Balanced AC power question

[mcochris]

I built a 2 kVA isolation transformer for my audio system. The secondary is wired for balanced output 60 VAC – 0 – 60 VAC rather than the standard 120 VAC hot–neutral arrangement. The center tap is bonded to a 10-foot ground rod.

This setup has run flawlessly with all of my existing audio gear and a PC. However, a new class-D amplifier I just ordered comes with a strong recommendation not to use balanced power—no explanation given.

Here are the caveats I already know about:

1. Use only equipment confirmed to work on balanced power. Nothing that relies on neutral as chassis ground or that lacks a 3-prong plug. Wall wart power supplies seem to work fine.
2. Don’t mix balanced and unbalanced-powered gear on the same circuit.
3. Be prepared for GFCI incompatibility. Some devices nuisance-trip or fail to trip when fed from a balanced source.

All three are easy for me to manage. My main goals were an independent equipment ground and extra surge immunity; I'm not expecting audible benefits.

Beyond the points above, are there any other reasons to avoid balanced power with modern class-D amplifiers?
I’d appreciate any first-hand experiences or technical explanations (EMI filters, leakage-current issues, warranty concerns, etc.). Thanks!

 

[kemmler3D]

This is way outside my area of expertise, but have you asked the manufacturer about that recommendation?

 

[Speedskater]

The center tap is bonded to a 10-foot ground rod.

NEVER EVER connect the Safety Ground to a different ground rod!
In fact the Safety Ground is always connected to the Neutral.

 

[kyuu]

I built a 2 kVA isolation transformer for my audio system. The secondary is wired for balanced output 60 VAC – 0 – 60 VAC rather than the standard 120 VAC hot–neutral arrangement. The center tap is bonded to a 10-foot ground rod.

I'm a little confused on this point. A transformer works by the ratio of turns, and if it's the same number of turns on both sides then you'll get the same voltage out as went in regardless of what the incoming voltage is. Unless you're saying this transformer is wired to give you two hots with 60VAC each instead of a 120V hot and a neutral? Is that what you mean by "balanced power"? I'm not sure why you'd want that? What benefit is this arrangement supposed to bring?

As speedskater said, you generally don't ever use a separate ground. Everything should be using a common grounding point, otherwise you could develop dangerous differences in potential between your grounds. That might not even be allowed by code.

 

[sergeauckland]

One reason not to use balanced power is that many (most?) equipment has the on-off switch and power fuse in the live side only. If you're working on a unit you may think it's switched off, but there's still power going to the other side if the mains transformer. Less of an issue with a floating i.e. isolated balanced supply, more so with one centre tapped.

I would only use a balanced floating supply for safety reasons on a workbench, when working on mains side of AC/DC vintage equipment. I don't see any benefit to one in a hifi system.

S

 

[RayDunzl]

What benefit is this arrangement supposed to bring?

You could roam around on this site to see.

Home - EQUI=TECH

The Pioneer Of Balanced PowerTechnology The Pioneer OfBalanced PowerTechnology “It all begins with clean power” – Robert Margouleff, multiple Grammy Award-Winning Producer “It’s like an IV drip for your studio” – Sean “Brycentenial” Bryson, Audio Engineer, Studio Owner “The cleanest power in...

equitech.com

 

[Speedskater]

Note that a balanced AC power system does note work the same way a balanced interconnect system does.
In a balanced interconnect system, the heavy lifting is done by the balanced input stage.
Almost no audio equipment has balanced input AC power.

 

[kyuu]

You could roam around on this site to see.

Home - EQUI=TECH

The Pioneer Of Balanced PowerTechnology The Pioneer OfBalanced PowerTechnology “It all begins with clean power” – Robert Margouleff, multiple Grammy Award-Winning Producer “It’s like an IV drip for your studio” – Sean “Brycentenial” Bryson, Audio Engineer, Studio Owner “The cleanest power in...

equitech.com

Oh... I see.

