R a d i o S y s t e m s , I n c .
M A - 4 M a n u a l
P a g e 4
Using Active Balanced Circuitry
Balanced lines have been used for many years and are in continuing use today because of their immunity to stray
pickup. Induced signals appear on both sides of the balanced line. The receiving end of the balanced line
responds only to the difference voltage between the lines which is the desired signal. Induced signals are
common to both and are balanced out.
Transformers have been the mainstay of balanced circuitry for decades. Unfortunately, transformers cause distortion
and ringing, and are susceptible to magnetic flux pickup. Further, good quality audio transformers are very
expensive.
The use of op-amp balanced circuitry has the advantage of transformers without the disadvantages. The only caveat
is that careful wiring practices are more important with active balanced than with transformers.
Active balanced outputs and inputs use three wires: +, -, and ground. The + and - terminals are both driven and
neither should ever be connected to ground. For best performance, a three-conductor shielded wire should be
used. The third wire completes the ground circuit. The shield should be connected to the ground at one end of
the wire only. If a two-wire shielded cable is used, it is important that a ground connection be made between the
sending and receiving units. A ground circuit through equipment chassis or through three-prong AC cord ground is
also acceptable.
Single-ended audio interconnections lack the interference immunity of balanced hook-ups. For the reason, keep
unbalanced connections short, direct, and well separated from AC power wires. To drive a single-ended load
from an active balanced source, use coaxial wire: + to center conductor and ground to shield, leaving the -
output unconnected. To feed an active balanced input from a single-ended source, use coaxial wire, connecting
the hot center conductor to +. Connect the shield to ground and put a jumper from ground to -.
When driving an active balanced input from a transformer balanced floating source, use two conductor shielded
wire. Ground the shield at the source end. Establish good ground between the chassis either directly or though
AC plug ground prongs. At the load, connect the + lead to the + input and the - lead to the - input. Put two 300
ohm resistors in series between the + input and the - input and connect their mid-point to the load ground. This
correctly terminates the source output transformer for optimum frequency and transient response (freedom from
ringing) and provides a low impedance return path for leakage and induced hum. If more than one active bal-
anced load is to be placed across a floating balanced transformer source, install this resistive termination once
only. From that location to the active balanced loads, run three-conductor shielded wire, shield continued from
the sources chassis, + from +, - from -, and ground from the mid-point of the terminating resistors.
To drive a balanced floating transformer load from an active balanced source, use shielded wire. Connect the shield
to source ground and leave the shield open at the load end. C to + and - to -, and establish a good
source ground to load chassis connection, either through a third wire in the interconnect cable or through chassis
contact or AC cord third wire ground.
Interconnections between pieces of stereo equipment require doubling the connections described above without
duplicating the ground connection. Between pieces of active balanced stereo equipment, then, 5 shielded
conductors should be run.
When testing active balanced equipment with single ended test equipment, do not connect the - to test equipment
ground. Most modern test equipment provides balanced inputs. In many dual-trace oscilloscopes, balanced
signals may be displayed by running the two inputs in the “add” mode with one input switched to invert. To
perform a test with single-ended equipment, + and - outputs must be tested independently and their results
added. Testing only a single output results in a 6 db loss in output level.
The active balanced equipment interconnection format makes possible state of the art fidelity. Careful attention to
detail and conservative practice will be rewarded with outstanding flat frequency response, low distortion, and
wide dynamic range.
