Solutions for Mixed Balanced and Unbalanced Systems

It is obvious from the vast quantity of literature that for fully balanced operation, the shield should connect to chassis ground at the point of entry. This is also true for unbalanced operation when a third shield conductor is available; connect the shield to chassis ground at the point of entry. However, this is only valid when 2-conductor shielded cable is used.

Shielded 2-Conductor Connectivity

Figure 5 shows recommended wiring for all combinations of balanced and unbalanced I/O interconnections when 2-conductor shielded cable is used. Figure 5 also includes the two most common manufacturer shield-grounding schemes; signal-grounding the shield and chassis-grounding the shield. Identifying these schemes for every unit in a system is essential to debug system hum and buzz. This is no simple task since chassis and signal grounds are connected together. The goal is to find out if the manufacturer connected them together is such a way that shield currents do not affect the audio signal. The dashed lines in Figure 5 represents the units' chassis boundary. Connections between dashed lines are functions of the cable. Connections outside these lines are the manufacturer's choosing, whether conscious or unconscious.

Figure 5 is arranged such that the top and left most figure (5a) is the theoretical "best" way to connect equipment with optimal results. The "best" way being, everything completely balanced with all shields (pin 1s) connected to chassis ground at the point of entry. As one moves down or to the right, degradation in performance is expected. Whether a system operates acceptably or obeys these theoretical predictions is too system-specific to predict accurately. However, one must start somewhere.

The quality and configuration of the input and output circuits are omitted from Figure 5 and the ensuing discussion, to focus on cable wiring and the internal wiring of the units. The I/O circuitry is assumed ideal.

Fully Balanced

Fully balanced systems (left column in Figure 5) provide the best performance when both ends of the shield connect to units with chassis-grounded shields (Figure 5a). When units with signal-grounded shields are encountered, disconnect the shield at the signal-grounded end (Figures 5b & 5c). This keeps the induced shield currents out of the audio signal ground. If both units involved have signal-grounded shields, you have entered the twilight zone (Figure 5d). This is perhaps the most common scheme. Most disconnect one end of the shield, specifically which end is disconnected creates strong political debates and is left for the individual user to decide [6]. Never disconnect both ends of a shield.

Unbalanced Output Driving Balanced Input

The second column in Figure 5 shows unbalanced outputs driving balanced inputs. Again, only shielded 2-conductor cable is used. The best case here has both ends of the shield connected to units whose shield is chassis-grounded (Figure 5e). Some may argue that the induced noise on the signal conductors may be injected into the "sending" unit through the unbalanced output stage. This is a function of the system and output circuit, and is quite likely. Disconnecting the shield at the unbalanced output might reduce this problem.

When units with signal-grounded shields are encountered, disconnect the shield at the signal-grounded end (Figures 5f & 5g). This keeps the noisy shield currents out of the audio signal ground. If both units involved have signal-grounded shields, you've entered the twilight zone again (Figure 5h). Support your one-end-only political party (Figure 5l).

Balanced Output Driving Unbalanced Input

The third column in Figure 5 is the most troublesome, balanced outputs driving unbalanced inputs. Since the input stage is not balanced, induced noise on the signal conductors is not rejected. If you must use an unbalanced input, use as short an input cable as possible. This reduces the induced noise. There's a reason it's hard to find and buy unbalanced RCA cables longer than 12 feet. Figure 5i shows both ends of the cable shield connected to units with chassis-grounded shields. If the units are far apart, the chance of the shield currents inducing noise on the signal conductors is greater. Keeping this cable very short reduces the shield current and therefore reduces the noise that is not rejected by the unbalanced input stage. Most systems may require disconnecting one end of the shield for the Figure 5i case. Even a small current in the shield may prove too much for an unbalanced input stage. Again, support your favorite one-end-only political position.

Disconnect the shield at units with signal-grounded shields (Figures 5j & 5k). If both ends have signal-grounded shields, run for your favorite one-end-only political party. (Figure 5l).

This scheme connects the balanced output's negative output to signal ground, rather than a high impedance input. Many balanced output circuits will attempt to drive this signal ground, causing great distortion and potentially damaging the output stage. Other balanced output stages are termed "floating" balanced. (Analog Devices SSM-2142 Balanced Line Driver chip is one example.) Also called a cross-coupled output, these circuits mimic the performance of fully balanced transformer solutions and are designed so the negative output can short to signal ground. If you find or use this scheme, be sure that the balanced output stage can properly handle signal ground on its negative output.

Full Unbalanced

Fully unbalanced systems do not provide a 3-conductor connector to enable proper use of a shield. In the unlikely event you run across one, use the wiring in the fourth column (Figure 5m-p). Again keeping cable lengths short will reduce noise problems, with or without a shield.

