Mar 1, 2002 12:00 PM, BILL WHITLOCK
AUDIO INTERCONNECTION BETWEEN BALANCED and unbalanced equipment seems to be a murky area for most users, technicians and system designers. The topic accounts for a surprisingly large portion of the calls received by the applications engineers at my company (Jensen Transformers.)
Confusion, even controversy, is further fueled by some published guides whose technical basis is dubious at best. This column will briefly explain the underlying engineering principles and offer some wiring diagrams for interfaces that solve the potential problems that await the unwary.
DEFINING OUR TERMS
what do the terms balanced and unbalanced really mean in a systems engineering sense? I've done considerable writing and occasional ranting about the true nature of balanced interfaces, so I'll just summarize the most important point: The status of balance is defined by the impedances of the two signal conductors with respect to another reference point, usually ground. [See Whitlock, “Balanced Lines in Audio — Fact, Fiction, and Transformers,” Journal of the AES, Vol. 43, No. 6, June 1995. Included in Shields and Grounds from the AES online store at aes.org/publications/other.cfm#5.]
An unbalanced interface exists when one of the signal conductors is tied to ground (zero impedance) and the other has some higher impedance. Unbalanced inputs and outputs are very popular in consumer electronics, electronic musical instruments and low-cost professional audio equipment (often called semi-pro). The ubiquitous RCA and other connectors are widely used for unbalanced audio interfaces. Examples of typical audio driver (output) and receiver (input) circuits are shown in the figures here.
Unbalanced interconnections are inherently prone to noise because the grounded conductor, which most often also serves as the cable shield, is not only part of the signal circuit, but also a path for small power line currents. These unavoidable power line leakage currents will always flow in any wire connecting pieces of equipment together. In an unbalanced interconnection, as these currents flow through the resistance of the grounded (shield) conductor, the resulting voltage drop is directly added to the signal voltage. This common-impedance coupling causes virtually all hum and buzz problems in unbalanced interfaces. [Whitlock, Interconnection of Balanced and Unbalanced Equipment, Jensen Application Note AN003, download at jensentransformers.com/an/an003.pdf.] Such problems are rarely due to inadequate shielding of the cable. This is an incorrect notion reinforced by many authors and cable suppliers.
A balanced interface exists when the two signal conductors have equal impedances to ground. Balancing is a powerful technique in eliminating potential system noise. In an earlier era, the presence of balanced inputs and outputs essentially defined professional equipment. Because the line impedances are equal, external noise or interference induces equal voltages in them, making it common-mode voltage. Since a balanced input responds only to the voltage difference between the lines, it responds only to the signal voltage and is unaffected by the common-mode noise or interference voltage. Of course, in the real world, this rejection is less than perfect for various reasons.
Nowadays, because of low cost, most balanced inputs consist of the simple differential amplifier circuit shown in the figures, or some variation of it. The pairs of resistors at each input are well matched, sometimes trimmed to within 0.01%, in order to nullify circuit response to common-mode voltages. When used in real-world systems, however, a major drawback of this circuit is its exquisite sensitivity to the slightest imbalances in the output (Rs) impedances of the line driver, which are effectively in series with its inputs. Very often, an input that touts impressive common-mode rejection ratio (CMRR) specs has a disappointing performance when connected to a real-world signal source instead of a precision signal generator.
Balanced output circuits vary widely, but the one shown in the figures is fairly common. This output circuit provides two symmetrical signals: i.e., they have equal magnitude but opposite polarity. The output impedances to the lines are equal. Note that symmetrical signals in this, or any other, balanced line driver have absolutely nothing to do with noise rejection.
Resistors typically have tolerances of ±5%, and capacitors typically have tolerances of ±20% or worse. As noted above, the balance of these output impedances strongly affects the actual CMRR of most balanced inputs. Few equipment makers seem to recognize the importance of this effect. Transformers enjoy a tremendous advantage over conventional active input stages with regard to their tolerance for these normal variations in driver impedances.
i'd like to direct your attention first to Figure 1B. This schematic shows a commonly used, but noise-prone, method using 2-conductor (shielded single conductor) cable and an RCA-to-XLR adapter at the input. Do not use this adapter — it results in zero ground noise rejection. All the potential noise-reduction benefit of the balanced input is wasted. Sadly, many commercial cable assemblies are wired exactly this way.
