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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