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|5th February 2003, 03:59 PM||#1|
Join Date: Aug 2001
Location: Columbus, Ohio
XLR Audio Input Wiring
I just made a post in the Aleph-X thread about a potential hum problem with how the audio input connector wiring is handled and thought this might be of interest to more than just Pass Fans.
In the past, pro audio systems have often suffered mysterious hum problems even when using shielded balanced wiring for all interconnects. These problems have been blaimed on ground loops, poor quality grounds, low quality cable, noisy power, etc. A few years ago it was finally understood that the root cause of most of these problems was the way that the gear manufacturers handled termination of the shield connection at the input connector. This is the notorious "Pin 1 Problem", if you haven't hear about it then do a google search on the phrase.
The standard 3 Pin XLR connector used in the PRO audio world has a ground connection on pin 1 and the balanced audio is connected to pins 2 and 3. The obvious thing to do with this inside a piece of gear is to wire a harness with 3 conductors (or perhapes a piece of shielded twisted pair if you to really be retentive about maintaining shield coverage) to the XLR jack and then connect the other end of the 3 conductors to the PCB. This previously standard practice has been proven to be a very bad idea.
Bringing a shield conductor carrying noise currents inside the enclosure and connecting it to a PCB right next to a high gain input greatly increases the likelihood that the noise will be induced into the audio circuitry. It has been determined that the best way to terminate a cable shield is to ground it to the metalic enclosure right at the input connector.
Note: Neutrik makes XLR jacks that connect the shield to chassis internally. This is the best way to go, since even a short loop of wire connecting pin 1 to ground at the jack can radiate noise inside your shielded enclosure.
How many of us are using gear that has harnesses with the cable shield grounded through the PCB? If you have a noisy sound system then check to see if this connection scheme was used inside your gear. If so you might try changing it (if possible) and see if the hum problem is reduced or eliminated.
Note: This problem could also occur with phono or RCA jacks if their sleeve connection is isolated from chassis ground or they are mounted on an insulated panel.
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