On Equipment Interface Options…

By Bryan Geyer

A summary of the means used to interconnect audio equipment chassis

In the beginning (and for ~ 35 years thereafter) the adopted “hi-fi” interface standard was the ubiquitous "phono plug," a.k.a. the RCA plug (and jack). This 1940 relic utilized a relatively small (undefined) diameter coaxial cable wherein the insulated center conductor is used for the hot side of the signal and the common (sometimes called ground) side of that signal is carried by a shield surrounding the center wire. The shield is intended to provide EMI noise isolation, and is generally braided, sometimes spiral-wrapped. It’s composed of copper, or tinned-copper, or, infrequently, aluminum foil. The hot center conductor is routed to the RCA plug’s pin contact, and the shield is routed directly to the shell. An outer jacket provides physical protection. Such cords are readily available in assorted styles and lengths, with various exterior Ø dimensions, dependent on shunt capacity rating. The latter ranges from ~ 15pf/ft. (special “low capacity”) to ~ 35-45pf/ft. (normal).

At some time in the early 1980s, an upgraded version of this RCA-type coax was introduced (as best I can recall) by Hitachi. In this new version the insulated hot conductor remains connected as before, to the pin of both RCA plugs, while a separate common/ground conductor (insulated) is routed along with it, inside the coax core, that connects with the outside shell of both RCA plugs. This wiring completes the intended signal transfer, and eliminates the risk that the impedance might vary if the common was routed only through a braid. A shield overlay is then added to repel EMI. To avoid creating unintended chassis-to-chassis ground loops, this shield is mated to only one RCA shell. That mated end then serves as the designated signal input side, so such cable is arbitrarily defined as being unidirectional*. RCA cords of this type are now prolific; there are many suppliers, e.g. Audioquest.

Any benefit conveyed with this upgraded style of unbalanced coaxial cable is directly related to length (cumulative net impedance). If the required run is fairly short (≤ 1 meter), the likelihood of benefit is minimal. For runs > 1 meter, this unidirectional style coax might possibly be superior to regular braided coax, but not nearly so nice as the balanced AES/EBU option that’s described next.

AES/EBU: In the early 1980s another form of analog interface evolved, as jointly developed by the Audio Engineering Society (AES) and the European Broadcasting Union (EBU). This cable was initially conceived for use in professional and commercial service, where there are often many more cables, frequent disconnects and reconnects, longer runs, and where extreme EMI exposure (strobed lighting, motorized generators) might prevail. This new standard utilized “Cannon barrel plugs”, a connector that was created (in 1950) by James Cannon. We now know these as “balanced” XLR plugs**. Although handicapped by an oversized Ø (~3/4”) that reflects its ~ 70 year old origin, these XLR connectors can provide useful advantages over ordinary “unbalanced” RCA coax cable; e.g…

…the hot and common/ground polarity terminal connections now engage simultaneously.

…the latched nature of the plug-to-jack mating is more secure; less likely to inadvertently detach.

…dependent on the related circuit design (see footnote**), an impedance-balanced XLR connection will facilitate full common mode EMI noise rejection. This latter feature can be a vital asset when heavy EMI is prevalent, and/or when the interconnect runs get lengthy (> 1 meter).

More recently, XLR plugs have been embraced by high-end audiophiles as general purpose interface connectors. This has come about despite ample evidence that basic RCA coax can provide fully sufficient noise immunity in any normal residential setting when the cable runs are ≤ 1 meter***.

Concurrent with creating AES/EBU, a new digital data transfer standard was developed to address the new digital media. This protocol is known as AES3, and it encompasses three distinct means of digital (only) data interface:

(a) Balanced, using a single XLR terminal (at each cable end) for full 2-channel stereo digital data transfer. One cable carries both of the encoded stereo channels.

