lower equivalent impedance.....

Do you have experience with such isolation transformer?

yes, i design and build them......

 

[Printer2]

So I am curious. Why is it you want to use isolation transformers? To reduce noise generated by one component from another? From external noise coming from the line? Do you have a goal in mind as how much isolation you need? Will isolation transformers actually do the job you want? Are isolation transformers good filters or are they really intended to combat ground loops?

 

[TonyTecson]

come to think of it, all our gears already comes with isolation transformers......gone are the days of the 5 tubes ac/dc tube radios.....

 

[johansen]

come to think of it, all our gears already comes with isolation transformers......gone are the days of the 5 tubes ac/dc tube radios.....

i was about to say something along those lines but didn't want to start a war over PSRR.

Tony you familiar with the fact that electrical machines scale to the 4/3rds power?
it took me the longest time to actually find that in print, in an old patent, which also explained how leakage inductance also scales to the same power, limiting the power density of certain smps topologies to an upper limit.

 

[TonyTecson]

i was about to say something along those lines but didn't want to start a war over PSRR.

Tony you familiar with the fact that electrical machines scale to the 4/3rds power?
it took me the longest time to actually find that in print, in an old patent, which also explained how leakage inductance also scales to the same power, limiting the power density of certain smps topologies to an upper limit.

no i am not, do you have links i can look at?

The Radiotron Designer's Handbook, chapter 5, discusses transformers in good detail....

the main reason for using isolation transformers is to avoid ground loops and for safety reasons, efficacy for audio i'd say YMMV.......

 

[johansen]

The reason I brought that up is because its pertinent to OP's question and justifies the use of one large one vs three smaller ones.

Take two identical transformers, scale one 2x in every dimension.
Core area is 4x, window area is 4x, volume of core and copper is 8x
Constant current density and same flux means 16x power for 8x losses.
In real life, you can only get half way there because surface area scales to the second power, unless you can cool it internally.

side note, power scales with flux squared, which is the biggest reason why cheap stamped E core transformers run 10-20% higher than they should, wasting 20-40% the cost to manufacture in electricity.. every year...

*still looking for that patent that explained the leakage flux, its been a while since i read it and it takes a little more math to explain.

 

[Printer2]

Just thinking out loud, since out equipment already comes with individual transformers how would sticking another transformer in front of it do any good? If RFI can get through the transformers we have already would they not get through the isolation transformer?

Don't know about you but I can turn my stereo up to the point where I hear hiss rather than EMI. If you want to get rid of EMI use a filter that gets rid of EMI. Did some work on getting rid of EMI among other things on a device years ago. If you want to filter your supply to get clean power use a filter. Transformers are for changing voltage or isolation.