Wednesday, February 13, 2013

Which Leg?

This post is part of a series on eliminating noise from audio systems. Click Here to see them all.  
In the continuing effort to help the Brethren of the Knob and Fader remove all traces of hum from their systems we address electricity today. Let's jump right in and talk about how the power that comes into your place is delivered.

There are two kinds of power commonly seen (and a few less commonly seen but we'll leave grounded B-phase a.k.a. "widowmaker delta" to the professional electricians). Three phase is common in commercial buildings. In those cases there are three hot conductors into the panel and three bus bars that distribute power to the branch circuits. Each row of breakers is on the next leg as you move down the breaker panel. The other kind, seen in residential service is called "split phase" and uses a center tapped transformer to take one of the three phases running down the street and splitting it to power two buses.

The difference between three phase and split phase isn't much if you just consider voltage going to standard outlets. The voltage between any of the phases and the neutral conductor is 120. When you start to compare voltages between phases you'll find that in your house you see 220 volts between the two legs and in commercial three phase you'll see 277 volts. Without getting too deep into it, the legs in a three phase setup have the 60 Hz waveform 120° apart from each other in phase. With split phase the two legs are of opposite polarity but with sine waves that's the same as being 180° out of phase.

OK. Enough with the science. Here's how you use the knowledge of branch circuits to quiet down your audio. Audio equipment will benefit from having everything in the system on the same leg of the panel. That means in a residential panel using outlets that are on the first, third, fifth, etc rows of breakers. In a commercial setting it's the first, fourth, seventh, etc. If your equipment is all on the same leg or phase you'll have a lot more immunity from hum caused by other devices like dimmers and appliances. 

A lot of care is taken to work this stuff out when power is set up for venues and studios. You can even rearrange the breakers in your house to make sure the washing machine and refrigerator are on the opposite leg from your bedroom so you can get cleaner tracks. But what if you don't have access to the panel or it's poorly labeled?

You can often tell if two outlets are on the same leg just by checking the voltage. Because the load on an electrical system is seldom balanced, the voltages vary between legs or phases. Setting up a home studio you have the time to sort things out but loading your bar rig into an unfamiliar venue you need to be quick on the draw, so keep your multimeter handy.

One way to solve the problem though is to just keep extra cable handy. If you're running a guitar rig in the other bedroom or trying to power up the mix position in an electrically noisy room, just bringing power from the control room or stage to the other gear can quiet things down considerably. While the extra length of power cable can be another way for noise into the system, keeping things all on the same leg will do more to reduce noise than gets in on the extra wire. 

That's all for today junior electricians. Tune in tomorrow as we wrap up this impromptu series on noise reduction with a quick lesson on star grounding!

No comments:

Post a Comment

You're the Scotty to our Kirk