Most home audio systems are fully unbalanced. Millions of these systems work virtually hum and buzz-free every day, due to their small nature, short cable runs and 2-conductor AC line cords. The headaches begin when one tries to add a balanced unit to such a system. In unbalanced home audio products neither of the line cord's conductors connects to the chassis, since plugging older, non-polarized AC plugs into an improperly wired outlet would place the "hot" wire on the unit's chassis. Lack of the third pin on the line cord prevents ground loops in home systems since a second path to ground, or between units, is unavailable. Professional audio equipment generally comes equipped with a 3-wire line cord. The third wire (green wire) is required to connect to the chassis. This provides the second ground path (loop) from one unit to the next.

Connector Choice

Connector type was purposely left out of Figure 5 and the above discussion since connector choice adds another layer of complexity to interconnection systems. The most troublesome culprit is the 1/4" connector. Mono 1/4" connectors are used on most musical instruments and in phone systems. Stereo 1⁄4" connectors are used for headphones, balanced interconnection, effect and insert send/return loops, relay switch closure points, and an extravagant collection of other miscellaneous connections. Murphy's Law tells us, if you provide such a diverse selection of 1/4" interconnection options, they will be hooked up improperly. The audio industry's problem is that many of these options are completely incompatible. A properly wired mono 1/4" connector has signal ground on the sleeve, a properly wired balanced 1/4" connector has chassis ground on the sleeve. Interconnecting this combination should not be achievable -- much like trying to connect 120 VAC to an RCA jack (see Figure 6). The 1/4" connectors low cost, high availability, and small size all contribute to its widespread and varied use. Undoubtedly the numerous interconnection uses of such a popular connector arose for these reasons.

Sadly, the possibility of including connector type in a recommended practices document is slim. The duplicate connectors on many audio components contributes to higher costs and wastes millions of dollars worth of connectors that are never used. Some manufacturers are attempting to eliminate the 1/4" connector to avoid the confusion and problems when 1/4" jacks are used. This is a step in the right direction, though the high density allowed by these connectors requires less valuable rear-panel real estate. Most marketing departments prefer thirty connectors per inch, making the currently available 3-pin (XLR) alternative markedly unpopular. What is needed is a 3-pin connector solution that requires less space than the traditional XLR connector. A locking, stackable 3-pin mini-DIN comes to mind.

Terminal block and Euroblock connector types are used when separate cable-end connectors are unnecessary or impractical. These connection solutions provide the user with the most wiring options when both signal and chassis ground terminals are available. This allows the user to decide which wiring practice to incorporate. This is the most desirable solution, though most studio equipment does not call for these connector types.

"Hidden" Balanced I/O Solution

An interesting solution for mono interconnection incorporates unshielded balanced stages, much like most telephone systems. Figure 7 shows this configuration. This allows off-the-shelf mono cables to be used to connect unbalanced or unshielded-balanced I/Os to a system. Though not as ideal as a shielded balanced interconnection, systems with mono connectors, like home theatre systems, benefit from this configuration. Keeping cable lengths short is essential and not difficult in a home environment.

One advantage of such a system, besides making it impossible, on fully balanced systems, to get signal ground on an external cable, is that it provides an easy upgrade path to balanced signal connections. The manufacturer need only change the connector to a 3-pin version. Also crucial for this solution is the need to have either cross-coupled output stages or an output that does not mind a grounded negative output, since the negative output may connect to signal ground.

A slight disadvantage lies with the common use of non-twisted pair cables in off-the-shelf mono cables. Using twisted cable with this unshielded balanced scheme greatly improves the achievable performance.

The Muncy Solution

Neil Muncy is an electroacoustic consultant and veteran of years of successful system design. His long standing solution to these issues provides real-world proof of the guaranteed performance achievable with fully balanced systems wired per the Audio Engineering Society recommendation. Mr. Muncy implements what I call the Muncy solution and alters every piece of gear so it has balanced inputs and outputs with both ends of the shield connected to chassis ground at the point of entry. Decades of this practice, and the early research and discipline to understand the basic physics required to implement it properly, have given Mr. Muncy the drive to tirelessly tour the country dispersing his findings. Mr. Muncy's seminars educate those who are ignorant of the "right" way to wire balanced equipment, and show the advantages gained when every piece of gear in the system is wired accordingly.

Current Manufacturer Solutions

Let's examine manufacturer's choices regarding signal-grounding or chassis-grounding balanced cable shields. The problems of signal-grounding balanced shields have already been covered. Users choose to live with hum & buzz, alter off-the-shelf cables by disconnecting one end of the shield or, even in fully balanced systems, use isolation transformers. All are senseless alternatives for inconsistent manufacturing methods. Their advantages and disadvantages are outlined in Tables 1 and 2.

