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Converting a Balenced XLR signal to Unbalenced Phone (or RCA) signal |
Many XLR cables are designed for use with unbalenced low impedance microphones - they assume an XLR connection on each end of the cable. The problem often crops up that you have only a 1/4 Inch Phone Jack (or an RCA jack) to plug into your Mixer (or Cam-corder) with. This question comes up a lot, and the answer to it needs to some consideration since some choices are much better sonically than others. Just because you are able to make the connections and get the signals to the right places doesn't mean that you will maintain the sound quality or Impedance compatability that you will need.
Looking at the backend XLR connection of a low impedance (300 ohms) balenced Shure SM58 microphone.
First off, XLR connectors have the abilty to have 3 wires in them and are often used because it allows for a balenced signal - this reduces noise because there are opposite (180 degrees out of phase) signals that are sent by the balenced signal source (a microphone, for example). For this to work correctly, it needs to connect to an input stage that is also balenced. There are a number of ways to do this. The simplest way would be to connect it to an input stage that is designed to match the impedance - this could be an analog balenced input amplifier, or it could be a transformer that is matched to this type of device. Many matching transformer solutions exist; a Direct Box is one, and pictured below is a Low Impedance Matching Transformer to High Impedance 1/4 inch Phone Jack Unit:
Low Impedance XLR to High Impedance 1/4 inch converter (Radio Shack part). NOTE: The green tape is only there to mark this particular device as mine - I've lost many of them in the past since these all tend to look alike and its easy to end up with someone elses components by accident.
 The Low Impedance Matching Transformer to High Impedance 1/4 inch Phone Jack Unit has a female XLR connector on it that can either be plugged directly into the Microphones male connector, or an XLR cable (that has one female and one male end on it - which is how typical audio XLR cables are made) can connect to the microphone XLR connector and the other end of the cable can plug into the XLR part of the Low Impedance Matching Transformer to High Impedance 1/4 inch Phone Jack. One problem with this is that the Low Impedance Matching Transformer to High Impedance 1/4 inch Phone Jack unit is heavy enough that it can cause the Printed Circuit board solder joints of the 1/4 inch connector on a mixer to break. Its also very inconvienent for hooking to some smaller device that only has an RCA connector, or a 1/8 inch jack (such as is found on a Video Cam-Corder). The other problem you might encounter is that some inputs are single sided Low Impedance (such as the Cam-Corder), so converting to Single Sided High Impedance won't accomplish what you need. Cam-Corder microphones are often pretty cheap, and more often than not they pick up the sounds of the tape drive motor and the breathing of the operator - they are also usually quite a distance from the sound source - its not uncommon to want to use a much better microphone with your Cam-Corder and place it nearer to the sound source or on an overhead boom. |
 XLR end view of Low Impedance XLR to High Impedance 1/4 inch converter |
If you need Low Impedance Balenced XLR to Low Impedance Unbalenced, you can convert the signal, however, it completely loses any low noise characteristics that you might have had with the Balenced signal (in other words, you have to keep length of this cable to 25 feet - approximately 7 1/2 meters - or less). This form of Low Impedance only cabling would be connected in the following fashion:
Balenced Low Impedance to Unbalenced Low Impedance Cables - Low Impedance Only!
This cabling is really only useful if you are connecting into a Low Impedance input (such as a Cam-Corder microphone jack). If you try use these cables to connect into a High Impedance input, you will have a lot of issues to deal with - all because of Impedance mismatch (the transformer mentioned above is the right way to correct that issue).
Attempting to run a low impedance signal into a high impedance input stage on a mixer or other type of amplifier will simply give you a very noisy signal that has to be boosted (turn up the mixer trim or volume level) substantially, and even then may have high frequency roll off (lack of treble response).
Summary
The rule here is to always match Impedances before trying to convert the Balenced Signal to an Unbalenced signal. You often lose something in the translation if you have to run long cables that are unbalanced, or the input is not compatable with the original signal.Questions? Comments? .
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