AXXIS AUDIO
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Interfacing Sound Systems
In any sound reinforcement or recording situation the most critical and most often misunderstood aspect is interfacing. Interfacing, put simply, is the process of connecting one or more pieces of equipment together. If your equipment is properly interfaced - hum, hiss, and buzz will be a thing of the past.
Connectors
Connectors are where about 50% of all sound problems begin. Although there is an international standard for wiring audio cables, the pin assignments may vary from manufacturer to manufacturer. The best way to insure that the cables are compatible with the device is to check the owners manual. If there is any doubt of how to wire an interface cable, follow these standards and 99% of the time they'll be right. The following pin assignments are the I.E.C. 268 standard.
3 pin (XLR) Connectors
Pin 1 - Ground (shield)
Pin 2 - Positive (Hot)
Pin 3 - Negative (Cold)
1/4" Phone Plug
Tip - Positive
Sleeve - Ground (shield)
1/4" Stereo Phone Plug
Tip - Positive (Hot)
Ring - Negative (Cold)
Sleeve - Ground (Shield)
There is a classic misconception that all XLR connectors are low impedance. This is not true; most all XLR connectors are balanced, but not all are low impedance.
Cables
Interconnecting cables are the lifeline of your system. If bad or improperly wired cables are used while connecting the system, the result will more than likely be your $20,000.00 system sounding like a $20.00 car stereo.
To properly connect any system, close attention must be paid to the type of cable used. Speaker cables should never be used to connect any signal processing or musical instrument. These cables are normally unshielded and are very susceptible to induced noise. The only cable that should be used to connect components is 2 or 3 conductor shielded cable.
Y Cables
Many people believe that the Y cord should have never been invented, and for good reason. Anything that can possibly be hooked up wrong will be. The Y cable should be used to split a single output into two inputs. This cable should never be used to mix two signals together. When two outputs are tied directly together, each one tries to drive the impedance of the other. This condition puts unnecessary strain on both devices and, more often than not, leads to failure of the units. This applies to all kinds of musical equipment, including microphones, keyboards, signal processing, and guitars.
Grounding
This aspect of interfacing is the most critical and, often times, the most ignored. I'm sure every one of you at one time or another, has touched another member of your band (or microphone) on stage and received a reminder of what electricity is all about. If a few basic guidelines are followed, this hazardous condition will no longer exist.
1. Always use grounded AC outlets. If a grounded outlet is not available, use an adapter - but make sure the tab is run to ground (the screw that holds the cover on is a good bet).
2. Never under any circumstances run the entire system without a ground. Besides the health hazard, there is an increased possibility for equipment failure.
3. Never pull or cut off the third prong of a power cable. If the unit is plugged into the wall backwards there is a good chance that the case of the unit could carry a jolt of 110 volts AC.
4. If you're using a device that only has 2 prongs on the AC line always make sure that it is plugged in properly (they will normally only go into the outlet one way).
5. Always make sure that an outlet is functioning properly before plugging any equipment into it.
6. As a final precaution against electrocution, connect all devices (except for lighting gear) to the same circuit.
In many cases excessive noise in a system can be traced directly to grounding (or the lack of). Ground loops can be easily avoided by following a couple of basic guidelines.
1. Signal ground (shield) should never be connected between units in a balanced system. The ground is not needed to make the system function, and most of the time just adds noise.
2. Signal ground should be connected only on the input of each unit. This keeps the shield of the connecting cable at ground but doesn't provide a path for the current flow.
By following these guidelines you'll eliminate grounding problems that destroy the advantages of a balanced system.
High or Low Z
There is often quite a bit of confusion as to what device is needed to plug a low impedance mic into a high impedance input or visa-versa. There are just a couple of things to remember when connecting devices to each other.
Guitars, basses, keyboards, line feeds from amplifiers, and high z mics must be run into a direct box before they can be run down 100' of snake. High impedance devices exhibit high signal losses over long distances of cable. This characteristic is not as noticeable in low z devices.
Low Z mics should have a transformer on the end of the cable when being plugged into High Z inputs.
As Musicians we deal in sound every day. Isn't your music worth the extra time required to properly connect your system? If you don't think it is, go check out your competition - I bet they do!
(We would like to thank Rapco for permission to use this information. As a dealer of Rapco Products we invite you to visit their website. You will find the link on our Product Lines Page.)
Axxis Audio - P.O. Box 75613 - Colorado Springs, CO 80970-5613
719-638-8777 office 719-638-1877 fax 719-930-8147 cell