Transformers: Auto- vs. Isolating

I'll have to use a 220/110 step-down transformer for my amp. What would be a difference between auto-transformer and isolating one for the mentioned application? Since the last ones are more expensive, how critical is it?

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

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...)
BTW, doesn't a transformer makes amp 'slower'? What about those effects why people buy aftermarket PWCs?
Any type of power conditioner or treatment can make an amp sound "slower" by limiting the amount of current available to the device as it is needed. In order to avoid this, select a device that is capable of providing at least 30% - 60% more power at any given time than what the device would draw under full load. As such, if you had an amp that is capable of drawing 8 amps continuous, you would want a device that could supply at least 12 - 15 amps continuous. Since ratings from manufacturers are "fudged" so many times, the more headroom that you can provide in terms of current capacity on the "filtering device", the less likely it is to impact the "apparent speed" of the device connected to it. Too many people try to draw too hard on a steady state basis from various filters or transformers and the distortion that they produce in such cases is passed on into the audio system. By keeping the load on the transformer or filter well below capacity at any given time, those distortions and current limiting are removed from the picture or at least drastically reduced. Sean
Sean, you mentioned vast difference in quality... Do you now anything good or bad about those offered on sites such as, etc?
On the mentioned above web site 1.5Kw transformer costs about $120, made in China. On the other side Hammond from Canada sells 1.5Kw for about $350. I guess there is something in that, how can it affect sound?
Dmitry: Those are not isolation transformers. They are voltage converters and there is a BIG difference. For one thing a "decent" 1.5KVA isolation transformer will weigh at least ( at the bare minimum ) 30 lbs by itself. I'm talking about just a transformer with no case or chassis weight added. The more "plates" in a transformer, the more it will weigh and the less likely it is to saturate under heavy loads. As a case in point, the 3.0 KVA transformers that i'm using weigh over 100 lbs apiece. This is bigger / heavier than some 5.0 KVA units that i've seen.

While there is more to finding a good isolation transformer than just sheer weight, it is a good starting point. The other major factor would be the internal capacitance rating ( a lower number is better like .00X ) and that of the rate of attenuation ( lower is better here too with -126 being better than -80 etc... ).

What are you trying to do and what country are you located in ? If outside of the USA, what voltage and frequency does your AC system run on ? Sean
Sean, thank for the reply. I'm in Canada, but I may have to go to 220V/50Hz area for some time. After my recent upgrade of almost whole system, I can't afford 'perfection' anymore :), and thus I'm looking for the economical way to feed my system with European voltage, meaning to compromise sound quality the least possible. Your advise?
Any more details, please!
This discussion is an example of a philosophy of sizing AC power components (transformers, power cords, house wiring, etc.) so as to provide the peak power requirement, which, as we all know, far exceeds the average. This is necessary because there is no device (other than the spinning momentum of an alternator) that can store AC power.

However, almost all the AC is used to generate DC power, and there are devices, capacitors, that store DC. At one time a popular Mod to audio power amplifiers was a bank of outboard capacitors. It seems to me that this is a better, and less costly, way to provide greater reserve power.
I'd say it's more about a possibility of compromise: if 'voltage coverters' such as those from may be used (virtually) without hurting the sound. There is no problem with sizing with them, unlike the real isolation transformers, which are much bigger, heavier, expensive, and less available.
Ok, it's 26lb, not 30... close though. Can't find how bad are they...