Variac vs. Isolation vs. Balanced Transformer? ALL


Hi,
I am looking into the different types of power transformers to fix a few things.

First is a slight hum from speakers using Class A ultra wide bandwidth amplifiers, caused by common mode noise between the Ground and neutral lines from my understanding.

This is not really all that critical as its well under noise heard 3 feet away, but we are picky otherwise we would not be on this site!

Second and maybe even more important is I am getting a pretty continuous 128 volt output from all my outlets. I would like to use a product or build one to step this down to the best possible 117 volts or so for most gear.

I see many variacs for about 100 bucks on the ebays etc... That can do at least 500 watts or 5 amps each.

I was considering just buying about 3 of these one for each mono block would cover the amps, and one or 2 more for the source components.

Anyway that's really the basis of the question, I will not pay 2000 for a used or new piece of gear, like a regenerator or powerplant etc...

I do not want to restrict the dynamics to the amps or other gear.

And I can not put just one unit at this point due to running power cables are in 3 different spots in the room and I will not be running 15 ft power cables all back to one point. Also due to the fact I prefer to split this all up over 2 or 3 circuits in the house anyway.

So where to go?
I see Balanced transformers, are these considered "Isolation" transformers as well? Also will these automatically put out a continuous voltage at the 115 to 117 range? Or would you still need a variac to adjust to the needed voltage and frequency?

Thanks
undertow

Showing 3 responses by gbart

Undertow, although on the high side, 128V is within the ± 10% utility company voltage tolerance in North America. The voltage at my house has always varied between 121.0V and 124.5V depending on the time of day. Coupled with the small no-load voltage boost from my isolation transformers around 1.5% and I'm typically looking at peaks of around 126.4V. I once called up the manufacturer of my equipment to enquire about maximum tolerable line voltage and was told not to worry, that the components are designed to tolerate up to 130V with no problem.
Undertow, you raise an interesting point about the relationship between line voltage and bias. I have high line at my house and its something I've taken into account when adjusting the bias of my amps. I don't understand though why gear should sound best between 115-118V when its designed to operate on 120V. In stock form, final bias readings would be set at the factory with line voltage held firmly at 120V, and obviously the amp should perform best at that voltage, not something lower.

Regarding the balanced isolation transformer, I don't understand why you would get a 5% loss just from going through it unless it has been designed that way. I have worked with quite a few different isolation transformers of various brands over the past few years. All of them in stock form start out with some percentage of no-load boost, not loss. I have ordered transformers and specifically asked that they be wound without the boost, to compensate for the high line at my house. A decent, appropriately sized transformer with good regulation should not lose 5 percent under load.
Jea48, after posting I did some research and found out that ANSI Standard C84.1 covers residential voltage regulation in North America and that the limits are ± 5 percent of 120V.