The Art & Science of Audio & Video

Distortion, impedance & the 10:1 rule.

PROBLEM: I am troubleshooting an audio system installation in a large conference room that can be divided into three separate rooms. Each room has a dedicated powered mixer with distributed ceiling speakers and the source equipment is in a roll around portable rack. The idea obviously is that the customer can take the source equipment to, whichever room is in need or it can be shared between any combinations of the three. The only problem is if one of the powered mixers is turned off because a room is not in use it causes distortion in the other areas. From what I can tell who ever did the original installation has simply paralleled the sources to all three mixers. How can we alleviate this distortion.

SOLUTION: Wow it sounds like these guys have left you with a mess. Okay there are several things wrong with the way this system is configured. We’ll start with what’s wrong and why then we’ll tackle how to fix it. First you (and by you I mean they) cannot simply parallel components at will. Just like speakers electronics have input/output impedances as well. The rule of thumb is to always maintain a 10:1 ratio between input/output impedance (i.e. input should be ten times greater than the output). This concurrently maximizes voltage transfer, which means maximum signal level. When paralleling equipment as Ohm’s law dictates the impedance will drop therefore current transfer increases and voltage transfer decreases resulting in a lower signal to noise ratio. There is also another potential problem with sharing, or paralleling, a source between systems like that. If each room is not on the same circuit you could possibly have different ground references causing hum. Second issue is when you turn any one of the powered mixers off that could be causing a short across that input. Normally this would not be a problem however in this case the source is now trying to output signal across a short and other additional inputs. So I think you can see how the problems are starting to compound here. The easiest and most cost effective way around this is to add a line level distribution amplifier to the source rack. However some system re-engineering is badly needed here and would benefit the customer greatly. One amplifier for all three rooms and independent source selection with volume controls in each room would probably be more appropriate. I see great potential for a sale here!


March 4, 2008 - Posted by | Troubleshooting | , , , , , , , , , ,

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