The Perils of Patchwork -- A Field Guide to Audio Interconnects and Patch Cables (March. 1978)

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Audiophiles who frequently find themselves "back of the box," capitalizing on the flexibility and features of modern high fidelity components, are all too familiar with the humble patch cord--and with … The Perils of Patchwork

Author: Clyde J. Schultz [Vice President Switchcraft, Inc., Chicago, Il. 60630]


Too often considered a "poor cousin" of the affluent audio system, the patch cord can be the weakest link in an otherwise powerful and responsive electronic network. When it's handled frequently or roughly, it can malfunction, causing signal losses, exasperating on/off operation-and even short circuits, if incorrectly matched.

So, unless you're a compact owner and are quite content just to plug it in and turn it on, it behooves you to know something about the construction, operation, and matching of patch cords--and to equip your music room and portable recording kit with the rudimentary adapters and connectors to handle any problems that may (and probably will) arise.

Basically, a patch cord is a length of cable with connectors at both ends designed to complete a circuit and carry current or a signal from one component function to another. The connectors used for over 90 percent of patch cords are: 1) phone plug (Y -in. finger dia.), 2) miniature phone plug (9/64 in. finger dia.), 3) subminiature phone plug (0.097 -in. finger dia.), and 4) phono plug (1/8-in. finger dia.). Occasionally other terminations are used, such as spade lugs, alligator clips, stripped-and-tinned leads, and banana plugs. Each common termination is described in the illustrated Glossary which accompanies this article.

Though it probably cannot ruin your audio system (unless you have a shorted speaker circuit), a poor, defective, or inadequate patch cord can diminish your high frequency reception, compromise your system's performance, prevent your using it to full advantage--or ruin a perfectly good evening.

Because audio connectors, it seems, follow their own corollaries of Murphy's Law, for example:

"Anything that can go wrong with an audio connector usually will." "Any patch cord on hand is an inch too short."

"You never have the right cord or connector with you when you're away from home--or when the audio stores are closed."

And it's not until you reach the most important passage of your favorite symphony or the critical point in a recording session that these rules seem to come into play. At this point, you would be willing to pay any price for a better connection and regret that it was here, of all places, that you decided to save a few dollars.

Rags and Patches

The fact is, there's no such thing as a "bargain" patch cord. As is often the case, bargain prices in connectors usually indicate corner-cutting in manufacture. Though two connectors may look identical, it's trouble-free operation over the long run that counts. Cheaply made plugs may operate well in a "static" mode, but pulling, twisting and frequent handling can mean breakage, short circuits, and frustrating, intermittent operation.

Displaced tips or poor concentricity can mean shunted jacks that won't open. And the junction between cable and plug is the most susceptible to malfunction and the weakest point in "throw-away" cords.

It's difficult to judge quality in products you normally cannot inspect because of blister packaging, but among the features you should look for (or ask about) are 1) positive strain relief, 2) a soldered or crimped bridge sleeve, 3) one-piece molded cable assembly and, 4) shielding, where required.

Keep in mind that each feature requires additional production steps, and therefore adds to finished product's costs, but it's an investment that will prove well worth the extra few cents.

Quality construction is not enough, however. You also must make sure the cord matches its application in such areas as capacitance and impedance. Here again, the salesperson and product catalogs will be of assistance, though specialty cables, such as low capacitance, probably will be clearly marked on the package itself.

Capacitance Impedance

Capacitance is the measure of the amount of electrically separated charges a device (such as a patch cord) can store in the insulator between two conductors, when a given voltage is applied. It's commonly measured in microfarads (µF) or picofarads (pF). Too -high capacitance can mean the loss of high frequencies, particularly with high impedance circuits and long cable runs.

And in CD-4 "discrete" four -channel systems, where frequencies up to as high as 45,000 Hz are critical, low capacitance cables (about 15 pF per foot or 50 pF for a 1-meter cord) are a must.

Impedance is another factor to be considered in selecting a connector, to be sure it's matched to audio levels involved. Impedance is a measure of a circuit's opposition to the flow of alternating current and is expressed in ohms. When it's important to an audio connection, as in microphones or speakers, it will be specified by the manufacturer.

High-impedance circuits themselves can act like microphones, picking up and amplifying stray signals from within or outside the system and introducing interference in the form of noise and hum. Here, low capacitance is important, but proper shielding is an even greater necessity. Shielding also is important in protecting against radio frequency interference (r.f.i.), which seems to be increasing daily as more and more citizens band radios take to the airwaves.

For microphone connections, specify a twisted pair of cables with good shielding (at least 90 percent). This, in turn, requires connectors with at least three terminations (including one for the ground shield) and fully shielded housings.

With a high audio level (not impedance) outputs such as speaker connections, hum and noise are usually no problem. Current-carrying ratings, however, may be. With a 100-watt amplifier, rms output can be as high as 30 volts at 3.5 amps, with much higher peaks. At levels like these, arcing can occur at connector contacts and introduce noise, so phono connections area poor choice for speaker applications; they simply are not designed for these levels.

For most speaker applications, a simple twisted pair or parallel two conductor cable is usually adequate, since shielding is not required. Number 18 or 16 gauge lamp cord is even suitable, but preferably with spade lug or banana jack terminations.

Phono pickup cables require limpness and flexibility to eliminate "pull" on the delicate tracking mechanism, and require shielding to eliminate hum pickup from nearby 60-Hz components. To be sure, always follow the manufacturer's recommendations when replacing phono pickup cables.

