Isolation transformers - where do I start?

I did this the easy way by purchasing an Audio Power Industries PowerWedge Utlimate 116, which features 6 isolation transformers for sources components (it isolates components from one another as well as from the line). It's been a long time since I compared using this to plugging straight into the wall (I do remember an improvement, otherwise I wouldn't have kept the unit), and it also puts the power through HF filters as well as transformers. But I can handily compare another power aspect, that of running balanced AC (+60v/-60v as opposed to +120v/0v), which is selectable/defeatable from tranny taps switched individually on the rear panel. Based on my experiences in this area, I would say if you're going to use isolation transformers, you should go the balanced route if you can.

Some general suggestions here.

"Old school" iron core transformers are superior to toroidals in terms of noise rejection. Toroidal designs do work, but are not as efficient in terms of total noise rejection.

Look for transformers that use the most iron for their cores and are rated the highest in terms of KVA. "Good" isolation transformers are phenomenally heavy and expensive. My bare 3KVA isolation transformers weigh over 100 lbs apiece. The cores are actually good for about 5KVA. As such, i might saturate the windings but i'd never saturate the core. The 1.8 KVA bare transformers that i have weigh 42 lbs apiece. I used to think that these had "healthy" cores until i picked up the 3 KVA units : ) The bare "little" .25 KVA transformers i have weigh 12 lbs apiece. Just the sheer shipping weight and cost of raw materials for these units might tell you why many manufacturers opt for lighter and less costly toroidal designs.

If you must share transformers between components, keep the digital separate from analogue and vice-versa. If using a transport and dac, further gains can be made by having an individual transformer for each unit. If possible, take that a step further and use individual isolation transformers for each component.

If you are going to use an isolation transformer on a power amp, the transformer should be rated quite a bit higher than what the amplifier would draw at the point of clipping. Otherwise, you might run into core saturation on the isolation transformer during momentary peaks or sustained low frequency passages.

Bare in mind that transformers produce a magnetic field around them. You will have to take steps to shield them if placed near the gear. Keep signal cabling FAR away from ANY type of power transformer.

Start with your digital gear and work your way down the line.

I've been told to stay away from Sola units as they supposedly do "bad things" sonically. Besides that, 3% distortion is pretty high. According to what i've read, the power coming off the grid is "supposed to be" less than 5% at any given time. If it is higher than that, your pole transformer is probably defective.

Short of purchasing a TRUE power regenerator ( expensive and in-efficient ), there is nothing that you can do to remove ALL of the noise coming in off of the line. Using good quality iron core isolation transformers will give you the most benefits with the least drawbacks, but they are sloppier and far less convenient to work with than commercial "audiophile" PLC's.

Keep in mind that this is just my take on the situation. Others will have varying opinions that are probably just as valid, maybe more-so. Sean
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Sean's advice is very good. I would recommend looking at EquiTech's wall mounted units. These are very large isolation transformers that take 220 in and then drop it down to 110 with two balanced poles (thus further rejecting the noise). I made one similar to these with a transformer I had gotten from a medical device company that was no longer used. It was about a 6.5 KVA unit (yes it was heavy). The only downside to the EquiTechs is that they are expensive, but I think they have really engineered a superior product (it's the brute force method--but works quite well). It's used by many recording studios, they're just not as well known in the home market.

Cdc, I was in a very similar situation this past winter. I looked at all the pieces recommended above and found the Hubbell to be one of the best out there. Rick at Virtual Dynamics has recently informed me that he is modifying Hubbell Iso Transformers with Cryogenic treatment, internal wiring tweaks and some other stuff you'd have to ask him about. My power needs have been met lately so I am not in the market for one of these... If you end up getting one please let me know what it does in your system.