Figure 1A shows a much better way to connect an unbalanced output to a balanced input. Sometimes called a pseudo-balanced connection, this method allows power line leakage currents to flow in the shield of a 3-conductor (shielded twisted pair) cable. This allows the balanced input to sense the signal at the unbalanced output and reject the common-mode ground noise. The 470-ohm impedance of the unbalanced output will degrade the 60Hz CMRR of the typical balanced input to about 30 dB, but that's still 30 dB better than using the adapter! For all intents and purposes, using a pseudo-balanced connection is the correct way to connect an unbalanced component to a balanced one. Now, let's look at the best way to add a transformer into the system.
Adding a Transformer. The interface in Figure 1C uses a conventional transformer to improve the impedance balance. This will reduce the CMRR degradation of the balanced input stage at low frequencies. In this circuit, CMRR will be about 55 dB at 60 Hz, but because of the inter-winding capacitance in the transformer, CMRR will gradually fall to about 30 dB at frequencies over 1 kHz. Compared to the correct adapter cable of Figure 1A, it doesn't improve buzz, which contains many high-frequency components, but it does reduce 60Hz hum by another 25 dB.
With rare exceptions, transformers used in hum eliminator boxes are conventional designs that don't include a Faraday (electrostatic) shield between the windings to prevent capacitive coupling between primary and secondary windings.
The interface in Figure 1D uses a Faraday-shielded, or input-style, transformer to improve the tolerance of the input stage to impedance imbalances. The input transformer, unlike the input stage itself, can tolerate large source impedance unbalances with very little CMRR degradation. In this circuit, CMRR will be about 100 dB at 60 Hz and 70 dB at 3 kHz, making it very effective at eliminating both the hum and buzz components. (Faraday-shielded transformers are used in most Jensen ISO-MAX isolator boxes.)
None of the interfaces shown in these figures provides the 12dB gain necessary to increase the nominal -10dBV (316 mV) consumer reference level to the nominal +4dBu (1.23V) professional reference level. In the rare case where the pro equipment doesn't have enough available gain, an active amplifier may be necessary.
One might ask, “Why not use a 1:4 step-up transformer to get the voltage gain?” A step-up transformer, even if ideal (i.e., without any loss) is not a viable source of gain in this application. Reflected impedances cause excess loading of the consumer output, leading to an actual loss of gain, compromised low-frequency response and increased distortion. [Whitlock, “Audio Transformer Basics,” Handbook for Sound Engineers, Third Edition, Glen Ballou Editor, Focal Press, 2002, Chapter 11, pp. 254-256.]
Connecting a balanced output to an unbalanced input is much more problematic. Balanced equipment uses a wide variety of output circuits. The type shown in Figure 2A and Figure 2B can be damaged when one output is grounded. Others, including most popular servo-balanced output stages, may become unstable and oscillate or produce distortion unless one output is grounded right at the driver. But such a ground, along with the existing one at the unbalanced input, simply reduces the interface to a completely unbalanced one. Therefore, all benefit of the balanced output is negated. That's why an external ground isolator transformer, such as the one shown in Figure 2A, is a foolproof method: It works with any kind of balanced or unbalanced output.
In the circuit of Figure 2A, CMRR will be about 60 dB at 60 Hz, but will decrease at higher frequencies. Again, conventional transformers are effective for hum but not for buzz. The 2-resistor pad provides 12 dB of signal attenuation to reduce pro levels to consumer levels.
The circuit of Figure 2B uses a 4:1 step-down (12 dB of attenuation) transformer with a Faraday shield, which results in an outstanding CMRR of about 120 dB at 60 Hz and 85 dB at 3 kHz, which is effectively an elimination of both both hum and buzz. Generally speaking, any Faraday-shielded transformer must be located close to the input it feeds because the capacitance of a long cable run can seriously affect frequency response.
Bill Whitlock is president of Jensen Transformers Inc. He has designed audio and video circuits and systems for 30 years. He can be contacted by e-mail at email@example.com.