A Clean Audio Installation Guide™

5.0 INTERCONNECT

5.1  Balanced Inputs

5.2  Balanced Outputs

5.3  Star Ground

5.4  Shield Wires

5.4.1 Forward Referencing

5.5  Jackfields

5.0 INTERCONNECT
And now for the nitty gritty. If you adopt the following rules for the interconnection of your equipment, hum free audio can usually be achieved the first time around.

5.1 Balanced Inputs
All equipment must have well-trimmed balanced inputs, preferably high impedance. This is absolutely necessary, as we have just seen, to reject the residual power line related voltage differences between pieces of equipment. For equipment that does not have balanced inputs from the factory, use an interface box such as the Benchmark IFA-1. Alternatively, install internal balanced inputs such as the Benchmark DIA-1 or DIA-2. This is sometimes preferable because of the control that you have over the interface grounds.

5.2 Balanced Outputs
All equipment should have balanced low impedance outputs. However, satisfactory performance from unbalanced outputs can sometimes be obtained using these rules of interconnection, provided the existing output is low impedance (50 ohm or less) and the line is not a long line in a high RF field.

Without the large build out resistor (the actual output stage is usually an op-amp with a 0 ohm output impedance) the output amplifiers would be fighting each other and oscillation is the result. This is the only advantage for increasing the output impedance. As we have seen earlier, there are very significant disadvantages to a high output impedance. In addition to the havoc that is wreaked on frequency response by the high output impedance and cable capacitance, another major problem will occur. If a differential input is used as the line receiver from such an output, the high output impedance will severely unbalance the diff-amp network and ruin the wonderful CMR that we paid dearly to achieve. The CMRR of a balanced receiver connected to an unbalanced source is determined by the ratio of the input impedance of the receiver to the output impedance of the source. It is better to play it safe, if at all possible, and add balanced, low impedance outputs where they do not exist. At a bare minimum, reduce the source impedance of the unbalanced output to approximately 30 ohm. Then add the same resistance to the output ground side. These two resistors should be matched as closely as possible, i.e. approximately 0.01%. This will provide a balanced output impedance to the differential input, even though the signal itself is not balanced. In order to really maintain the CMRR of the input diff-amp, an instrumentation type input stage is needed with identical input impedance on each leg of the input stage. (See figure 4)

5.3 Star Ground
Interconnect the analog reference from all pieces of equipment with a separate insulated ground wire (#14 or larger is preferable) to form a "star" or single point ground system. The hub of this star could be a 1/4" thick copper plate perhaps 4 by 6 inches in size and located physically at the center of the audio signal hub, i.e. an audio console, distribution amplifier or routing switcher system. The copper plate needs to be insulated from any other ground. An additional wire must be added to this star to connect the copper plate to the system's power ground located within the power box, or, better yet, go directly to the power company's local ground point, typically a ground stake. Be sure to use your own clamp.

At times, this "star" configuration must be created using the third wire (safety ground) of the power cord. This is, most assuredly, inferior to a separate conductor from the power system star ground, the reason being that the "ground reference" third wire may have a power related signal induced into it by the parallel runs of current carrying mains. Not only that, but ferrous conduit, while ideal from a shielding standpoint, will raise the impedance of the ground wire, and render it ineffective at all but power line frequencies. However, if the equipment does not have an analog signal "ground" reference within the chassis that can be made to float from the chassis, as is often the case, then this may be the only recourse. Do not be tempted to eliminate the third wire safety ground. It has been included to protect your life in the event of a power line component failure! Third wire safety grounds should have a separate wire that returns to the power box from every isolated type outlet (Hubbel #IG5362, or General Electric #GE8300-IG). Do not use the metal conduit as a substitute for the separate third wire. This separate third wire arrangement forces a star configuration. These third wire grounds should tie together in the power box via a copper buss bar that is floating from the box, which in turn is tied to the power system ground point via a 4-0 cable. (See Ref. 1 for more information)

In any case, all signal references must be tied together with a separate wire other than the drain of the shielded pair used to interconnect the signals, and, hopefully, these references will be tied together with only  one external wire.

Ideally, all chassis will be tied via a brute force system by the equipment racks in which they are installed, and the signal references will be tied together via a star wire system. Better equipment manufacturers provide a terminal strip with the signal reference on one terminal and the chassis ground in the adjacent terminal in their equipment to allow for proper grounding practice.

With large systems, such as in network facilities where there are many systems that must be interconnected, a star of stars is the correct approach to take. The hub of the stars is again the central point of the signal distribution, usually the distribution amplifier or routing switcher system. The time taken in carefully thinking through this signal reference system will pay large dividends.

5.4 Shield Wires
Connect all inputs and outputs together without connecting the drain (or shield, if you prefer) at one end. The reason for not tying both ends to the shield is that we do not want any power line related currents to flow between equipment through this wire and risk inducing a signal into our audio inputs. Edgar Lee Howard⁽¹¹⁾ has shown that a technical advantage exists for tying the shield of an interconnect cable at the send end rather than at the receive end. The potential advantage is a reduction in coupling of high frequency noise to the audio signal due to "line to shield" capacitive unbalances.

5.4.1 Forward Referencing
Balanced outputs, of course, are connected in the normal one-to-one manner. However, unbalanced outputs must be connected so that the signal is forward referenced (See Figure 4). This is accomplished by connecting the non-inverting (+) input to the output terminal and the inverting (-) input to the "ground" (signal reference) terminal of the unbalanced output. Now, any difference of potential that still remains between the two pieces of equipment will be ignored by the input as a common mode signal.

5.5 Jackfields
When using jackfields, be sure to use a fully balanced jackfield even when connecting to unbalanced outputs so that forward referencing can be maintained.

The jackfield presents an interesting problem in light of the above shield connection recommendations because the shield lines of the jacks are not usually switched. If you wire the jackfield so that the drain wires tie across from output jack to input jack as a part of the "normaled" wiring and do not buss the grounds together, everything is "according to Hoyle" ...until you patch. Then, all at once, you have grounds tied together that you did not want. Perhaps the best way to approach the wiring of a non ground-switching jackfield is to tie all drain wires at the equipment ends, both input and output, but not at the jackfield. Then, at the jackfield buss all grounds together and tie them to the system ground as though it were a separate piece of equipment, using a separate insulated ground wire.

However, the best approach to the problems of ground loops with patch bays is to use ground switching jacks. The following devices will switch the ground when a patch cord is inserted: ADC JJ-075; Switchcraft MT-336A; Audio Accessories 322A - Mil JJ-075. While it is not common to run microphone lines through a patch bay, occasionally it is necessary. In such a case, it is our firm recommendation that ground switching jacks be used.

Go to: Section 6.0