Hum and Buzz In Unbalanced Interconnect Systems
Jensen AN-004

In contrast to a balanced system, an unbalanced system uses only two wires, one for signal and one for ground. Its use still prevails in consumer audio, probably because it is cheap to make and it performs acceptably well in very small systems such as typical home stereo setups. However, any unbalanced scheme has an inherent problem called common impedance coupling. From Ohm’s law we know that when current flows in a resistance, a voltage drop appears across that resistance. With the exception of superconductors, any conductor (wire) has resistance. If two different circuits share the same conductor or wire, a current flowing in either circuit will produce a voltage drop across the wire. As shown in Figure 2, a partial schematic of the simplest possible system, the shield conductor of the interconnecting cable becomes the offending common impedance.

Since the cable shield is effectively connecting the grounds of the devices together, it carries a current derived from the power line as well as the audio signal current. Although this fact is often overlooked or ignored, it is fundamental to this discussion. Whatever voltage is present between its inputs, points A and C in Figure 2, will be amplified by Device B. It cannot tell the difference between signal and hum, and will amplify both if they are present. To determine what the input “sees”, we must trace the circuit loop from point A to B to C. Since the voltage A to B (shield voltage drop) is in series with the voltage B to C (the signal), the voltages will directly add. Clearly,
it would be very desirable for the shield voltage drop to be zero to avoid contaminating the signal. In the real world, regardless of shield construction, material or gauge, we cannot make the shield conductor resistance zero. Our only remaining choice is to somehow reduce the interchassis current, I in Figure 2, to an acceptable level.

Note the Vx ± Vy term in the single capacitor equivalent circuit. The ± accounts for the fact that most consumer equipment has a twoprong AC plug which can be inserted into an outlet either of two ways. A special example illustrates how extreme the effects of plug reversal can be. Consider the case where CPS1 is ½ the value of CPS2 and CPS3 is ½ the value of CPS4 (this condition would be highly unlikely in the real world). If the two plugs are connected to the AC line as shown in the diagram, each pair of capacitors forms a voltage divider with a 3:1 division ratio, making chassis voltages Vx and Vy each 40 volts AC with respect to ground. Since no current will flow in a wire connecting two points of equal voltage, current I will be zero. However, if one of the AC plugs is reversed, the chassis voltages will no longer be equal and current I will flow. Except for this special case, plug reversals simply cause a change in the interchassis current, rather than the total cancellation seen in this example. For this reason, reversing AC plugs will almost always change the hum level in a system..

It is also very unlikely that the two capacitances, CPS1 and CPS2 or CPS3 and CPS4, would be exactly matched in any piece of equipment. Mismatch ratios of two to one are common. Since all utility 120 VAC power in this country is distributed asymmetrically with respect to earth ground, one side called “neutral”, is grounded. The other,
called “hot” or “line”, is at 120 volts with respect to ground.[4] Recently, proponents of a scheme called “Balanced AC Power” have claimed that “[hum reduction] results are often quite dramatic”.[3] Balanced power uses a center-tapped transformer to make each side of the line 60 volts with respect to ground. Although intuitively attractive, this approach can completely cancel interchassis currents in a system of three or more devices only in the case where each of the devices had such matched capacitances. This would be an extremely rare occurrence. Although 10 to 15 dB hum reductions, which would be more routinely achieved, might be considered“dramatic” in a video system, this author cannot recommend this or any other “line conditioning” method as a cost effective solution for audio system hum problems.

to a ground equivalent to power line neutral. The safety ground of any modern outlet is convenient for this. It is required by safety codes to be tied to earth ground, as is the neutral. The current measurement should be taken under four conditions: device "off", device "on", then repeated with the device’s AC plug reversed. Taking the highest reading will give us a "worst case" number which can then be used, along with Figure 4, to estimate the system hum levels produced when this device is connected to others via cables.

The author has tested a variety of consumer devices, including CD players, cassette decks, tuners, receivers, and power amplifiers, for chassis current to ground. The broad categories of typical ground currents developed from the testing were used in Figure 4. Ground current is generally related to AC power consumption of the device, since this dictates the size of its power transformer and, to some extent, its interwinding capacitances. Ground current L, 5 µA RMS, is typical of "low power" consumer gear drawing under 20 watts. This includes most CD players, cassette decks, and turntables. Current M, 100 µA RMS, is typical of "medium power" consumer gear drawing 20 to 100 watts. This includes most tuners, low to medium power receivers or power amplifiers, and some small TV receivers. Current H, 1 mA RMS, is typical of "high power" consumer gear drawing, or capable of drawing, well over 100 watts. This includes most high powered amplifiers or powered subwoofers and large screen or projection TV receivers. Figure 4 shows the calculated effect of these currents when they flow in interconnect cable shields in an unbalanced audio system. A contact resistance of 50 mS per connection was used and the 0 dB reference level is 300 mV RMS or about !10 dBV. All results have been rounded to the nearest dB.

