Setting forms Monday!

[RICK FITZPATRICK]

Most homes have single phase 240

That is correct I beleive. But they have two phases of 120. Most homes have three wires from the pole to the breaker box. Two "hot" and one "neutral" Voltage measured from each "non grounded conductor(hot) to the grounded conductor(neutral) is 120v, but 180 degrees out of phase with each other., Measured across BOTH non grounded(hot)conductors is a single phase of 240. At least that was the way it was explained to me. Three phase 240( or 208), has 4 wires. Three hots, and a neutral. Voltage measured from any of the 3 hots to neutral gets 120v, but each is out of phase with the other, measured across any two of the three hot wires gets 240v, but each set of 2 is out of phase with the others. Hence-3 phase. And usually this is only available in commercial buildings. I wired 3 phase 240 and 3 phase 208 shops here.
fitz.

There's no such thing as double phase.

I'm not quite sure about that. I've seen "240v" motors that required the neutral as well as the 2 hot wires to run. That would provide double phase 120v. I've also seen motors that DID NOT require the neutral, which would provide a single phase of 240v. But someone correct me on that as I am no expert either. This is just an observation and I came to this conclusion. It may be incorrect. Hense, I'm not sure.

But let me add this for a little clarity. When you hook up "240" to your cloths dryer", you are actually hooking up 2 phases of 120v. As it requires the neutral to work. The motor uses one phase of 120v, the heating coils use the other phase of 120v. They never use the 240 available. At least, thats how the ones I've worked on are wired.
Thats "double phase" at work.

Let me also add this. That is why knightfly said to use one phase(120v) for the studio, and one phase for everything else. IF you have both phases in the studio, not only will it make for noise, it also creates a potential for a 240v MURPHY possibility. ie, a guitar amp pluged into one phase, and your mic/mixer on another phase creates a potential for AHHHHHHHHHHHHHHHH!!!!!!!! I've SEEN it knock people to the floor with burnt lips More than once on stage at large concert halls where the house pa was on one phase, and the amps were on another.

 

[John Sayers]

knightfly - as I understand it (and please - OZ is 240V - one hot - one neutral (240 across them) and one ground, also linked at the board to neutral so we are unbalanced)

We run a separate earth line for each outlet back to the board. When we use a shielding around the cables like in a metal duct or pipe the duct/pipe is also earthed BUT back at the board.

One common way of doing telescoping shields in a studio is to use the mixer as the common shield tie point

This is the way to go. If you reference everything to one point you can avoid loops. If say an ADAT is earthed and the console is earthed and you connect the two you create a loop. Therefore de-earth the connector at the ADAT end and the console becomes ground.

Balanced power is definitely the way to go if you can afford it.

cheers
john

 

[TexRoadkill]

This is the way to go. If you reference everything to one point you can avoid loops. If say an ADAT is earthed and the console is earthed and you connect the two you create a loop. Therefore de-earth the connector at the ADAT end and the console becomes ground.

Doesn't that contradict the whole purpose of star grounding your outlets. Can you really just remove the earth connection and rely on the audio ground for the whole unit?

If that was the case couldn't you simply ground lift all devices that were hooked up to your console and leave the console as the only grounded device? Then why go through the hassle of worrying about grounding your outlet boxes?

 

[knightfly]

Tex, I think John was referring to the AUDIO wire shield connector being lifted, NOT the ground that is required for the POWER connection. the NEC requires ELECTRICAL grounds for each and every piece of gear; these can be a star system, as in each outlet has its own ground that is ONLY connected to the outlet and at the STAR connection, which would be the bus bar inside the breaker panel (this bus bar is then connected thru a GROUNDING conductor, directly to the real EARTH ground. That way, if there are no other grounding anomalies, each outlet box has the exact same ground reference as every other one, so there should be no difference of potential between outlets and therefore no current flow in the ground system. Spiffy...

