Transformers: Auto- vs. Isolating

An auto transformer has the primary and secondary windings electrically joined and the isolation transformer has the primary and secondary windings...isolated.

The AT will transmit electrical noise while the IT will not (as much). A shielded isolation transformer offers the highest level of power line noise isolation. How "critical" depends on your application - if your wiring system is noise-free, an AT is sufficent. If you have a lot of computers, digital, motors, EMI, RF etc. in your system, then an isolation xfmr would be the way to go. Best way to do it is whole-house or whole circuit rather than at the equipment.

Excellent response Gs. I would only add that there are vast differences in quality between various isolation transformers. As a general rule, "old school" heavy iron transformers work better than toroidals for this purpose. With that in mind though, some "iron core" transformers are much better than others at isolation due to the way that they are wound and internal capacitance. The lower the internal capacitance, the less coupling / better isolation. Besides that, you want to look for transformers with the biggest cores, as this reduces the effects of saturation and hysterisis distortion.

I went overkill and have the dedicated AC system isolated at the A/V breaker box and then isolated again at component level as needed. I did this using smaller individual iso transformers near the gear. This keeps the AC from other parts of the house and the outside lines from feeding into my audio & video systems and the secondary transformers at component level keep the AC noise generated within the components from flowing back into the system and polluting other components on the same line. As such, they are filtered from both inside and outside the system.

I am currently working on a similar system for my Dad. Luckily, he just has one combo 2 channel / HT system : ) Sean
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PS... Yes, i am finally beginning to realize that i am insane.

Thanks guys! I'd just ask if you can recommend some specific brands or sources of the transformers available in North America (or if you have any NOT recommended, also to mention them...)