Rane MP26 Operator's Manual

Also see for MP26: Operator's manual

Contents

Rane MP26 Operator's Manual Manual pdf 20 page image

Interconnection-2Ground LoopsAlmost all cases of noise can be traced directly toground loops, grounding or lack thereof. It is importantto understand the mechanism that causes groundingnoise in order to effectively eliminate it. Each compo-nent of a sound system produces its own ground in-ternally. This ground is usually called the audio signalground. Connecting devices together with the inter-connecting cables can tie the signal grounds of the twounits together in one place through the conductors inthe cable. Ground loops occur when the grounds of thetwo units are also tied together in another place: viathe third wire in the line cord, by tying the metal chas-sis together through the rack rails, etc. These situationscreate a circuit through which current may flow in aclosed “loop” from one unit’s ground out to a secondunit and back to the first. It is not simply the presenceof this current that creates the hum—it is when thiscurrent flows through a unit’s audio signal ground thatcreates the hum. In fact, even without a ground loop, alittle noise current always flows through every inter-connecting cable (i.e., it is impossible to eliminate thesecurrents entirely). The mere presence of this groundloop current is no cause for alarm if your system usesproperly implemented and completely balanced inter-connects, which are excellent at rejecting ground loopand other noise currents. Balanced interconnect wasdeveloped to be immune to these noise currents, whichcan never be entirely eliminated. What makes a groundloop current annoying is when the audio signal is af-fected. Unfortunately, many manufacturers of balancedaudio equipment design the internal grounding systemimproperly, thus creating balanced equipment that isnot immune to the cabling’s noise currents. This is onereason for the bad reputation sometimes given to bal-anced interconnect.A second reason for balanced interconnect’s badreputation comes from those who think connectingunbalanced equipment into “superior” balanced equip-ment should improve things. Sorry. Balanced inter-connect is not compatible with unbalanced. The smallphysical nature and short cable runs of completelyunbalanced systems (home audio) also contain theseground loop noise currents. However, the currents inunbalanced systems never get large enough to affectthe audio to the point where it is a nuisance. Mixingbalanced and unbalanced equipment, however, is anentirely different story, since balanced and unbalancedinterconnect are truly not compatible. The rest of thisnote shows several recommended implementations forall of these interconnection schemes.The potential or voltage which pushes these noisecurrents through the circuit is developed between theindependent grounds of the two or more units in thesystem. The impedance of this circuit is low, and eventhough the voltage is low, the current is high, thanks toMr. Ohm, without whose help we wouldn’t have theseproblems. It would take a very high resolution ohmmeter to measure the impedance of the steel chassis orthe rack rails. We’re talking thousandths of an ohm. Sotrying to measure this stuff won’t necessarily help you.We just thought we’d warn you.Figure 1a. The right way to do it.+–GTRSREDBLACK 2-CONDUCTOR SHIELDED CABLE2-CONDUCTOR SHIELDED CABLE2-CONDUCTOR SHIELDED CABLESHIELDREDBLACKSHIELDREDBLACKSHIELDREDBLACKSHIELDREDBLACKSHIELDREDBLACKSHIELDCHASSISGROUND CHASSISGROUND SIGNALGROUNDSRTG–+BALANCED OUTPUTS BALANCED INPUTSMALE FEMALEMALE FEMALE123C132 2C 31213