How Could This Happen?

To fulfill their users' desires to "go" balanced, hi-fi designers started upgrading equipment to balanced. From an unbalanced designer's mind-set, connecting the new balanced circuit's shield to ground is almost subconscious. The issue of which ground connects to the shield is alien or unknown. The old unbalanced "shield" (really the return signal conductor, not a true shield) is already "grounded." Without appropriate balanced interconnection research, this hi-fi mind-set may not think to add a chassis-grounded shield around the existing 2-conductor cable. This redefines the "old" return conductor as a "new" negative signal carrier, not as a shield. It was perhaps the convenience of the situation and this mind-set that started improper signal grounding of balanced shields in the first place. Little treatment of this subject is given in educational institutions, and many systems happen to work satisfactorily even with improperly grounded shields.

Other designers, upgrading to balanced interconnections, may have realized that by connecting the shield to signal ground, interfacing to unbalanced equipment is made simpler since signal ground (needed for unbalanced interconnection) will be available on the cable. (This unfortunately allows easy use of 1/4" mono connectors.) This still creates the same problem, signal-grounded balanced shields. Signal-grounded shields on balanced equipment create ground loops in the audio path and modulate the audio signal ground, wreaking havoc with most systems. This practice penalizes those who want to realize the superior performance of balanced interconnections and has given balancing a bad reputation.

A third possible reason for signal-grounded balanced shields arises if designers change phantom powered microphone inputs to balanced line-level inputs, and do not use caution. The phantom power return currents travel through the shield, requiring shield connection to the signal ground. When changing this topology to line-level balanced inputs, the designer may not think to change the shield connection to chassis ground. This issue is further complicated by manufacturers who incorporate ground-lift switches in their products. These switches disconnect chassis and signal ground. Thus care should be taken to ensure that phantom power return currents always have a return path to their power supply, regardless of the ground-lift switch position.

Manufacturers who started in balanced fields, such as the telephone and broadcast industries, used chassis-grounded shields when maximum protection from electromagnetic interference (EMI, including RF) was necessary. Perhaps users from these balanced fields assumed that all balanced equipment had chassis-grounded shields. When improperly-wired manufacturer's equipment was installed, they discovered hum and buzz problems. They solved them with isolation transformers, by disconnecting one end of the shield, or by simply not using that manufacturer's equipment. The feedback to inform manufacturers of their improper shielding practices never developed. Manufacturers may have suggested isolation transformers or cable rewiring solutions instead of addressing the cause of the problem: signal-grounded balanced shields. Again, some systems with signal-grounded shields work acceptably, causing further and future bewilderment.

The History Lesson

The lesson to be learned from this account involves keeping in mind these audio interconnection issues when specifying, designing, or upgrading other connectivity systems such as AES3 (formerly AES/EBU), SPDIF, and other electrical interfaces. Balanced and unbalanced systems are not designed to interface together directly. As the audio industry embraces more digital products, interconnection systems must be clearly designed and specified for use within the limits of their electrical interfaces. Multiple conductor connectors, carrying either digital or analog signals, present even more challenges. The distance between units is an important issue. Keeping interconnects balanced and chassis-ground-shielded provides the best possible immunity from electromagnetic interference, regardless of cable lengths. Unbalanced interconnection may be less expensive to manufacture and sell, but is perhaps more expensive to install -- hum and buzz-free.

The Audio Engineering Society is to be applauded for assembling and disseminating this information to those who may be unfamiliar with it. Manufacturers and, more importantly, users will eventually be rewarded.

Chassis Ground vs. Signal Ground

Let us examine the distinction between chassis and signal ground in audio devices. Chassis ground is generally considered any conductor which is connected to a unit's metal box or chassis. The term chassis ground may have come about since units with 3-conductor line cords connect the chassis to earth ground when plugged in to a properly wired AC outlet. In units with a 2-conductor line cord (consumer equipment), the chassis does not connect to earth ground, though the chassis is normally connected to the signal ground in the box in both unbalanced/consumer and in balanced/pro equipment.

Signal ground is the internal conductor used as the 0 V reference potential for the internal electronics and is sometimes further split into digital and analog ground sections. Further signal ground splits are also possible, though it is important to remember that all "divisions" of signal ground connect together in one place. This is usually called a star grounding scheme.

It is easy to confuse chassis ground and signal ground since they are usually connected together -- either directly or through one of several passive schemes. Some of these schemes are shown in Figure 3. The key to keeping an audio device immune from external noise sources is knowing where and how to connect signal ground to the chassis.

First let's examine why they must be tied together. We'll cover where and how in a moment. There are at least two reasons why one should connect signal ground and chassis ground together in a unit.