Digital Applications
As with digital data processing applications, AC power harmonics can cause data corruption. Short of a complete system failure, there is a “grey scale” effect of signal degradation that differs somewhat from analog applications. The main difference is the the frequencies at which problems appear.
For example in 16 bit audio, 16 bit “chunks” of data are processed at 44.1kHz. The bit stream rate in audio is about 700,000 bits per second (16 x 44,100). Various other digital clocking functions may run at MUCH higher frequencies and are too subject to high frequency AC noise. How this sounds to our ears is, well, subjective…but is also very measurable.
For fun (yes, this is how we have fun), we recently completed some tests on a well-known manufacturer’s DAT machine. The results were wildy interesting. First, under standard power, peak jitter was measured under tests in a live performance situation. That resulted in a peak jitter of 18ns with an average jitter at a measured 6ns. We repeated the test using balanced power that yielded peak jitter at 6ns and an average of only 3ns. When high frequency interference is present, proportionally there will be timing errors. Digital jitter appears in a manner not unlike intermodulation distortion in analog circuits. Jitter is compounded as more equipment is added to the digital signal chain the same way low frequency noise compounds in analog systems. Digital jitter is essentially “digital hum” and will adversely affect results.

 

[Devnull]

NEVER EVER connect the Safety Ground to a different ground rod!
In fact the Safety Ground is always connected to the Neutral.

No. Ground is actually connected to the "physical ground, i.e. dirt" in most homes in multiple places. As examples, the metal chassis of a furnace is tied to ground through the ground wire and also the gas pipe. Same with a gas stove. An electric water heater through the water pipes. This assumes metal pipe. In fact, if you have a secondary building with a sub panel it's required to have a ground rod. This panel does not have the neutral and ground connected.

I believe you are thinking about one connection between ground and neutral. This should be at the location where the power enters, usually in the the main panel. It's also why you remove the bond between neutral and ground on the generator when you have a generator hookup for your house. It's also why you don't have neutral and ground connected in subpanels.

 

[kyuu]

After a bit of review, this "balanced power" system might be considered a separately derived system and therefore could have a separate ground per code. It also seems these systems require GFCI protection on all outlets per code. The EQUI=TECH units linked above include them I believe.

 

[amirm]

No. Ground is actually connected to the "physical ground, i.e. dirt" in most homes in multiple places. As examples, the metal chassis of a furnace is tied to ground through the ground wire and also the gas pipe. Same with a gas stove. An electric water heater through the water pipes. This assumes metal pipe. In fact, if you have a secondary building with a sub panel it's required to have a ground rod. This panel does not have the neutral and ground connected.

He is 100% right about OP's situation. An earth connection to main panel in order to trip the breaker is not effective as it has too high an impedance. The breaker will likely not trip, leaving the energized chassis of the equipment live. That connection to earth doesn't actually do much of anything.

There is no good solution to this problem and hence the reason balanced power is not allowed in residential settings.

 

[amirm]

After a bit of review, this "balanced power" system might be considered a separately derived system and therefore could have a separate ground per code. It also seems these systems require GFCI protection on all outlets per code. The EQUI=TECH units linked above include them I believe.

Regardless of how it is done -- and manufacturers know this - is not to code. Here is article 647 of NEC: https://thenecwiki.com/2021/02/article-647/

"General. Use of a separately derived 120-volt single- phase 3-wire system with 60 volts on each of two ungrounded conductors to an equipment grounding conductor shall be permitted for the purpose of reducing objectionable noise in sensitive electronic equipment locations, provided the follow‐ ing conditions apply:

1. The system is installed only in commercial or industrial occupancies.
2. The system’s use is restricted to areas under close supervision by qualified personnel.
3. All of the requirements in 647.4 through 647.8 are met."

There is no good reason to use these systems. To wit, I have reviewed the EQUI=TECH btw: https://www.audiosciencereview.com/...s/equitech-1-5rq-balanced-power-review.24948/

 

[kyuu]

"General. Use of a separately derived 120-volt single- phase 3-wire system with 60 volts on each of two ungrounded conductors to an equipment grounding conductor shall be permitted for the purpose of reducing objectionable noise in sensitive electronic equipment locations, provided the follow‐ ing conditions apply:

1. The system is installed only in commercial or industrial occupancies.
2. The system’s use is restricted to areas under close supervision by qualified personnel.
3. All of the requirements in 647.4 through 647.8 are met.

Thanks, good to know. I hadn't found the section limiting it to commercial/industrial settings.

 

[mcochris]

NEVER EVER connect the Safety Ground to a different ground rod!
In fact the Safety Ground is always connected to the Neutral.

Do you mean don't connect two different grounds together? I'm not doing that. The transformer primary and secondary grounds are not connected.

