2. tech-talk
  3. 1243823399

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 jea48

jea48
4,313 posts
 
Undertow,

128V is pretty high. You must go through a lot of burned out light bulbs.

Are you sure the volt meter you are using is giving you a correct reading of the voltage? Is the meter displaying true RMS or peak?

I am looking to keep the system self contained and moveable vs. hardwired built in. I will have to take advantage of existing circuits for the time being. it
Undertow 06-02-09:

You still could get by using one main isolation transformer and a small panel. Especially if your main electrical panel is in a basement and is exposed and easy to connect to.

Of course you will need to hire an electrician to do the job.

Install the transformer and new panel close to the existing panel. The electrician can easily move the existing audio branch circuits to the new panel.

I would suggest buying a transformer with a 240V primary and 120V secondary. You may have to get one with a 120/240V secondary and just parallel the two secondary windings, 120V out.

Here is the kicker though... You will need one more smaller transformer. This transformer will be a buck transformer connected between your main electrical panel and the primary winding of the main iso transformer. The buck transformer, (configured as an autotransformer), will lower your now 256V mains down to 240V.

Balanced / isolation transformerÂ… It will have about a 5% loss bringing my wild voltage down to about 122, and in theory in turn give me 61 volts per phase on the output plenty low enough now being split into a balanced load for any gear to run much more optimally for the most part. Nothing fancy, nothing current limiting or adjustable just one continous clean output more or less regardless what the wall is doing, each leg will not vary much more than between 58 to 64 volts.........
5% loss? Please explain the 5% loss.

NEC 2005 Code Article 647. Same for 2008.
http://www.equitech.com/support/647.html

Note, the neutral is connected to ground. The 120V out for power use is two ungrounded hot conductors.
.
jea48
4,313 posts
 
Gbart, well hopefully there is no loss! But for some reason there is a claim of up to like 5% maybe saturation or something with most transformers? Still it should be better due to obviously cutting the voltage in half on and spreading on 2 phasesÂ…
I might be wrong, but if you feed the transformer with 128V you are going to get 128V out on the (2) hot ungrounded secondary leads. Just because the center tap of the secondary winding will be connected to ground will not have any effect the output voltage.
jea48
4,313 posts
 
06-03-09: Undertow
.... actually this is a "Balanced transformer" so it will output if there is no loss 128 volts / 2 = 64 volts to the hot and 64 volts to the neutral on the duplex.

Balanced is a fancy word for split phase..... The same type of transformer the utility power company uses to feed your home. (120V 0 120V)

128 volts / 2 = 64 volts to the hot and 64 volts to the neutral on the duplex.
Equals 128V.... The center tap of the secondary of the transformer is the neutral and it is connected to ground.
The two wires that will connect to the power contacts of the receptacle are both hot. Neutral from the transformer does not come into play. The equipment ground of the GFCI receptacle will connect to the same point as the grounded neutral conductor.
More to discover