Balanced Input or Stereo Input? Mic Input or Line Input?
I want to discuss a topic that I get asked about constantly. It’s the confusion surrounding balanced inputs verses stereo inputs and the true differences between mic and line inputs. First and most obvious are the balanced and stereo. This is something that has become a more prevalent problem in recent years because of the popularity of portable MP3 players and the use of laptops running special software for athletic programs and multimedia presentations. So here’s where the problem usually starts, someone takes the convenient 1/8″ stereo mini and assumes that the fact that it has a Tip, Ring and Sleeve translates into a balanced input of pin 1(G), 2(+) and 3(-). Well as you’ve probably figured by now this is not the case.
A TRS stereo 1/4″ jack or 1/8″ jack is essentially two unbalanced signals with a shared common. Typically Tip = Right channel, Ring = Left channel and of course the Sleeve in ground. Balanced on the other hand is looking for the exact same signal on pins 2 and 3 but in opposite polarity. It uses a method known as “Common Mode Rejection” to resist interference. Any noise induced on to the cable or passing from one source to the next will be rejected if it’s not the same on both pins. Therefore if you wire you stereo output to a balanced input the signals on pins 2 and 3 will obviously be quite different because you now have right channel information on pin 2 and left channel information on pin 3. Well needless to say they obviously are not going to match (100% of the time anyway) and will attempt to cancel each other out. Your best course of action here is to split out the left/right to two different channels on the console or sum them to mono. It is possible with most modern equipment to combine the left and right channels on pin 2 and tie the commons together on pin 1. Notice I say possible with “most” equipment because doing this changes the electrical properties. So checking the input ratio from the source component to the consoles inputs still retains a 10:1 impedance ratio for maximum voltage transfer. This yields a strong signal level and keeps your signal to noise ratio in check.
I find that a lot of technicians think the only difference between mic and line level inputs is the input sensitivity. The misnomer is that simply reducing the input gain will allow you to use mic inputs for either mic or line. However the input sensitivity is only one difference. Equally important if not more important is the difference in the impedance. Microphones have very low output impedance typically in the 50-200ohm range. In return the input impedance on the mixer may only be 1-2 kohm. Now should you try to plug a line level source in to the mic input you now have a source with an output impedance of 1kohm or more connected to an input with impedance that’s relatively the same. So when considering a mic input with a line level input you may only have a 1:1 or possibly 2:1 impedance ratio also known as an “impedance mismatch” (BTW the term impedance mismatch is used very loosely by many and it’s meaning can very drastically so always get details). On the other hand the line inputs will have an input impedance of 10kohm to 100kohms meaning your 10:1 rule is safely in tact. The logic behind the impedance matching is that per ohm’s law as impedance goes up so does voltage and the current drops. As you lower the impedance your current transfer increases but voltage transfer will dramatically decrease. If you’re not completely confident in your knowledge of voltage and current and how they relate to one another I offer you an analogy. You will see this same analogy in many books and used in classes as well. Imagine a garden hose with no nozzle on it. You turn on the faucet opening it all the way and the result is a lot of water moving through the hose gently flowing from the end with very little pressure. Therefore the amount of water you are moving through the hose is equivalent to “current”. Now let’s put a nozzle on the hose and try again. Now the water is combating the resistance being cause by the restrictive path of the nozzle causing it to exit the hose under extreme pressure, the equivalent of “voltage”, but you are now moving less current. So now you can see how all three are directly related. Little resistant (no nozzle) increases the current (flow) and lowers the voltage (pressure). Increase the resistance and you decrease your current (flow) and increase you voltage (pressure). For my disclaimer I must state that this does not mean you should start wiring your systems with garden hose!
If you need additional assistance when wiring various types of connections to one another I strongly suggest you refer to the Rane white paper available on their web site, RaneNote 110, for guidance on any and all connections.
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