(The alternative to the star system, otherwise known as daisy-chaining, just loops several outlets on one cable, including the ground - not good for low noise)

Where the problem comes in, is if there actually IS some slight difference of potential between chassis in a rack, for whatever reason - now, if besides the main ground that ties back to the power panel, you ALSO have a complete path between chassis by way of the shields on audio cables, that gives you a complete circuit thru which AC current can flow. This is where hum comes from.

The simplest way to combat this, is to not have a second possible path for ground current to flow - this is done by disconnecting the shield of any audio cables, typically at the mixer since it is central. As long as the audio has a complete path (differential in the case of balanced lines) the signal will be recieved.

With single ended audio cables, there are a couple of ways to make sure there is a return line for the audio without there being a complete path for the shield. One way is to use shielded twisted pair, connecting the hot and ground of the unbalanced line via the two center conductors, then cut the shield at one end and connect the shield at the other end to ground. Now there is no possibility of current flowing in the shield, since it's only connected at one end - still, the actual audio signal has a return path but it is NOT a shield.

In cases where (audio) connected units have a common star ground, you can usually do a telescoping shield even on unbalanced lines, having just a center conductor carrying the audio signal, with the return being accomplished by system electrical ground.

Generally, though, it's best to not go to telescoping shields (this term means that only one end of the shield is connected, so that it is STATIC) unless you have a noise problem.

Rick was right about the power, and yes sometimes it's referred to as two legs of a single phase - Technically, home power is two phases but they are 180 degrees out of phase. If you measure the RMS voltage between the two "legs" or "phases" of home power, you get 240 volts. Each leg to ground measures 120 volts. With commercial, 3 phase power, the phases are 120 degrees apart. If you measure the voltage between phases with 3 phase power, it is LESS than half the nominal amound due to the fact that any two phases are 120 degrees apart, not 180.

If Americans weren't so wimpy, we could have 240 volt balanced power right from the mains as John said - However, I don't think it would be as clean as using a balanced power conditioner, since it's been proven that the power usage meter itself screws up the waveform of the AC power and needs to be conditioned just to get the power back to a reasonable sine wave.

For permanent installation, doing a star-grounded power system with telescoping audio shields is doable - for live sound, it isn't safe and is a nightmare to try and keep track of which shields are clipped where, etc - in that case, its' easier just to carry a bucket full of ground liftable DI boxes, and break the loop wherever necessary... Steve

 

[TexRoadkill]

Thanks for clearing that up. Anybody recomend a good power conditioner under $500 for a studio installation? Are the Furmans really worth buying?

 

[knightfly]

You're welcome - Haven't tried the Furman stuff, my plan for the next go-round is balanced power fed from a 2200 VA line-interactive UPS (you can't do it the other way 'round) , but I really WISH I could accomplish that for $500 - more like $3k, for one 15 amp circuit...

If I come across any comments on this I'll post back, and PM you so you don't miss it... Steve

 

[John Sayers]

If Americans weren't so wimpy, we could have 240 volt balanced power right from the mains as John said

You're not the only one here - England is on 240 (which is where we got ours from) but on a construction site it's illegal to have 240V gear - it all HAS to be 110.

BTW ours isn't balanced because the neutral is connected to the ground.

tell you what a 240V hit is a bit of a wollop!! I've had a few

cheers
john

 

[Transputer]

Michael any pics of your construction. Plan looks superb BTW.

Cheers

 

[Michael Jones]

Michael any pics of your construction. Plan looks superb BTW.

Cheers

The forms are set, and today they are bagging the perimeter for the foundation grade beams. There's about a 3' drop from the rear finished floor to the existing grade outside.
They've hauled in all the select fill for the foundation, but they haven't spread it out yet or set the steel.

My once park-like backyard now looks like a full blown construction site! I have giant ruts along the driveway where the dump trucks ran off while backing in to deliver the fill dirt. And it's going to get worse before it gets better.
I'll take some pics and post them this evening, but, it's pretty ugly out there right now.