Specifications

Radio Systems Millenium-D Digital Console Page 45
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 nei-
ther 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 receiv-
ing 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 unbal-
anced 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 balanced load is to be placed across
a floating balanced transformer source, install this resistive termination once only. From that location to the active bal-
anced 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. Connect + 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.