(b) Unbalanced, using a single 75Ω coaxial cable, normally with RCA-type connectors, and generally labeled “coaxial S/PDIF” (Sony/Philips Digital Interface). Such cable is also offered with BNC terminals, provided the BNC connectors conform with 75Ω (not 50Ω) coax. One cable carries both stereo channels. The standard coax most commonly used is type RG59A/U, which has a 22 AWG compacted-copper center wire. Type RG59B/U, with a 22 AWG solid copper-clad-steel center wire, can also be used. The latter has slightly lower loss, but less flexibility. Both have Ø ≈ 0.24 inch. These are professional quality cables, as used by commercial recording studios. They are available on-line, in various lengths, directly from U.S.-based supplier L-Com, at prices that are far less (often some 80% less) than the selling price from retail high-end audio sellers’ sites.

(c) Optical digital data transfer, via an optical fiber (usually plastic, preferably glass) cable, using Toshiba’s F05 style “Toslink” terminals. Also known as “optical S/PDIF”.

While all three of these digital data transfer means are practical and effective, most people use option (b), the unbalanced 75Ω coax, using either RCA plugs or (less frequently) BNC (75Ω) connectors.

TRS plugs: Three conductor TRS (tip-ring-sleeve) plugs were derived from the old (circa 1930s) 1/4” diameter two conductor telephone switchboard plug. The third wire contact was added via an isolated ring, with the intent that a single plug could then serve for (unbalanced) analog stereo (left-hot, right-hot, plus a common/ground) connector applications. Smaller variants (3.5mm & 2.5mm Ø) soon followed. This basic 3 wire plug can also serve for balanced analog connections, where its smaller (than XLR) size often proves advantageous. As with the XLR plug, dual TRS connectors are needed for balanced analog stereo service. The TRS configuration is not quite so fully shielded as XLR, but it’s generally fine for home stereo; also for most commercial service. Full-sized (1/4 inch) TRS plugs are the normal commercial standard for guitar amplifier inputs/outputs; also for some dynamic mics.

A note about the photos: These plugs (a plug = male, a jack = female, a socket denotes either gender, but is chassis-mounted) are shown side-by-side to give you a sense of the relative size (big!) of an XLR plug. That size becomes X2 when utilized for balanced analog stereo (it requires separate L/R feeds). You can appreciate why a smaller size connector might be of interest when EMI is benign.

BG (September  22, 2019)

*There’s persistent chatter on audio forums (and in Audioquest’s sales pitch) supporting the assertion that wire exhibits directional properties. No proof of this belief exists (just aural hearsay), and prominent engineers don’t concur. This folly stems from irrational groupthink. Wire is not directional; its conductivity is fully bilateral.

**An XLR connector is inherently “balanced” by virtue of its three wire symmetry. However, this “balanced” state does not always extend to the associated active circuitry. Sometimes these XLR connectors are simply wired in direct parallel with unbalanced RCA inputs, so no phase cancellation (noise) benefit is then derived. It’s far better to connect the hot signal (XLR pin 2) to the RCA hot input, and then wire the cold input side (XLR pin 3) in series with a selected (adjustable) resistance that’s precisely equal to the measured input impedance appearing at pin 2. This creates an impedance balanced input that’s fully consistent with effective common mode EMI noise rejection. Do appreciate that this impedance balance can be achieved with virtually any equipment, provided it has an internal means (generally a simple potentiometer) to set the desired pin 3-to-pin 2 impedance match. In very rare and costly instances, entirely symmetrical balance can be automatically assured (no internal adjustment required) by implementing a fully complementary circuit design, with precisely matched differential stages throughout, from input-to-output. Such equipment might, or might not, truly exist; the aspirations of audiophiles know few limits. In any case, fully balanced common mode EMI noise rejection can be accomplished by either means, and both would be equally effective.

***There’s really no need to use analog stereo connectors that were expressly developed to cope with the EMI demands of a rock concert (XLR) when the site is a private home. Good coaxial cable with RCA plugs will yield comparable noise performance when the line-level runs to other components are ≤ 1 meter. (This length restriction does not apply to low bass passband signals routed to the line-level inputs of any self-powered subwoofers. Those RCA coax runs can safely extend some 5 or 6 meters without hum or noise impact.)