Manley TNT MICROPHONE PREAMPLIFIER - Manual (Page 18)

The ¼” transformerless output has a switch on the
back panel that allows it to PROPERLY drive +4 dBm
balanced inputs, +4 dBm (pin 2 hot) unbalanced inputs
(rare enough), and –10 dBu unbalanced inputs. In each
case, a very stable and very low output impedance is
given that does not suffer and cause common mode
rejection problems when driving less than perfect
cables and inputs. Typical cross-coupled line drivers
magnify their source resistors while attempting to
deal with imbalances, which only magnifies common
mode rejection problems. For example, a great input
stage may have a CMRR spec of 60 dB at 1kHz, (and
not in the specs is that this typically drops to 30 dB
at 20K, may or may not be 80 dB at 60 Hz, and may
be 30 dB at 10 Hz), but CMRR is more dependent
on the output impedance match of the device driving
it, and that 60 dB spec can drop to 20 dB when there
is a 10% difference between source resistance from
Pin1 & Pin2. Cross-coupled line drivers create CMRR
problems for the sake of idiot-proofing and people
using funky cables and adapters. The TNT just uses
a switch.
Transformers have traditionally been great at the idiot-
proofing aspect and are very “forgiving” and solve
more practical connection issues than they create.
However, the back panel switch does have an effect
on the XLR transformer output. For both ¼” and XLR
outputs, in the “+4 balanced” mode, headroom reaches
+30 dBm comfortably, but drops to +24 dBm when
the switch is set to “+4 unbalanced”, which probably
close to maximum level of most A to D converters.
This might be used to choose which box clips first and
best. The impedance on pin2 and pin3 remains equal
and constant and low, so it will work better than most
balanced outputs into a balanced input even though
it is labelled “unbalanced”. In other words, when
the switch is set for “unbalanced”, it will drive both
balanced and unbalanced inputs fine, and unbalanced
inputs at the right level (no 6 dB drop), but does have
6dB less headroom, which should still be good enough
in most situations (balanced into balanced is the best
choice generally). In the “-10 unbalanced” mode the
¼” jack drops the appropriate 12 dB (not 14) and the
XLR drops 6 dB. Much semi-pro and most consumer
gear is happiest with –10 dBu signals, but you may
need a ¼” to RCA adapter. Again, this can be used in
some situations other than “standard procedures”. One
can use one output to drive an input in one room, the
other to a different room (like an amp in the studio or
a remote truck) and avoid a lot of hassle with splitter
boxes or ground loops and interaction.
Please note that the tube (T) channel has a different
output arrangement, uses different technology, and
has a ¼” output that is only unbalanced and doesn’t
have the back panel switch, yet the ¼” output will still
be OK to drive balanced inputs, especially if one is
trying to avoid transformers today.
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