One reason is to decrease the effects of coupling electrostatic charge on the chassis and the internal circuitry. External noise sources can induce noise currents and electrostatic charge on a unit's chassis. Noise currents induced into the cable shields also flow through the chassis -- since the shields terminate (or should terminate) on the chassis. Since there is also coupling between the chassis and the internal circuitry, noise on the chassis can couple into the internal audio. This noise coupling can be minimized by connecting the signal ground to the chassis. This allows the entire grounding system to fluctuate with the noise, surprisingly providing a quiet system. Further coupling reduction is gained when the chassis is solidly bonded to a good earth ground -- either through the line cord, through the rack rails or with an independent technical or protective ground conductor. This provides a non-audio return path for any externally induced noise.

The second reason to connect signal ground to chassis is the necessity to keep the signal grounds of two interconnected units at very nearly the same voltage potential. Doing so prevents the loss of system dynamic range where the incoming peak voltage levels exceed the power supply rails of the receiving unit.

Unbalanced units connect successive signal grounds together directly through each interconnecting cable -- the sleeve of each RCA cable. This, and the fact that the chassis is generally used as a signal ground conductor, keeps the signal ground impedance of unbalanced systems very low. Many may agree that unbalanced systems are helped by the fact that the chassis are normally not earth grounded. This allows an entire unbalanced system to float with respect to earth ground. This eliminates the potential for multiple return paths for the audio grounding system, since there is not a second path (ground loop) through the earth ground conductor. Low signal ground impedance between units is essential for acceptable operation of all non-transformer-isolated systems, balanced and unbalanced.

The design of balanced interconnection does not connect signal grounds directly together. The negative conductor provides the required signal return current. To avoid loss of dynamic range, balanced systems use a different method of keeping signal ground potentials small.

Since the cable shield already connects the two chassis together, simply connecting signal ground to the chassis in each box keeps the signal ground potentials between units small. The key is how to connect them. Since the cables between units also provide the shortest (and therefore the lowest impedance) path between two units, using the cable shield to minimize the signal ground potentials between units is quite effective.

Now that we know why one must connect signal ground to chassis, let's discuss how to connect them. The schemes in Figure 3 appear straight forward enough, but what is not shown is precisely where and how the conductors connect together.

It all comes down to paying close attention to where currents flow. As discussed above, the shield noise currents flow through the chassis and shunt to earth ground on units with 3-conductor line cords. The key issue is that these noise currents do not flow through a path shared by any audio currents. It seems so simple, and is -- especially to draw (see Figure 3 again). The hard part is implementing the proper layout scheme.

Connecting signal ground to the chassis in each unit can only be done in one place in each unit. If done twice, one leaves the possibility open that the noise currents will flow through a path shared by audio.

There are two schools of thought on where to connect the signal ground to the chassis. They are both versions of the star ground scheme mentioned above. The first connects a trace (or wire) directly from the audio power supply ground terminal and connects to the chassis ground point (see Figure 4). It is important, in both "schools", that no other signal currents be allowed to flow through this trace. Do not allow this trace to share any other return currents from other signal-grounded circuit points, such as the input or output circuit's ground. This keeps chassis noise currents from flowing through the same trace which is a return path for an audio signal. Also keep in mind that this trace may contain noise currents and should be kept away from noise sensitive circuitry. This is a star grounding scheme which uses a point originating at the output of the power supply as the center of the star. There are two common locations in the power supply for the star's center: the output terminal of the power supply and the point between the AC filter capacitors.

Another school of thought on where to connect signal ground to the chassis simply moves the center of the star ground to the input jack's ground. This scheme makes the most sense for unbalanced units and balanced units equipped with 1/4" connectors where use of mono plugs is possible.

Manufacturer Issues to address

Implementing their users' desires to "upgrade" to balanced, traditionally unbalanced manufacturers are faced with an important issue: How do you solve the balanced/unbalanced incompatibility problem? If you sell your product to a mixed balanced/unbalanced market, a suggested method of interconnection must be available. Isolation transformers and active interface boxes are the best solution and should be offered as the best interconnection alternative. However, persuading unbalanced customers to buy an expensive interface solution is much harder than the lower performance option of rewiring their cables. (The "add-on" transformer solution is analogous to a software company releasing a new software revision which renders your existing files incompatible unless an additional file conversion program is purchased.)

Through careful rewiring of the cables, acceptable interconnection solutions are achievable in some systems. (One of Rane's most popular RaneNotes, Sound System Interconnection, is one example of the "custom" wiring needed in some systems.) This same cable re-wiring solution holds whether the equipment is wired with signal ground or with chassis ground on the balanced circuit's shields.

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