between 3 and 6dB. This isn’t much of a problem by
itself, but if any of the mixers is for some reason un-
plugged or switched from ‘Microphone’ (1200 ohms
input impedance) to ‘Line’ (10k bridging impedance),
this sudden change in microphone level can trigger
feedback/howlround in the PA system, requiring
some frenzied adjustments.
Second, and more important perhaps, the micro-
phone’s frequency response is changed. At the mi-
crophone’s resonant points, the response (with loads
below the rated 1200 ohms) may dip or peak by sev-
eral extra dB, causing a substantial change in the way
the microphone sounds. This upsets the engineer’s
judgement as to which microphone to use, and re-
quires extra EQ’ing effort in an attempt to recover
the intended sonic characteristics.
Because the response changes can involve changes
in quite sharp dips and peaks, not many console EQ
sections will be capable of fully or satisfactorily com-
pensating the results of the microphone’s excess load-
ing. Again, if the ‘Microphone/Line’ settings of any
of the 2,3 or 4 mixers across the microphone are sub-
sequently changed, the frequency response will be
upset, possibly causing howlround, and certainly
causing the microphone’s sound to change again, re-
quiring further EQ’ing. Even without Microphone/
Line switching, level and tonal changes can occur
when pad switches are operated, as in some consoles
their setting significantly alters the input load imped-
ance.
Overall, as the PA crew have little control over what
broadcasting and recording engineers do with the
consoles in their trucks, this explains why broadcast and
recording link-ups are regarded with some trepidation...
Why Microphone Splitter Transformers
don’t help.
Microphone ‘splitter’ transformers are commonly
employed when microphones are connected to more
than one or two mixers. They usually have 1:1 ratios,
eg. 1:1:1:1 for a three-way split. It’s a common belief
that they alleviate the mixers’ loading effects. This
however is wrong, unless the transformer is combined
with active electronics.
Usually for reasons of cost, splitter transformers are
stand-alone, passive devices. If so, they can only ap-
portion the microphone’s impedance equally between
the mixers, exactly as would happen without the
transformer!
Splitter transformers are still useful, as
they galvanically isolate the grounds breaking poten-
tial loops, and also restricting the potential for dam-
age and death if any circuits are accidentally con-
nected to the mains power. However, Ground isola-
tion can be achieved more cheaply by breaking shield
connections with switches, or using low value resis-
tors for a “soft float”.
Since good transformers are highly expensive, and
more affordable transformers will always add signifi-
cant frequency response, phase and distortion aber-
rations of their own, it’s really best to avoid them
unless galvanic isolation is essential for safety, for
example, in outdoor events where wet weather is
possible, or where insurance requires it.
Benefits of an Active Split
Because an Active Microphone Splitter makes each
feed immune to changes caused by the loading pre-
sented by additional mixer connections, one of its psy-
chological high notes is that harassed live sound en-
FIG.2: An Active Microphone Splitter allows several
consoles to be used without affecting microphone loading.