XTA DS8000/D - Manual (Page 27)

Page 27 DS8000 Operator’s Manual
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Passive splitter with No Isolation
Consider the scenario below — this shows the most basic method of splitting the signal and
distributing it to all the required inputs. This “passive” splitter is so called because it does
not contain any “active” (powered) components and so requires no power to function.
Low Level
Mic Signal (~-40dBu)
Signal
Ground
-40dBu
-40dBu
-40dBu
Local Ground
Local Ground
Local Ground
Outside
Broadcast
Truck
Front of House
Mixing Console
Monitors
Mixing Console
Whilst this method will work, there are several drawbacks:
•
Being passive by nature, there can be no amplification of the signal levels to decrease
their susceptibility to noise pickup. Bearing in mind that microphone signals are very low
level, when dealing with long cable runs, any noise picked up will be amplified at the
mixing desk when the preamp is used to boost the signal to a useable level.
•
Long microphone cables runs normally means higher capacitance — this phenomenon
creates a filter that will decrease the high frequency content present, leading to dull
audio. Adding equalisation at the mixing desk to compensate will also accentuate the
noise content of the signal.
•
The input impedance of each “feed” (so in this case the two mixing desks and whatever is
in the OB truck) will all appear in parallel, producing a significant additional loading effect
on the signal, reducing its level even further for all connections.
•
Phantom power to feed the microphone is normally fed by a pair of 6k8 resistors to limit
the current draw available. Given that there may now potentially be three sources of
phantom power, if these are all unintentionally enabled, the increased current and voltage
available may cause damage to the microphone.
•
The grounding scheme is just a single interconnected system where every ground is
directly connected to every other one. Every engineer knows that multiple grounds
normally mean, at the very least, hum problems! For example, the mixing desks above
might well share a common ground with no problems, but the OB truck will probably have
a generator and so create its own local ground, causing possible issues.