Isolation transformers - where do I start?

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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I use Equitech and www.b-p-t.com products

Storytime: Interesting update on the issue Sean raised above concerning my use of the API PW Ultra 116 PLC (although it admittedly doesn't have to do with isolation transformers). To recap briefly, I have everything plugged into this one unit, the source components into the individual isolation transformers, the VTL MB-185 tube monoblocks into the filtered but non-isolated high-power outlets. The 116 Ultra is then plugged into a single wall outlet via its own cord (and a grounded 3-prong adaptor). Sean mentioned that running everything from this one AC cord could be choking off the whole system's power somewhat, and suggested that I run the power amps to the wall separately. The thought had occurred to me before, but as I explained I don't have the outlets near the system to accomplish this, in addition to which API recommends plugging everything into their unit (VTL doesn't agree, FWIW).

Tonight, I was doing some lowish-volume comparisions between my old C-J MV-55 amp and the VTL's, as part of some auditioning I've been doing over the past few days just to get a handle on where I am with the new amps, now that their re-tube complement is fully burned in and I've installed a pair of Shunyata Sidewinders PC's on them, and before I decide to either send the VTL's away for a while to be upgraded and/or sell the C-J after 3 1/2 great years. Both the VTL's and the C-J were plugged into the Ultra's amp outlets (it has four), but of course were not all powered up at the same time, since one or the other was without a speaker load until I switched the cables over.

Listening to several audition cuts while A/B'ing the amps had me feeling pretty good about my upgrade once again, after not having heard the old amp for a while. The C-J obviously doesn't have the power to go loud like the VTL's (which was the main instigator for my upgrading, because I moved to a larger listening room), but it held its own on transient articulation and freedom from sounding mechanical or hard. Overall though, even at these late-night levels, it could not match the new amps in terms of frequency extension both up and down the spectrum, transparency to fine detail, resolution of images' spatial context, image separation, relative lack of imposed timbral colorations, bass authority and tautness, dynamic independence of images from one another throughout the soundstage, depth of field, and the smooth cleanliness that indicates relative freedom from noise and distortion artifacts. Next to the VTL's (at about 2 1/2 times its price when new), the C-J sounded smaller, flatter, coarser, more colored, more veiled, more congested and prone to glare, looser, more sonically changeable in the face of its input signal and load demands, less extended spectrally at both ends of the scale, and also dynamically at the small-scale end (didn't get into high level dynamics during this session, but of course the VTL's rule there as well). Don't get me wrong, the MV-55 was still enjoyable in its own right (and represented a huge improvement over my previous Classe Seventy SS amp at the time that I got it), but in short it was confirmed once again as sounding less natural and imposing more of its own character on the sound than the much more powerful mono's that are replacing it.

About the only possible negative on the side of the VTL's that I could find in these direct comparisions was a slight suspicion that they might be just a tad lean tonally through the midrange, but I couldn't be sure whether this might seem to be so simply because in A/B'ing the two amps, they were more extended and present at both frequency extremes, with less thickening congestion through the center. When I had last done comparitive auditioning on the VTL's, it was with the Sidewinders vs. the stock cords, and though I generally found the Sidewinders were a pretty universal improvement in terms of laying bare the soundstage and improving dynamic liveliness, I did wonder whether they could be just a little too bright-sounding in my setup. I chose not to get overly concerned about this for the time being however, since the VTL 'sound' had struck me as having been just a smidge dark and lacking in sparkle before replacing the cords. But now, I was wondering about the cords again, when Sean's admonition came back to me. Maybe if the amps were plugged into their own outlets, they might sound slightly heartier in balance.

That's when I suddenly remembered something I had forgotten before - I still had my old original, non-'Ultra' Power Wedge downstairs somewhere. This smaller model didn't have the balanced AC option on the isolated source outlets like the current version, but I could use its filtered amp outlets. I went and found it, and with its power cord I could stretch far enough to reach the AC duplex on the adjacent wall, and then use it for plugging in the monoblocks alone. The mono's would still have to share one main AC cord to the wall, but not in addition to the source components. The outlet in question was also on a different circuit breaker, something confirmed when the amps' turn-on current in-rush dimmed the lights on that side of the room much more than when they're plugged into the Ultra along with everything else as usual.

So, with the mono's on their own cord and circuit, I powered them back up, expecting to hear at the very least sound that was merely equally good as before, and hopefully better in some way. But when I played the last cut over again, I was startled to discover that it sounded as if something had broken. The VTL's were now rendered as sounding much closer in almost all ways to the old amp, losing most of their considerable advantage at the flick of a switch. I couldn't believe it. How could amps that should by rights be so easily superior suddenly sound not all that much different in a head-to-head, no matter where I plugged them in? In some ways, like smaller image and stage sizes, and sounding pinched and glarey on unision horn charts, they now sounded worse if anything. Feeling freshly vulnerable to the whims of fate, I started the track again from the beginning, but could only get halfway through it before I needed to jump up and put things back the way they had been.

With the MB-185's restored to drawing juice from the 116 Ultra with the rest of the system, I again heard the sound they ought to have, no problem. Whew! I checked both outlets with a voltmeter, both read 120v. I'm not gonna dwell on this episode too extensively right now, either concerning its possible causes, or to extrapolate it to when (or if) I decide to get new multiple dedicated lines and outlets installed. Although I believe the old Power Wedge isn't supposed to have quite as good an AC filter as the Ultra does, I really don't think that this could account for everything I heard. I tend to think that either it's because of something having to do with the way the outlets are wired, or because API is right about the advantages of plugging everything in centrally, but I can't know for sure. All I know is that for now, I'm sticking to the one main AC cord for the system, even if Sean is technically correct (and he always is), and am not going to lose any sleep over it. Good night. :-)