For the manufacturer, several shield connectivity choices are available.

1. Keep or change shield connections to chassis ground.
Manufacturers who chassis-grounded balanced shields originally must still recommend isolation transformers, cable altering and the technical support that go with these hum and buzz solutions. This is unfortunately necessary, since not all balanced equipment has chassis-grounded shields. Ideally, if all balanced equipment were suddenly and miraculously chassis grounded on both ends at the point of entry, off-the-shelf cables could be used in every system, leaving only the I/O circuitry to dictate system performance.

2. Change shield connections to signal ground.
Though this would be a step backward, it is still a choice. Most equipment is connected this way and most users have found their own costly "add-on" interconnection solutions.

3. Offer the shield connection choice to the user.
Provide both options. Two independent screw terminals (one signal, one chassis), a switch or a jumper option permit the user to wire as they please. More on this later.

Manufacturer Solutions for Efficiently and Effectively Connecting Balanced Shields to Chassis

Printed Circuit Board Mounted Jacks

The printed circuit mounted jack provides manufacturers with the most cost-effective solution for transferring cable signals to a printed circuit board. On the board, most manufacturers connect the balanced shield conductor (to signal ground) with a board trace. For optimum balanced performance connect the shield to chassis ground at the point of entry. This means that the shield conductor, to avoid spraying any induced RF energy into the box, never passes the chassis' outer plane. This is not a simple task. Currently no printed circuit mounted 3-conductor connectors provide this optimum solution.

Terminal Strips

When both signal and chassis ground terminals are provided on terminal block or Euroblock connector types, the user decides which wiring practice to incorporate. This is a desirable solution, though a lot of equipment does not call for these connector types. Providing a Pem nut, screw and toothed washer near the cable terminals, instead of an additional chassis-grounded screw terminal, prevents the shield conductor from entering the enclosure -- supplying the ultimate interconnection solution. (This is why Rane terminal strips and Euroblock inputs and outputs have a PEM nut, screw and tooth washer above the shield connection.) Users select their preferred wiring practice, and the shield can not spray RF into the enclosure. Maintaining the shield around the signal conductors all the way to the I/O terminals is important. Keeping the Pem screw near the terminals is therefore essential.

Panel Mount Jacks with Wires

Panel mount jacks require the manufacturer to connect a wire from a terminal pin to the printed circuit board or chassis. This is a good solution for chassis-grounding a shield, though this allows the shield to enter the enclosure. Keep the wire short, the gauge large, and the path to chassis away from sensitive circuits. "Wire" is a four letter word to many manufacturers, and some consider them too costly due to their labor intensive nature. Achieving consistent results with hand-wired connections is difficult, making the PC mounted jack solution more desirable.

L-Bracket or Standoff Solution

A circuit board trace run to a nearby chassis-grounded point is another option. Use of an L-bracket, standoff or similar mechanical connection to the chassis provides mechanical stability, but also consumes valuable rear panel and/or PC board real estate at the same time. Important here is avoiding long traces and keeping the trace away from sensitive areas since it acts as a noise source when shield currents are large or noisy.

Jumper Options

Not as "friendly" as the screw terminal solution, an internal jumper option provides user configuration of internal shield connection points. This allows the use of XLR or 1/4" connectors yet still gives the user control of shield wiring practices. Providing a separate, external switch for this function is not cost effective. Two issues arise with this solution. The first is that there is no external visual indication showing shield connection point. The second issue to address is which position to ship the jumpers in.

The first problem is nothing new. Most manufacturers do not specify where their shields are connected. The unit's manual or schematic, if available, may indicate what ground connects to the shield. The schematic symbols used for grounds are not standardized, though there is an Audio Engineering Society standards group addressing drafting symbols to solve the dangling triangle mystery. Proper schematics indicate which symbols represent signal and chassis grounds. The second issue's answer is clear -- chassis-grounding the balanced shield is the "best" default option, though offering the choice supplies an elegant solution for parties on both sides of the fence. For fully balanced systems, defaulting the shield jumper to chassis provides the best solution, but only when all interconnected units have chassis-grounded shields. Other units with signal-grounded shields short-circuit the shield currents to signal ground when connected, causing potentially nasty modulation of the signal ground. This makes the other guy appear the culprit, but does nothing to solve the problem. Clearly users must be able to determine manufacturer shield wiring practices. Additionally, to support both "one-end-only" shield connection parties, separate input and output jumpers must be provided (see Figure 8).

