Interconnection-5
buzzing, there is an incompatibility somewhere in the
system’s grounding configuration. In addition to these
special cable assemblies that may help, here are some
more things to try:
1. Try combinations of lifting grounds on units sup-
plied with lift switches (or links). It is wise to do this
with the power off!
2. If you have an entirely balanced system, verify all
chassis are tied to a good earth ground, for safety’s
sake and hum protection. Completely unbalanced
systems never earth ground anything (except cable
TV, often a ground loop source). If you have a mixed
balanced and unbalanced system, do yourself a favor
and use isolation transformers or, if you can’t do
that, try the special cable assemblies described here
and expect it to take many hours to get things quiet.
May the Force be with you.
3. Balanced units with outboard power supplies (wall
warts or “bumps” in the line cord) do
not
ground the
chassis through the line cord. Make sure such units
are solidly grounded by tying the chassis to an earth
ground using a star washer for a reliable contact.
(Rane always provides this chassis point as an exter-
nal screw with a toothed washer.) Any device with
a 3-prong AC plug, such as an amplifier, may serve
as an earth ground point. Rack rails may or may not
serve this purpose depending on screw locations and
paint jobs.
Floating, Pseudo, and Quasi-Balancing
During inspection, you may run across a ¼" output
called floating unbalanced, sometimes also called psue-
do-balanced or quasi-balanced. In this configuration,
the sleeve of the output stage is not connected inside
the unit and the ring is connected (usually through a
small resistor) to the audio signal ground. This allows
the tip and ring to “appear” as an equal impedance,
not-quite balanced output stage, even though the out-
put circuitry is unbalanced.
Floating unbalanced often works to drive either a
balanced or unbalanced input, depending if a TS or
TRS standard cable is plugged into it. When it hums, a
special cable is required. See drawings #11 and #12, and
do not make the cross-coupled modification of tying
the ring and sleeve together.
References
1. Neil A. Muncy, “Noise Susceptibility in Analog and Digi-
tal Signal Processing Systems,” presented at the 97th AES
Convention of Audio Engineering Society in San Fran-
cisco, CA, Nov. 1994.
2.
Grounding, Shielding, and Interconnections in Analog
& Digital Signal Processing Systems: Understanding the
Basics
; Workshops designed and presented by Neil Muncy
and Cal Perkins, at the 97th AES Convention of Audio
Engineering Society in San Francisco, CA, Nov. 1994.
3. The entire June 1995 AES Journal, Vol. 43, No. 6, available
$6 members, $11 nonmembers from the Audio Engineer-
ing Society, 60 E. 42nd St., New York, NY, 10165-2520.
4. Phillip Giddings,
Audio System Design and Installation
(SAMS, Indiana, 1990).
5. Ralph Morrison,
Noise and Other Interfering Signals
(Wiley, New York, 1992).
6. Henry W. Ott,
Noise Reduction Techniques in Electronic
Systems
, 2nd Edition (Wiley, New York, 1988).
7. Cal Perkins, “Measurement Techniques for Debugging
Electronic Systems and Their Instrumentation,”
The Pro-
ceedings of the 11th International AES Conference: Audio
Test & Measurement,
Portland, OR, May 1992, pp. 82-92
(Audio Engineering Society, New York, 1992).
8. Macatee,
RaneNote
: “Grounding and Shielding Audio
Devices,” Rane Corporation, 1994.
9. Philip Giddings, “Grounding and Shielding for Sound and
Video,”
S&VC
, Sept. 20th, 1995.
10. AES48-2005:
AES standard on interconnections —
Grounding and EMC practices — Shields of connectors
in audio equipment containing active circuitry
(Audio
Engineering Society, New York, 2005).
Band-Aid is a registered trademark of Johnson & Johnson
Winning the Wiring Wars
• Use balanced connections whenever possible, with
the shield bonded to the metal chassis at both ends.
• Transformer isolate all unbalanced connections
from balanced connections.
• Use special cable assemblies when unbalanced lines
cannot be transformer isolated.
• Any unbalanced cable must be kept under 10 feet
(3 m) in length. Lengths longer than this will ampli-
fy all the nasty side effects of unbalanced circuitry's
ground loops.
Summary
If you are unable to do things correctly (i.e. use fully
balanced wiring with shields tied to the
chassis
at both
ends, or transformer isolate all unbalanced signals
from balanced signals) then there is no guarantee that
a hum-free interconnect can be achieved, nor is there a
definite scheme that will assure noise-free operation in
all configurations.
