Rane CP 52S, Installation Manual

Interconnection-2

Ground Loops

Almost all cases of noise can be traced directly to ground 

loops, grounding or lack thereof. It is important to under-

stand the mechanism that causes grounding noise in order 

to effectively eliminate it.  Each component of a sound 

system produces its own ground internally. This ground is 

usually called the audio 

signal

 ground. Connecting devices 

together with the interconnecting cables can tie the signal 

grounds of the two units together in one place through 

the conductors in the cable. Ground loops occur when the 

grounds of the two units are also tied together in another 

place: via the third wire in the line cord, by tying the metal 

chassis together through the rack rails, etc. These situations 

create a circuit through which current may flow in a closed 

“loop” from one unit’s ground out to a second unit and back 

to the first. It is not simply the presence of this current that 

creates the hum—it is when this current flows through a 

unit’s audio signal ground that creates the hum. In fact, even 

without a ground loop, a little noise current always flows 

through every interconnecting cable (i.e., it is impossible to 

eliminate these currents entirely). The mere presence of this 

ground loop current is no cause for alarm if your system 

uses properly implemented and 

completely

 balanced inter-

connects, which are excellent at rejecting ground loop and 

other noise currents. Balanced interconnect was developed 

to be immune to these noise currents, which can never be 

entirely eliminated. What makes a ground loop current an-

noying is when the audio signal is affected. Unfortunately, 

many manufacturers of balanced audio equipment design 

the internal grounding system improperly, thus creating bal-

anced equipment that is not immune to the cabling’s noise 

currents. This is one reason for the bad reputation some-

times given to balanced interconnect.

A second reason for balanced interconnect’s bad reputa-

tion comes from those who think connecting unbalanced 

equipment into “superior” balanced equipment should 

improve things. Sorry. Balanced interconnect is not compat-

ible with unbalanced. The small physical nature and short 

cable runs of completely unbalanced systems (home audio) 

also contain these ground loop noise currents. However, the 

currents in unbalanced systems never get large enough to 

affect the audio to the point where it is a nuisance. Mixing 

balanced and unbalanced equipment, however, is an entirely 

different story, since balanced and unbalanced interconnect 

are truly 

not compatible

. The rest of this note shows several 

recommended implementations for all of these interconnec-

tion schemes.

The potential or voltage which pushes these noise cur-

rents through the circuit is developed between the indepen-

dent grounds of the two or more units in the system. The 

impedance of this circuit is low, and even though the voltage 

is low, the current is high, thanks to Mr. Ohm, without 

whose help we wouldn’t have these problems. It would take 

a very high resolution ohm meter to measure the impedance 

of the steel chassis or the rack rails. We’re talking thou-

sandths of an ohm. So trying to measure this stuff won’t 

necessarily help you. We just thought we’d warn you.

The Absolute Best Right Way To Do It

The method specified by AES48 is to use balanced lines and 

tie the cable shield to the metal chassis (right where it enters 

the chassis) at both ends of the cable

.

A balanced line requires three separate conductors, two 

of which are signal (+ and –) and one shield (see Figure 1a). 

The shield serves to guard the sensitive audio lines from 

interference. Only by using balanced line interconnects can 

you 

guarantee 

(yes, 

guarantee

) hum-free results. Always use 

twisted pair cable. Chassis tying the shield at each end also 

guarantees

 the best possible protection from RFI [radio fre-

quency interference] and other noises [neon signs, lighting 

dimmers].

Neil Muncy

1

, an electroacoustic consultant and seasoned 

veteran of years of successful system design, chairs the AES 

Figure 1a. The right way to do it.

+

–

G

T

R

S

RED

BLACK

2-CONDUCTOR SHIELDED CABLE

2-CONDUCTOR SHIELDED CABLE

2-CONDUCTOR SHIELDED CABLE

SHIELD

RED

BLACK

SHIELD

RED

BLACK

SHIELD

RED

BLACK

SHIELD

RED

BLACK

SHIELD

RED

BLACK

SHIELD

CHASSIS

GROUND

CHASSIS

GROUND

SIGNAL

GROUND

S

R

T

G

–

+

BALANCED OUTPUTS

BALANCED INPUTS

MALE

FEMALE

MALE

FEMALE

1

2

3

C

1

3

2

2

C

3

1

2

1

3