By TJ Cornish, September 2014
In part one of the series, we covered some methods to find multiple circuits in a venue. This next section covers how to use them best.
Dividing the load
The primary reason to seek out multiple circuits is because the demand of your system exceeds what a single circuit can supply. The two main factors governing how you use this power are the current consumption of your equipment and the location of these heavy-demand devices relative to the sources of power. Chances are the venue power won’t be where you want it, so you’ll likely be running a couple cords. This isn’t a huge problem, as long as you minimize the length of the cords and can run them so they are out of the way of walkways and doorways. The larger issue is the potential for ground loop problems due to the different paths of the circuits you are using back to the service panel.
We encounter two kinds of grounds in audio equipment: the equipment ground and the signal ground. The equipment ground is a safety device – the third wire of the power cord that is designed to ensure that a wiring fault will not cause a device to become energized and a shock hazard. The signal ground is a reference point for audio interconnections to reduce noise and interference. In most devices, the signal ground is connected to the equipment ground. This connection is necessary for the noise immunity that the signal ground provides.
A ground loop is an electrical circuit that is formed when grounded devices (the equipment ground) are plugged together via signal connections. The current that flows in this circuit has the potential to create noise in the system, and in extreme cases, such as if a receptacle is miswired, can cause equipment damage or injury.
This current is induced in the ground conductors from power flowing through the nearby current-carrying conductors in conduits and cables. The amount of current flowing on signal grounds depends on the path length, and how much current is flowing through the current-carrying conductors.
Ground loops are inevitable and are not necessarily problematic, however significant ground loop currents can cause problems. Ground loop issues can be addressed either by isolating the devices’ signal grounds from each other via a ground lift or isolation transformer, or by working to reduce the voltage differences between grounds of the circuits you are using. It is never OK to defeat the equipment ground of a device. Though cutting the ground pin off a power cord or using a cheater 3 wire to 2 wire adapter may seem to help reduce noise in some cases, this practice is very dangerous.
A centralized power source – either running your system off a single circuit if possible or using a power distro – can significantly reduce the ground loop currents in signal grounds because you now only have one grounding path back to the venue power distribution system, and therefore any ground loops in the audio system will be shorter in length, and will be contained within the audio equipment.
Many venues do not have the provision for a large high-current distro; the only available power comes from regular receptacles. How do we fight off excessive ground loop currents in buildings with normal power? “The Poor Man’s Distro.”
The Poor Man’s Distro
The “Poor Man’s Distro” is a device that connects the ground conductors of multiple circuits together, significantly reducing ground loop currents, while leaving current-carrying conductors separate. This device provides many of the benefits of a larger high-current distro, while leaving the venue’s power systems untouched, and using code-compliant, off the shelf devices.
The easiest way to understand the concept is to think of each section of the above graphic as a power strip. Each block of receptacles is powered from a cord that is plugged into a different circuit in the venue. The PMD leaves the hot and neutral conductors of each section isolated from the other sections, but connects the ground conductors together.
There are a number of forms a Poor Man’s Distro can take. Here is a manufactured product based on Neutrik Powercon devices. The blue connector is the power inlet, and the white connectors are the power outlets. In this product, each section is bonded via a removable ground jumper.
An even more basic PMD can be created using a number of rack-mount power strips in a rack with metal rails. The chassis of the rack-mount power strips is metallic and bonded to the ground conductor. If you sand off the paint of the backside of the rack ears of the power strips and the front side of the rack rails, you create a very good low-impedance ground bond between devices through the rack rails. An added bonus is you haven’t modified any devices, built any assemblies that may have code approval problems, or in any way done anything other than use off the shelf devices in their intended manner.
A great place to put your PMD is your amp rack. This is likely the place where your greatest power consumption occurs, and you can pre-wire your amplifiers to the PMD.
Using the PMD is incredibly simple. Identify your available circuits, test them for proper functioning, run cords from those receptacles to the PMD, and run cords from the PMD to the rest of your equipment. The only rule is that you must plug all of your equipment into your PMD, including your soundboard, which may require running a long power cable back to stage.
What a Poor Man’s Distro is Not
A Poor Man’s Distro doesn’t give you any more power than you previously had. It will not allow you to run more equipment in a given room. It doesn’t somehow change or improve the nature of the power – if the room has voltage sags or spikes, you will still have those with a PMD. The single function of the PMD is to bond the grounds of all of the circuits you are using together so that any ground loop currents caused by power usage external to your audio setup flow on the ground conductors of the PMD, and not the signal grounds of your audio equipment. This, however, is a significant benefit; you will have less trouble with ground hum and hiss.
In certain cases, the PMD can also protect against a wiring fault. If one of the receptacles in the venue is miswired and has either a hot/ground swap or a reverse-polarity bootleg ground (a condition where the neutral terminal is jumpered to the ground terminal and the receptacle is wired in reverse polarity so the hot conductor feeds the neutral and ground terminals), the PMD will short out the mis-wired circuit. If you have tested all your receptacles as you should have, you would have found this issue before plugging in your PMD, but it’s nice to know that if you missed testing one, the damage will be limited to blowing a breaker rather than burning up your snake or other gear from the miswired receptacle.
NOTE: I don’t know the original source of the concept or the name of the “Poor Man’s Distro”. I first heard of it on the ProSoundWeb forum. If someone would like to claim credit for this, please let me know and I will attribute it properly.