The AM Forum

 In normal wiring there is one neutral wire which is always at (almost) ground potential and other wire which carries the 120V AC voltage. There is also a separate safety ground wire.

In balanced power there is two out of phase 60V lines rather than normal 120V and neutral. The voltage difference between those out of phase 60V lines is 120V, so the equipments will get full 120V between their power input pins. The balanced power is generated from normal AC voltage by using an isolation transformer with centre-tapped 120V output and the centre-tap of the output is grounded.

That expensive power balancing equipment is just a 1:1 mains power isolation transformer with a centre tap to ground, so that you have two legs of AC which are 180' out of phase with respect to one another (with respect to ground), so that your ground currents cancel out. The important fact not noted is that the centre tap is also situated as a the centre of the output voltage swing. The key in a balanced power system is that the a.c. power line supplying the studio is treated just like a balanced audio line.


Ground loop problems and how to get rid of them (http://www.epanorama.net/documents/groundloop/)  (Check out the links at the bottom of the page)

Hmmmm. I remember when I was on ship, we generated two legs of 55 VAC. There wasn't any neutral. I would suspect they were out of phase to produce 110VAC.

why not just run everything on 240 volts... which already has the grounded centertap and two 120-volt out-of-phase lines?  ???

Those centre taps will never work here in the USA.   ;D

Because the grounded neutral in the center is incredibly noisy. You want to isolate sensitive equipment from it, and the way to do that is to use a center-tapped isolation transformer and connect its secondary center-tap to building steel or a *good*, separate ground.

A good experiment is to connect a scope or AC voltmeter between the power mains neutral and a ground...Particularly in an office building, you'll see VOLTS of crud on the neutral. Harmonics from switching power supplies, motors starting, fluorescent lighting, ground faults, spikes, you wouldn't believe it. I ran that check on my own 3-story office building, nothing special, a camera store and radio station sharing three floors. I saw 10 volts of crud on the white neutrals with reference to the building steel.

Was your scope battery powered?

An isolation transformer would have 240 primary without CT to produce a balanced output of 240 with a CT to ground.  Still the load needs to be balanced or you just set up another ground loop with the new CT. 

You don't have to use a 240 volt primary.  Just use the 120v mains as  voltage source.  The 1:1 ratio isolation transformer then converts the unbalanced line to balanced, with the grounded midtap on the secondary side.

Apparently, isolation transformers with balanced 120v secondary are expensive and hard to find, but I have a couple of Faraday-shielded isolation transformers  with split primary and secondary, each side designed for 240v (series connection) or 120v (parallel connection).  I plan to try using the series connection on 120v in order to get a balanced midtap @ 120v.  That will de-rate the power-handling capability and regulation of the transformer somewhat, since the maximum current rating will still apply but at half voltage, and the winding resistances at 120 volts will be quadrupled, but both transformers are rated at least 1 kva, so either one should still be more than sufficient to run all my low level audio stuff, which is where the ground loops tend to be problematic.

Right now I use a couple of 400 V-A TV service bench isolation transformers, with the secondary windings simply floating free.  That already reduced, but not eliminated some of the hum problems.  I plan to experiment with the balanced grounded secondary.

As the webpage explains, there is a fundamental conflict between U.S. safety/fire/code requirements that dictate an unbalanced neutral at near ground potiential, vs ground loop avoidance and noise abatement  with audio and rf equipment.  The isolation transformer with 120v midtapped secondary is probably the best solution.  Ideally, the ground would be completely separate from the entrance panel ground, even though the NEC says the two grounds must be bonded together for safety.

In addition to the balanced a.c. line, another recommendation is to use star grounding for individual units.  That means each piece of equipment in the audio chain has its own separate ground lead to one and only one common grounding point, even when the equipment is all mounted in the same metal rack.  Daisy-chaining chassis grounds, or depending on the metal rack for grounding is asking for hum problems, since capacitive and resistive leakage may produce significant noise and hum currents on one or more of the ground leads.

HUZ ;

So that explains why my center tap doesn't work in England. 8)

(Epsilon Lyra)

Maybe it's the wrong colour.

once and a while at work I use an isolation transformer to get rid of line noise.
I just float the whole secondary so there is no common mode current possible except for the P to S  C of the transformer. A shielded transformer reduces common mode currents even lower..

Huz,
good one.

That, or you didn't earth it properly.

Curious...in a transformer designed purposely as an isolation transformer is the secondary wound bifilar or 'conventional'? Anyone ever had a look?

Worked on an array of e probe antennas for vlf/lf a few years back that was powered from an ac operated dc supply. The ground loop/conducted noise issues from the power line at LF (very common with these antennas) was pretty bad in certain LF frequency ranges. Found that a power transformer with a bifilar wound secondary virtually eliminated the problem. Stands to reason since balance of the bifilar wound transformer should be much better. 