 

[mcochris]

I'm a little confused on this point. A transformer works by the ratio of turns, and if it's the same number of turns on both sides then you'll get the same voltage out as went in regardless of what the incoming voltage is. Unless you're saying this transformer is wired to give you two hots with 60VAC each instead of a 120V hot and a neutral? Is that what you mean by "balanced power"? I'm not sure why you'd want that? What benefit is this arrangement supposed to bring?

As speedskater said, you generally don't ever use a separate ground. Everything should be using a common grounding point, otherwise you could develop dangerous differences in potential between your grounds. That might not even be allowed by code.

I'm using a multi-tap transformer which allows one to create different primary and secondary voltages. Signal SU-2 specs attached.

 

[mcochris]

Regardless of how it is done -- and manufacturers know this - is not to code. Here is article 647 of NEC: https://thenecwiki.com/2021/02/article-647/

"General. Use of a separately derived 120-volt single- phase 3-wire system with 60 volts on each of two ungrounded conductors to an equipment grounding conductor shall be permitted for the purpose of reducing objectionable noise in sensitive electronic equipment locations, provided the follow‐ ing conditions apply:

1. The system is installed only in commercial or industrial occupancies.
2. The system’s use is restricted to areas under close supervision by qualified personnel.
3. All of the requirements in 647.4 through 647.8 are met."

There is no good reason to use these systems. To wit, I have reviewed the EQUI=TECH btw: https://www.audiosciencereview.com/...s/equitech-1-5rq-balanced-power-review.24948/

As far as I know, balanced AC power is primarily used in audio/video studios, broadcast facilities, and other sensitive electronic environments to reduce electrical noise and improve ground isolation. I’m not claiming any audio quality improvements myself, this was just a hobby project, and I built the unit out of personal interest.

You must get tired of saying "no good reason"

 

[wwenze]

I haven't read through the entire thing but the first thing I can think of is SMPS which class D modules like to use.

You know how SMPS simply takes the AC120V and rectifies it into a DC170V and 0V behind the rectifier? Basically it has a clearly-defined high voltage side and a low voltage side.

If you used balanced AC power with this, you will technically get 85VDC on the high side and -85VDC on the low side. In the absence of other information and in a universe where voltage is relative, this doesn't give any issue on its own. However I'm thinking if some guy did the isolation design of the PSU and his isolation between the low side and the ground (or the outside world, anything) is based on the low side being close to 0V and not -85V, this may cause issues, especially if there is carelessness when the user decides which node to connect the ground to. That being said, I know some EU countries that uses balanced AC in their homes by default, any electrical authority wants to comment?

Even if safety isn't a concern, there's also the issue of noise which may not flow the right way. And class D are well-known for generating common-mode noise.

 

[MaxwellsEq]

As far as I know, balanced AC power is primarily used in audio/video studios, broadcast facilities, and other sensitive electronic environments to reduce electrical noise and improve ground isolation.

Can you give examples? Balanced audio is used throughout studios.

 

[mcochris]

Can you give examples? Balanced audio is used throughout studios.

I'm familiar with balanced audio signals. Recording studios that I've heard that use balanced AC power are Skywalker Sound (California), Masterdisk (New York), Abbey Road (UK), and Blackbird Studio (Nashville).

I did consult at a water testing lab that used balanced AC power for their extremely sensitive (down to parts per billion) measurement equipment.

 

[mcochris]

I haven't read through the entire thing but the first thing I can think of is SMPS which class D modules like to use.

You know how SMPS simply takes the AC120V and rectifies it into a DC170V and 0V behind the rectifier? Basically it has a clearly-defined high voltage side and a low voltage side.

If you used balanced AC power with this, you will technically get 85VDC on the high side and -85VDC on the low side. In the absence of other information and in a universe where voltage is relative, this doesn't give any issue on its own. However I'm thinking if some guy did the isolation design of the PSU and his isolation between the low side and the ground (or the outside world, anything) is based on the low side being close to 0V and not -85V, this may cause issues, especially if there is carelessness when the user decides which node to connect the ground to. That being said, I know some EU countries that uses balanced AC in their homes by default, any electrical authority wants to comment?

Even if safety isn't a concern, there's also the issue of noise which may not flow the right way. And class D are well-known for generating common-mode noise.

I see your point. I do use balanced AC power with my computer equipment (all SMPS workstations, servers, monitors, etc) and everythinng has been working fine for over 10 years. I've used a class D amp on balanced AC power without problem, but did not check if it was only using 60 VAC.