Please note that this characterization of chassis current applies only to devices with two-prong AC plugs. Three-prong plugs effectively connect the device chassis to safety ground, making the chassis a voltage source. System effects of this will be discussed later.

To eliminate hum, we must effectively eliminate interchassis ground current. We could eliminate it by simply breaking the chassis to chassis shield connection. Of course, this alone would not solve our problem. We must break the signal line as well and insert a device which will sense the voltage at the output of device A and regenerate it into the input of device B, while ignoring the voltage that exists between the now disconnected device grounds. These properties generally describe a differential responding device with high
common-mode rejection, usually called a ground isolator. See reference [2] for more information on this subject.
Two basic types of differential responding devices, active differential amplifiers and audio transformers are available at reasonable cost. We won’t consider active optical or carrier modulated isolation amplifiers here because such devices which also have acceptable audio performance are still quite expensive.

Active differential amplifier circuits are used in a number of commercially available devices. To a greater or lesser extent, they all share several disadvantages: they can further complicate the ground system by contributing interchassis currents of their own, since they require AC power; they cannot handle ground voltage differences over about 10 volts RMS; they use semiconductors or integrated circuits which are prone to degradation or failure caused by power line or lightning induction voltage transients; and, worst of all, they are exquisitely sensitive to source impedance. This sensitivity limits hum rejection, even in a balanced system (for which they are intended), but it makes them nearly useless in an unbalanced system.[2] A typical example of such devices is the popular Sonance AGI-1 (which uses the Analog Devices SSM2141). Lab measurements on this unit, shown in Figure 5, reveal that over the 200 S to 1 kS range of source (output) impedances typical in consumer equipment, its hum rejection is only 15 to 30 dB.

High quality audio transformers are, by their nature, relatively insensitive to source impedance and exhibit excellent hum rejection performance in either balanced or unbalanced systems. Under the same conditions and same range of source impedances, the passive transformer based ISO-MAX® model CI-2RR measures 90 to 110 dB. As shown in the Figure 5 graph, its measured hum rejection is over 70 dB better than the active device. It requires no power of any kind and can handle ground voltage differences up to 250 volts RMS without malfunction, degradation, or damage. There is a widespread belief that all audio transformers have inherent limitations such as high distortion, mediocre transient

1 mA x 0.68 S = 0.68 mV for TV to preamp cable,
1 mA x 0.55 S = 0.55 mV for preamp to sub-woofer, and
1 mA x 0.11 S = 0.11 mV for preamp to power amplifier.

!113 dB at preamp input from TV,
!115 dB at sub-woofer input from preamp, and
!99 dB at power amplifier input from preamp.

!110 dB at preamp input from TV,
!110 dB at sub-woofer input from preamp, and
!99 dB at power amplifier input from preamp.

DO NOT DEFEAT SAFETY GROUNDS!

Figure 7: THE EXAMPLE SYSTEM “AFTER”

The worst case is two safety grounded devices operating from two different branch circuits of the building’s AC power. The best case is two safety grounded devices located very near each other and operated from the same AC receptacle. Magnetic pickup loops can easily be created by improper routing of cables. Any closed loop of wire in the proximity of an AC magnetic field will have a current induced in the loop. When this current flows in the shields, it produces voltage drops which contaminate the audio just as interchassis currents do. Common sources of strong AC magnetic fields are power transformers, motors, fluorescent lights, AC power wiring (even inside walls and conduit), TV sets, and
computer CRT displays. A simple stereo pair of interconnect cables between two devices forms a loop because the shields are tied together at both ends. Since loop pickup is directly proportional to the area inside the loop, the cables should be dressed as physically close together as possible. In fact, all cables that connect between the same two devices should be bundled together in order to minimize loop area. The bundles should, of course, be kept as far as possible from magnetic field sources.

CONCLUSIONS AND TIPS

TChoose shielded cable for low shield resistance. A cable with 6.5 mS/ft (6.5 S per 1000 ft) shield resistance and 90% shield coverage is much preferred over one with 25 mS/ft shield resistance and 100% shield coverage.

T Use high quality, transformer based audio ground isolators to eliminate high interchassis currents.

T Bundle together all audio cables connecting the same two devices and keep the bundle away from power cabling or other AC magnetic fields.

T NEVER, NEVER DEFEAT THE SAFETY GROUNDING of any device having a 3-prong power cord. The results of doing so can be deadly to you and/or your customer.

T If the system contains more than one safety grounded device, use audio ground isolators to eliminate current flow through audio cables connecting them.

T Add external grounding, if possible, to devices without any other system ground path in order to reduce commonmode voltage at the isolators.

T Use a transformer type RF isolation device, if necessary, to prevent ground currents that may result from the cable TV connection.

Figure 8

REFERENCES:

[1] B. Hofer, “Transformers in Audio Design”, Sound & Video Contractor, March 15, 1986, p. 24.

[2] B. Whitlock, “Balanced Lines in Audio Systems: Fact, Fiction, and Transformers”, J. Audio Eng. Soc., vol. 43, pp. 454-464 (1995 June).

Figure 9: contact information