Aphex
Thermionics
- Division of Aphex Systems Ltd.
Model 1100 MkII Mic Preamp - Owner’s Manual
The fact is that, as long as the hum and noise is
induced into both wires equally, the hum will be
rejected even if only one of the wires actually has
the wanted signal on it. That may be a hard fact
to swallow, but look at it this way. The reason we
twist the wires together tightly is to make them
occupy the same thin line in space on average.
Now, if both wires have the same impedance to
ground at both ends, then they will electrically
appear like two identical antennas in space. Both
wires will therefore pick up equal interference
from all surrounding sources. Remember that
when both wires carry identically the same inter-
ference, the interference will be cancelled.
To carry the explanation one last step, look at
figure 2-7. This is the model for an impedance
balanced transmission line. Notice that one wire
is fed from an output amplifier through a resistor
while the other goes to ground through an equal
resistor. If we consider that the amplifier’s output
is essentially a zero ohm source, then we can con-
sider it to be the same as a ground connection
in terms of impedance loading on the wire. This
makes the circuit of figure 2-7 the same as figure
2-6 for the purpose of noise rejection.
So that explains how an impedance balanced
output gives us full noise rejection, but how can
you just drive one line and receive it OK? Well,
that’s really simple. If one wire is always at zero,
and the other has a signal, then the difference
between the two wires will be the signal, right?
Right.
Look at figure 2-8 showing a conventional line
driver where there are two output amplifiers to
drive the balanced line in counter phase through
two equal resistors. The only real difference
between the counter phase driver of figure 2-8
and the impedance balanced driver of figure 2-7
is that to make up for a zero signal on one wire,
the impedance balanced driver must double the
voltage output to the signal wire.
In other words, if we need two volts peak output
balanced, a conventional output stage would
drive negative one volt to one wire and positive
one volt to the other wire. Our circuit just drives
positive two volts to the signal wire. The bal-
anced input circuit in the first case sees positive
1 volt minus negative one volt and gets 2 volts.
In the second case, the balanced input stage sees
positive two volts minus zero volts and still gets
2 volts.
Now here’s why impedance balancing is such
a good deal.
Let’s say you want to patch the
output of the Model 1100 MkII into a line insert
of your studio console. Some of these inserts
are balanced, but most are unbalanced. With
the Model 1100 MkII all you have to do is make
sure pin 2 wires to the patch plug tip, and you’ve
got full signal. You don’t lose half the signal
amplitude and with it 6dB of precious preamp
gain. You also don’t lose the 6dB of peak output
headroom you lose with a counter phased output
stage running unbalanced. In effect, the imped-
ance balanced output stage acts a lot like a trans-
former balanced output but without the sonic
degradation associated with audio transformers.
There is complete information on interfacing the
impedance balanced output of the Model 1100
MkII in section 4 of this manual.
Figure 2-6 Common Noise Rejection Model
Figure 2-7 Impedance Balanced Line
Figure 2-8 Conventional Balanced Line Driver
Page 2-5
