XTA DS8000/D - Manual (Page 28)

DS8000 Operator’s Manual Page 28
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Passive Splitter with Transformer Isolation
This second system uses a passive splitter again, so no amplification on stage, but does
utilise transformers to allow the grounds to be isolated from each other. The input is
connected to one side, the primary, of the transformer, and multiple secondaries provide
isolated outputs for each feed.
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 set-up is considerably better in performance than a basic passive (parallel) split
arrangement, to achieve good quality results, it is dependant on the transformers themselves
being very good quality, and with quality comes increased expense.
The ground isolation will help prevent hum loops and associated noise issues, but the
following problems still remain with an additional one also being introduced:
As there is still no active circuitry, the signals are still being distributed at low levels, which
are susceptible to noise pickup — noise that will be amplified by the mixing desk’s mic
preamps.
The problem of cable capacitance and high frequency loss still remains, as most transformer
systems offer a 1:1 ratio so the microphone loading is not altered. Using a transformer with a
higher ratio is possible (perhaps 3:1) although the “trade-ff” here is that the gain is reduced in
the process, so making the already low level signals even lower. This is only really useful for
line-level signals where a degree of gain loss is not quite so much an issue.
A new problem is introduced regarding phantom power. As DC is not passed through a
transformer, locally derived phantom power must be generated for any microphones
requiring a 48V supply. This may be via a direct passive parallel connection to one mixing
desk (losing the full isolation) or via a separate supply.