Neutrik Solution

Neutrik AG, Liechtenstein, offers snap-in, printed circuit mount jacks with metal brackets which pierce the inside of the chassis when external mounting screws are installed. This chassis-pierced bracket also has a separate pin available through the printed circuit board. The sharp piercing tab provides the electrical connection between the chassis housing and printed circuit board. This solves the problems of the labor intensive wire and the need to connect to a chassis point, providing the best solution for manufacturers and users. [Neutrik's popular "combo" receptacles -- combined female XLR & female 1/4" connectors -- provide this piercing tab feature.] Unfortunately, depending on the available height in a given unit, these jacks have trouble fitting in a single rack space unit due to their slightly larger height. Hopefully other jacks with this built-in feature will become available, providing manufacturers with a cost effective solution to this grounding problem.

Other Suggestions

Many years ago RCA developed their own guidelines for rear panel I/O practices. Some manufacturers and users practice their own methods of left to right interconnection customs. AC and speaker level I/O on one side, microphone and lower level signals on the other side. This permits easier rack wiring and decreases crosstalk between cable runs in the rack and along cable paths. While the recommended practices document may not dictate product design at such a basic level, this type of thinking benefits everyone. With multi-manufacturer standardized network-controlled products popping up everywhere, now is the time to address these basic features. Users with "standardized" interconnection systems, designed with the user in mind by informed engineers, will spend less time debugging and installing systems. This allows more installations per day, generates better, quieter systems and provides more business with smiling users and manufacturers.

Fiber is the Future

Digital fiber optic interconnection solves all the above problems of electrical interconnection systems, though one must face a new set of problems. However, when one adds up the debugging costs of eliminating hum from electrical systems, fiber may not seem as expensive.

Conclusion

Balanced and unbalanced interconnection are two very different beings. The incompatibility between these two configurations, whether using analog or digital signals, must be considered when designing, specifying, installing or upgrading equipment and systems. Literature on the subject of grounding and shielding audio devices dictates chassis-grounding balanced shields. Most manufacturers, however, signal ground their balanced shields. Speculation about how and why this practice materialized was explored. The Audio Engineering Society is developing a recommended practices document which also condones chassis-grounding balanced shields, among other things. It was shown that the manufacturer choice of signal-grounding or chassis-grounding balanced shields does not affect the cable re-wiring and other technical support solutions normally recommended when interconnection of balanced and unbalanced equipment is needed. Therefore manufacturers need not hesitate in addressing their "pin 1 problems," and should provide users with the real benefits of balanced interconnection by providing chassis ground on balanced shields. Efficient and effective ways of doing this were also discussed.

Also covered was the importance of reducing signal ground voltages between interconnected units by carefully and properly connecting chassis ground to signal ground, in one place, in each unit. Vitally important is the manner in which one connects these two grounds together. The same care must be taken when connecting I/O cable shields to the chassis ground. One must avoid common impedance coupling in the shield-to-chassis trace to ensure optimum performance from balanced interconnection.

The goal of the Audio Engineering Society in recommending these balanced interconnection solutions is to reduce or eliminate the need for interconnection work-arounds through education and information sharing. This is the mission statement of the Audio Engineering Society in the first place. Systems installed with chassis-grounded balanced shields on all units, with well-twisted interconnection cables operate hum and buzz-free, leaving only the input and output circuit topology specifications to dictate system performance.

The Audio Engineering Society recommendation's purpose is not to create another "pin 2 is hot" war. In reality, users and installers have found acceptable solutions for "the pin 1 problem" of signal-grounded balanced shields and are unlikely, nor will they be able, to suddenly change over to not using alternatives. Manufacturers specify I/O connector type on data sheets, similarly, we should specify shield connection practices in equipment specifications, on the chassis, or at least in the manual, thus providing users with required information for proper system configuration.

References

1. Ott, Henry W., Noise Reduction Techniques in Electronic Systems (John Wiley and Sons, Inc., NY, 1976).

2. Morrison, Ralph, Grounding and Shielding Techniques in Instrumentation (John Wiley and Sons, Inc., NY, 1967).

3. Morrison, Ralph, Noise and Other Interfering Signals (John Wiley and Sons, Inc., NY, 1992).

4. Giddings, Philip, Audio System Design and Installation (Howard W. Sams, 1990).

5. Jung, Walt and Garcia, Adolfo, Op Amps in Line-Driver and Receiver Circuits, Part 2, (Analog Dialogue Vol. 27, No. 1, 1993).

6. Whitlock, Bill, "System Problems and Equipment Manufacturers" (Systems Contractor News, September 1997).

7. Perkins, Cal, Measurement Techniques for Debugging Electronic Systems and Their Interconnection, (Proceedings of the 11th International AES Conference, Portland, OR, May, 1992).

8. Sound System Interconnection, (Rane Corporation, Mukilteo, WA, 1985).

9. Metzler, Bob, Audio Measurement Handbook, (Audio Precision, Portland, OR, 1993).


A version of this RaneNote was published in the Journal of the Audio Engineering Society, Vol. 43, No. 6, June, 1995.

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