Performing live with prerecorded track on laptops running sequencing programs such as Ableton Live etc. is getting increasingly popular. Whether you are running through your own PA or need to plug into a venue’s house system, there exists the possibility of doing irreparable damage to your expensive laptop due to phantom power or improper cable connection from a stereo source to a mono channel. Noise can also be a problem. Here are the solutions:
If your laptop has RCA audio outputs from the sound card, male-to-male RCA or RCA to 1/4" cable is all you need. Hopefully, the tape-in section will have its own level control so that you can make it louder if necessary. As long as the mixer (powered or otherwise) is near you, there shouldn’t be a problem. Things can get dicey, however, if one of three conditions exists; your mixer is more than ten feet away from you; you want to plug into two mic/line channels for more control over the sound, or you’re playing in a venue where you must use the house system and plug into a stage snake.
Okay, We’re A little Unbalanced …
First, a little tutorial on balanced vs. unbalanced cables. An unbalanced cable consists of a single wire surrounded by a braided shield, which prevents the cable from picking up radio frequencies and noise from electromagnetic fields — but only for short lengths of cable. A balanced cable has two wires of opposite polarity surrounded by shielding. Signal is transmitted over one wire and received back on the other. No signal passes through the shield, which enables it to function more effectively (if properly grounded). The operating principle depends on induced noise such as EMI (electromagnetic interference), RFI (radio frequency interference), AC hum, or all of the above appearing equally on each wire. Each wire in a balanced line should have equal impedance relative to ground, which guarantees each wire will have equal susceptibility to noise. As signal travels into a preamp or mixer, its balanced input stage only amplifies the difference between the wires and therefore rejects noise that is common to both.
Phantom power employs the same concept by sending a DC voltage equally through the two signal lines of a balanced mic cable. Since the balanced signal is only the difference between the two signals, which are equal and opposite in polarity, the power is effectively invisible to balanced equipment that doesn’t use it. Connecting a microphone with a balanced output that does not require phantom power will not damage the mic since it doesn’t "see" the phantom power, and our story ends happily. Scary version: Your iPod, does not have a balanced output. It can "see" the phantom power and worse yet, the phantom power can see it too. Connected to a channel with phantom power activated, your iPod will find itself with +48 volts stuck up its little high-tech butt, which will make the little critter so unhappy, it’ll burn out on you just to see the look on your face. The soundcards of laptops are also not a balanced output, and tend to be integrated into the motherboard. Trust us, taking +48 volts up the pipe will also fry your motherboard, and you know what happens when you make a bored mom unhappy—you’re grounded for life, and not in a good, balanced audio way either. Also, you’re out a few hundred or more of your favorite dollars.
Return Of The DI
Now, lets get back to our three scenarios in paragraph two. The first scenario is if you’re going to be more than 12 feet from your mixer. Beyond this length, unbalanced cables can become microphonic and otherwise noisy. Beyond 25 feet they will lose signal strength. Using a direct box (DI) will convert the unbalanced output your iPod, MP3 player, or laptop to a balanced signal. Once balanced, long cable runs without signal loss are possible, noise is not a problem, and the world becomes a magical place of art and beauty. Scenario number two: Converting the unbalanced output of the iPod/laptop to balanced by virtue of a transformer-isolated DI will ensure that if phantom power is inadvertently turned on, or you get accidentally plugged into the wrong channel, your iPod/laptop will be safe — music and merriment ensues. The same holds true for scenario number three. A stage snake is, 99.9% of the time, XLR balanced going to the balanced mic inputs of the console. In all three cases, connecting to a transformer-isolated, passive stereo DI, such as the Radial ProD2, or two passive DIs such as the inexpensive but very effective Whirlwind IMP-2 will enable you to connect to the console safely. Your iPod/laptop is now cleared for takeoff, no unwanted phantom passengers grounding you (grounded as in sitting on the runway for life), and no noise issues. The aforementioned 1/8" to 1/4" Y-cable or stereo RCA to 1/4" cable will work in all three scenarios.
Going Stereo A Mono
If you can only plug into one channel, or your mixer is set for mono output, just using a single mono cable or one side of the Y cable won’t work. You’ll lose half of your stereo mix (Our next tip covers stereo to mono compatibility for live performance). If you want to run mono (especially for long distances, you need a DI that will sum the stereo outputs to balanced mono and provide transformer isolation to protect your equipment from the dark side of phantom power. One such device that handles all of this beautifully is the Radial ProAV1, a passive direct box with 1/4", RCA, and 1/8" stereo connectors for instruments, iPods, and computers that are summed to mono via a resistive mixer. Never, we repeat, never try to combine two signals into one with a Y cable or use a single mono cable. Here’s why. Outputs are low impedance and always must be connected to a high impedance input. If you tie the two outputs together (with a Y cable or mono cable), each output will try to drive the other, which can force them beyond the safe current limit and possibly into short circuit. At minimum, you will experience a severe loss of signal. Worst case; you can damage your iPod or laptop. Remember, a Y cable that splits one signal into two is okay (stereo to 2 x mono). Combining two signals into one is right out!
That pretty much wraps it up. All it takes is a little understanding and the right DI and you’re ready to gig.