why balanced power last before component?

I've always been curious about the idea of balanced power. Funny thing is though, the balanced transformer systems I have seen use a center tap which goes to ground.

Its very well known that this will degrade its noise performance. If there is noise common to either side of the AC line it will get though easier if there is a grounded center tap.

This is because no center tap is truly a center tap- they are always off by just a tiny amount. This is why in most balanced systems no center tap is used.

I have read the PDF from the Furman site; it appears that there is likely a better way to do this than use a center tap. It could yield another 20 db lower noise...

Atmasphere,

My isolation transformer is not balanced. It profoundly improves SQ. I am not saying balanced is not good -- and maybe even better than non-balanced. But I have never done an A/B. I believe there may be an overdose of marketing with balanced products.

Grounding of the center tap in balanced power systems is to keep voltage stable, and for quick clearing of ground faults in equipment operated on such systems. It is an NEC requirement. The same applies to a household supply. The incoming neutral (power company transformer center tap) is grounded at the service panel for the same purpose, to keep line voltage stable, and for fast clearing of ground faults in branch circuits. Any equipment being operated on balanced power needs to have the same quick clearing of ground fault current. I have used balanced power for more than 10 years, and find the benefits to be significant.

Here are the NEC requirements. You're not running balanced power everywhere, just the audio system, right? At any rate you might want to read all the way through:

NEC 647, which defines the requirements for balanced power systems, places some important restrictions on both their installation and use.

1. Conductors must be sized so that the IR drop does not exceed 1% of the line voltage under a load equal to 50% of the branch circuit current rating, and so that the
combined IR drop of the feeders and the branch circuit wiring does not exceed 2%.
2. A dedicated Equipment Ground conductor must be run to all equipment and each receptacle.
3. All receptacles must be protected by a GFCI.
4. The neutral must be bonded per NEC 250, and must also be connected to the grounded conductor of the circuit that feeds the system.
5. Balanced power systems are restricted to industrial and commercial occupancies.
6. All outlets “shall have a unique configuration” and must be identified using specific language called out in NEC 647.7.
7. There must be a receptacle having a grounded circuit conductor (i.e., conventional unbalanced power) within 6 ft of each receptacle for the balanced power system.
8. All lighting fixtures connected to balanced power must be specifically rated for 60/120 VAC balanced power, must “have a disconnecting means that interrupts all
ungrounded conductors,” and must be permanently installed.
9. Isolated ground receptacles are permitted. Balanced power systems are expensive, and their noise reduction capability is limited to about 10 dB. Isolated ground systems are generally a far more effective and less costly solution.

This might be old hat for you, but that final point is where I was going...

The final point is interesting.. but misleading -if I understand it to mean the maximum noise attenuation that can be provided by a balanced power system 10db. Can we agree that due to the physics of balanced power there is the potential for a range of "quality" for noise reduction achievable by balanced transformers? I'm surprised the NEC would make a 'blanket' assessment that "noise reduction capability is limited to about 10db". They're not identifying if they're talking about differential or common mode noise; nor specifying any frequency range(s)--both of which are necessary to make a meaningful statement. Don't you agree?