207d
instruction Manual
Aphex Systems Ltd. Model 207D
Page 20
207d
2-ch tube mic preamp with adc
Page 21
Aphex Systems Ltd. Model 207D
wire is the source and which is the return alternates
accordingly. In this regard, balanced and unbalanced
lines are the same. They both need two conductors.
What makes a system unbalanced is when one of
the wires is formed into a tube that wraps around
the other conductor, without touching it, such that
the outer conductor can be said to “shield” the inner
conductor. This describes all of the coaxial cable used
for video, cable-TV and radio as well as most of the
high fidelity audio cables.
Balancing
If both conductors are identical insulated wires that
are twisted together, then they form a balanced line.
This describes telephone lines, microphone cables,
and most professional audio cables. Typical balanced
cables include an additional shield wrap around the
twisted pair, but this is not strictly required for bal-
anced lines to work properly.
Many people, because they have more experience
with unbalanced wiring, think that balanced is con-
fusing. Believe it or not, balanced lines are really
easier to understand than unbalanced. There is no
grounding issue with balanced, and the way it works
is perfectly natural and simple. Balancing naturally
rejects hum and noise and eliminates all sorts of com-
plications in interfacing.
Balanced transmission works something like this.
7.0 Appendices
Appendix A: Balanced and Unbalanced
Lines and Operating Levels
Interfacing all types of equipment with balanced
and unbalanced lines and can sometimes be trouble-
some. First you have to somehow connect balanced
to unbalanced and then you have to deal with dif-
ferent levels. This tutorial will teach you about the
principles of balanced and unbalanced lines, wiring
standards, and how to effectively interface them.
Standards
Professional audio equipment usually comes equipped
with inputs and outputs that are balanced using
3-pin XLR connectors and sometimes 1/4 inch phone
jacks as well. This equipment most often is designed
to operate at +4dBu, a professional industry stan-
dard. That translates to a magnitude of 1.23 volts
RMS (Root-Mean-Squared).
Consumer gear has unbalanced I/O as standard, usu-
ally on RCA jacks. The normal operating signal level
follows the IHF (Institute of High Fidelity) standard of
-10dBV, or 0.316 volts (316mV) RMS. Converting to
dBu dimensions, this works out to be the same as
-7.79dBu. There is therefore a difference of 11.79dB
between pro and consumer operating levels.
Grounding
There is the notion that some king of earthly “ground”
exists out there that sinks all the noise and acts as
some kind of a shield. You see wires connected to
ground rods and water pipes that are supposed to
get a good ground. This is not a correct interpreta-
tion of grounding from an audio standpoint. Proper
grounding of equipment and wiring is important and
you will gain a better understanding of that as you
read along.
Balanced -vs- Unbalanced
Every audio signal is connected through a circuit.
The circuit must contain two conductors to create a
complete return path. In other words, a signal volt-
age is conducted to a piece of equipment by injecting
a current into a wire. That current has to flow to the
destination through the wire and return back to the
source through another wire. Since audio is an alter-
nating voltage, swinging through negative and posi-
tive polarity, the current through the two conductors
changes direction each alternate half cycle. Which
Figure 1 Balanced Line Model
Figure 2 Unbalanced Line Model