The Art & Science of Audio & Video

Balanced or Stereo Input?

Q: I have a CD/Cass combo with unbalanced outputs going to a balanced input on my mixer and the sound is extremely low and very distorted.

A: Look at the input on your mixer. You need to verify whether it’s an actual balanced input or is it a left / right input with a shared common. Balanced inputs use a principal known as common mode rejection in order to prevent noise. When sending left and right information to pins 2 and 3 the input is receiving voltage on both pins of the same polarity. Because of the fact that the left and right channels aren’t identical they will not completely cancel each other out. Instead the result is a low and distorted signal. Balanced inputs are looking for the same voltage on pins 2 and 3 but opposite polarity. If noise such as RFI is induced on to the cable it will be induced on to both conductors equally as a result of the twist in the cable resulting in the same voltage and same polarity on both pins. If you are dealing with a left/right input it is expecting positive voltage on both pins 2 and 3 whereas pin 1 is used as a common for both channels. So this is consider an unbalanced input even though there is three pins or terminals. This leaves you with two options. If you do have actual balanced inputs you can either combine the left and right positive on pin 2 with the left and right negative on pin1 or used two separate inputs/channels. So one channel would have left(+) on pin2 and left(-) on pin1 another channel will have right(+) on pin2 and right(-) on pin1. For the first option there are careful considerations to be made. You must consider the ratio of the output impedance of the source device verses the input impedance of the input it’s going to. If the ratio is not high enough you must use some sort of combining device or summing transformer. A good rule of thumb is a 10:1 ratio but the higher the better.


February 23, 2008 - Posted by | Troubleshooting | , , , , , ,

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