Isolation transformers - where do I start?

Just FYI, the API PowerWedge Ultra 116 (sorry, not 'Ultimate' like I typo-ed above) incorporates 6 iron-cored iso trannys, 5 of which are 120w rated, one 150w rated, and all are capable of running in *balanced AC* mode or having their grounds lifted at the flick of a switch. It also provides 4 current-unlimited outlets without iso trannys for power amps. All the outlets employ line filtering as well, and there are MOV's and circuit breakers for protection. Everything in one 65lb. nice looking, amp-sized component (smaller versions with fewer outlets are available). The only quirk in this product is that the AC mains connector is a special Neutrik twist-lock variety, which cramps your ability to play with cords going from the wall to the unit - you basically have to use one of API's or have a custom one made up. One of the nice things about this approach though, is that ground paths are all more nearly the same length for all components, reducing ground loop potential. Since none of my source components is rated to draw above around 5w-30w nominal or so, I can't see needing a more complicated setup than this unless I want to try running my amps from isolation transformers, but this seems like it might be more trouble than it would be worth for relatively high-powered tube mono's. The unit has been most beneficial for my digital front end components and FET preamp; it didn't do all that much for my tube preamp for some reason (and my phonostage is battery powered). No sonic downside that I've heard, but the PW Ultra 116 costs over $1,500 new, and usually 1/2 to 2/3 that much used when you can find one. A PS Audio Power Plant might do all this and more, but would cost more for the same number of outlets, draws a lot of power, and produces a lot of heat, and as Sean alludes to, I for one have never measured worrisome voltage conditions on my lines. (Interesting side note: The Ultra was designed in conjunction with Kevin Halverson of Muse fame.)

Z: That sounds like a very nice product. However, i would still bypass this device and run the amp direct to the wall. Otherwise, you're trying to pull enough current through one power cord to feed the entire system. As such, the amp WILL pull hard enough to create sag on the line for the other components being fed if hitting the volume control. Other than that, it sounds like it would be a very convenient and clutter free device. Sean
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Luke Manley has suggested the same thing concerning the VTL's, but at the moment I am not set up conveniently near enough outlets to do this (I assume only a different duplex could make any difference in this regard provided the single main AC cord is comparable in guage to the in-wall wire). At some point I intend to test this proposition however, especially since API maintains that their filtered outlets will improve the wall power, and direct comparisions using my smaller stereo amp tended to bear this out. I am contemplating getting the listening room rewired for dedicated lines.

Doing more research, not all isolation transformers are the same.
1) "Multi-shielded IT with interwinding capacitance .005 pF max and common mode noise attentuation 126dB. 120 volt / 4.17 amp. $360.00"
2) "Low leakage (hospital type): leakage current primary to secondary rated less than 50 micro-amps and is measured at less than 10 micro-amps. Power: 250 VA $133.00"
3) "Isolation transformer with static shield 150 VA $36.80."
4) "VA 150 capacity / 7.0 pounds / $45.34. The case and core and electrostatic shield are internally connected to a third conductor of NEMA standard cord and receptacle for safety"

So far #1 and #2 hospital grade looks best. Any thoughts?
I checked on ebay but not really sure what I would be getting / not returnable / get what you pay for / etc.

You are right about transformers being "different". Not only are we talking about the design of the unit, but also the quantity and quality of materials used. The 3 KVA transformers that i have use as much iron as other 5 KVA units that i've seen. Obviously, the smaller one is built a LOT "beefier" than the other even though it is rated for only 60% as much power. Retail price on the 3KVA transformers when new was up around $3000 and i've seen them selling for appr $900 in electronic surplus outlets. The only reason that i can afford the things that i have is because i am a careful shopper and know what to look for. I would imagine than many of you here are in the same position, hence the perusal of Audiogon's used gear listings. As such, I picked up the two above mentioned transformers for $210 : ) Then again, shipping set me back another $130 due to the high ( 230 lbs ) weight : (

When it comes to looking at electrical specs with these types of transformers, the models with the lowest stray capacitance is typically going to do a better job of isolation. Increased capacitance will couple ( you've heard of "capacitively coupled" ??? ) the main side of the transformer to the secondary side in a more direct fashion. In effect, the higher the capacitance per winding, the more of a "leaky" isolation transformer. The end result is only partial isolation if using a transformer high in capacitance.

Think of the amount of leakage as being compared to a signal to noise ratio ( S/N ratio ). The greater the isolation, the less leakage and the less noise gets by. Just as a higher s/n ratio is rated at a higher number in terms of dB's, so is the isolation factor. An isolation transformer with 146 dB's of isolation is "better" or more isolated / less leaky than a transformer that is rated at 126 dB's. At the same time, a transformer with a bigger iron core can pass more current without saturation or distortion. If you can achieve these two things in a transformer, you are most of the way there.

One should also take into account that any transformer generates a magnetic field. If you have multiple transformers within the same chassis, it is possible for the magnetic field of one transformer to "modulate" or "super-impose" its' signal onto another transformer. This occurs because the fields inter-act with each other due to close proximity. This is the very same reason that we do not want signal cables near power cords, etc...

As such, transformers that have "end caps" or shields over the windings are normally preferred over those that are expose the windings. While it is true that toroidals do produce a smaller field around them, they are also not quite as efficient at reducing / isolating noise to begin with. As such, one must pick and choose their trade-offs accordingly. If you have limited space and want convenience while feeding several different components, one can shoot for one chassis with several toroidals or fully shielded yet smaller sized iron core transformers in it. While one stands the potential for greater inter-action between them, the benefits would typically far outweigh the drawbacks in terms of having to use several different chassis, taking up tons of space with the associated power cord nightmare, etc... This is especially true if pulling minimal power from the transformers i.e. to feed line level sources, a preamp, etc... It would be a different story if you had multiple large transformers with their bigger magnetic fields trying to feed power amps, etc.. Sean
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