Been curious about isolation transformers ever since.     

dont know what your talking bout steve.. MY CENTER TAP SEEMS TO WORK WHEREVER I STICK IT

Jay,
Bifilar is tuff for voltage brake down. Windings side by side would be balanced.
Usually the best thing to do is have a copper Faraday shield between primary and secondary.
On a ship you never want any AC current flowing through the hull. 

Frank said:
Thanks Frank, that explains it.

I'm surprised that none of the audio pholks have not jumped in on this one. Balancing the 110VAC is extremely expensive.

Phred

Mike,
I'm working part time on a Navy job at work and learning all the things you would never think of that come up on a ship. They avoid hull currents so things don't become magnetic. When my boss told me about degaussing a ship my jaw dropped. Then I thought about my buddy who was into ASW and MAD gear.

But I did.
Unfortunately it was over an electric fence.  :o

That could drive up the prices of balanced isolation transformers just like it did for 2A3's, 211's and 845's.  I can see it now.  $8,000 Faraday shielded, balanced 250 V-A isolation transformers wound with gold plated, oxygen-free copper wire.  $25k for the 1000 V-A version.  $100k for a dual secondary version large enough to run the electrical system in your entire house.

How about using back-to-back pole pigs?

In the 80's Hewlett Packard used to include an isolation transformer in most of their high-end minicomputers.  Most of them had taps on the input to run anywhere from 208 to 240; the secondary was 240 center-tapped, and the windings were well shielded.  I still have several left over from stripping some of the old retired computers.  I use an 8KW generac, and run 240 from the back of the lot to the transformer, feeding the transfer switch at the panel with isolated 120/240 winding.  This balances the 120 and 240 loads, as the generator only sees a 240 load.  The transformer is rated for 5KVA CCS.   Weighs over 125 Lbs.  Thinking of putting one in the bottom of my BTA-1M and eliminating the neutral line to the rig.

Also have a couple Deltec Super Isolation transformers, 120 in, 120 out, about 4KVA.  These are great as an auto-transformer or isolation transformer.  I used to use one of these to derive the center tap ground at the panel for my 5KW Onan military generator I got from Homestead AFB via Army Mars.

Several of these transformers are excess to my needs.  Cannot ship, but available for pickup northwest of Atlanta, PM or email if interested.  I attend quite a few fests up and down the east coast, and would consider bringing one if there is interest.

BTW, I also have some rather heavy 120/240 input power transformers from HP disk drives, with multiple windings out, up to around 60 or 70 vct out, might be good for class-E or other solid state projects if anyone is interested.  Weight probably around 40 or 50 Lbs, easily good for a KW.

Unfortunately I do not know the oxygen content of the copper, I do not know if they are Bifilar wound, and the end plates are not triple-gold-plated.  But they are guaranteed to stay on the ground during a tornado or hurricane, the centre tap is the correct colour, and if you have a boat they will definitely function as an anchor.  They run cool and quiet at full input voltage, so there is sufficient iron for 24-hour use.

I have a large, buzzardly, Sola 240v to 120v isolation transformer.  No taps on either of the windings, but I have tested it under a 2 kw+ load, and the output regulation out in the smokehouse (the ham shack ;D) is superior to that of either one of the 120v legs feeding the same load directly.  It would at least isolate the load from the neutral.  That near-ground potential neutral is the culprit in many  hum and ground loop problems.

One possible alternative to an expensive hard-to-find  midtapped isolation transformer would be to use a standard untapped isolation transformer with an external midtapped choke across the secondary to establish the balanced ground connection.  An easily available source for the tapped choke would  be an ordinary variac, using the wiper contact as the tap with each end of the winding connected to one side of the 120v isolated line.  Then you would adjust the variac for best balance or minimum hum in the audio equipment.  I suspect that if you could find a hum null, it would occur in different audio units at slightly different settings of the variac.

An isolation transformer should also serve as a surge suppressor of sorts.  Since the core of any transformer will saturate at some point after which any further rise in primary voltage will deliver no further increase in secondary voltage, it would limit lightning spikes and other abnormal voltage surges as long as the amplitude was not high enough to possibly arc through the insulation of the transformer from one winding to t'other.

Ordinary MOV surge suppressors are said to allow something like 400% to 600% nominal voltage to pass through before they fully clamp the circuit (and maybe explode like a firecracker).  An isolation transformer probably would not allow much more than that even if the surge didn't crap it out first.

Don:

Sola transformers are resonant devices...They almost act like a tuned circuit for 60 HZ so they attenuate a lot of spikes and other garbage.

I think Sola made both isolation and constant voltage. The CV output waveform is distorted but it does regulate.