Between preamp and amp, the leads are usually short, so resistance is not critical, but shielding and capacitance always are. How much shielding is "enough?" This will vary geographically (as in high CB-use areas) and with the totality of your system. The best test, of course, is to plug it in-then if you encounter hum, noise, or r.f.i. you'll know more shielding probably is required.

There's a trade-off between total shielding and flexibility at this stage of development. Belden, for example, sells a 100 percent shielded cable, but it's very rigid and totally unsuited for microphone use, where movement is important. On the other hand, it may be just the thing for amp-preamp connections where cable flexibility is not nearly as critical.

Combinations and Permutations

At first glance, the variety of audio connectors on display at your stereo outlet or catalog seems bewildering, and usually is enough to send the budding audiophile running for a salesperson. But there's good reason for this seemingly endless proliferation of patch cords and attachments.

A prime factor in their design and construction is "idiot proof" operation. An array of connector sizes and designs, together with color-coding, theoretically prevents one from inserting a plug in the wrong jack-with possibly disastrous, and certainly non-functional, results.

Technically, one plug/jack style could handle all of the outboard demands of an audio system. The history of connector design since World War II is one of increasing specialization and, particularly, miniaturization.

The DIN (European) design made its appearance just before the War, and the phono plug was introduced (along with the 45.-rpm record) shortly thereafter. Since then the trend has been shrinking size, as the demand for compactness and portability has increased. Today, most commercially available connectors are variations on only three or four types, as the glossary that follows this article should make clear.

Why not standardize? First, there's really very little need for it, if the audiophile will learn the ABCs of connector language and logic. Second, adapters are readily available to "marry" any two types.

And, third, the enormous amount of hardware already in use would make changing over a slow, painful, and perhaps impossible job.

There has been some movement recently within the Electronics Industry Association to standardize dimensions within the various categories, but even here progress is slow, despite the worthiness of the goals--proper mating, better connections, and greater interchangeability of components.

The Complete Circuit

After questions of suitability have been answered by identifying the correct terminations, length, and electrical characteristics that you need for your particular application, it's time to ask the most important question: "How about quality?" We've all heard stories about the spacecraft or airplane that was mysteriously sabotaged--by a 124 washer or 94 screw. Sound systems operate on the "weak link" principle just like other systems, and performance equals only the "worst case" ability of the most inferior component. And an inferior component can as easily be a cheap patch cord as an imperfect amplifier capacitor.

Electronic designers choose parts based on "cost/benefit ratio." Simply, this means spending what is reasonable for the necessary desired level of performance; no more--and certainly no less. Audiophiles who have been involved with electronics for a while and experienced salespeople can regale you with stories of plugs that stayed in jacks when cables didn't stay in the plugs, intermittent open/closed circuits that would give Sherlock Holmes nightmares, CB reception on hi-fi systems when they were turned off (caused by cabling), and many other practical examples of the perils of patchwork. Less dramatic, but even more insidious, is the very subtle and almost undetectable degradation of ultimate sound quality that can come from inferior connectors and cables.

All it may mean is that the extra $150.00 you spent on your system in a burst of enthusiasm was wasted because the added performance you bought is negated by patch cord caused sound quality reduction.

Relieving the Strain

Some of the specific advantages that are yours with a quality patch cord have to do not with the connector or cable alone, but with their integration. For example, insertion/withdrawal life is the measure of how well a patch cord will perform a few years in the future, when you've finally installed your system "for good"-after plugging and unplugging every connection who-knows-how many-times. It is a function of internal and external cable strain reliefs, extra strong molded construction, and other features that would not be apparent off-the-shelf with even the most sophisticated test equipment. The best measure of this quality is a reputable, manufacturer's name on the package.

Top features include extra-long handles for ease of insertion and withdrawal and minimum strain on internal connections. (Let us say here, since it hasn't yet been mentioned explicitly, that one should never put any strain on the cable; always pull only upon the plug.) Special construction, such as right-angle plugs for cramped back-panels, color-coding for easier-to-follow rat's nests, specific lengths to minimize clutter, all are benefits that come with a premium patch cord. And patching is one place that seeming aesthetic considerations like neatness and proper length also affect ultimate performance by minimizing interaction between signal and power cables.

The ultimate guide should be to choose exactly what is right and needed for your particular job.

The discussion of insertion/withdrawal life, for example, wouldn't apply to the assembly of a compact, where connections are made just once, usually for the life of the unit. The value of high -quality patch cords becomes more evident with a component system, where components are rearranged, replaced, and moved, and mechanical flexibility is a decided asset.

For the Odd Couple

Adapters are available in most configurations for the connection of unmatched terminations. For example, DIN connectors on some European equipment, microphone connectors, or other terminations may not match your equipment. Rather than splicing on a new connector (time, trouble--and impossible on borrowed equipment), the correct adapter ensures continuity of all connections--including ground/shield conductors--through the circuit.

Special adapters, such as "Y" adapters, headphone-to-speaker-terminal connectors, remote headphone control centers, and so on will greatly expand both the possibilities and performance of your system.

Most adapter problems can be solved by a "basic" kit such as Switch craft model 331AP1, which it is estimated, will handle 90 percent of the most common patching problems.

The kit is especially valuable to field recordists who have to make quick connections to unknown equipment for temporary use. Of course, for more permanent work, one should always obtain the correct patch cord at the beginning.

When you choose the proper patch cord, your reward will be freedom from the concern that anything is in the way of optimal performance by your system. That trouble -free performance will continue for a long time, during which you'll be able to do exactly what you bought your system for, Enjoy it.

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GLOSSARY

Adapter -- A device which converts one type of connection to another. For example, a phone plug to phono plug, or microphone plug to phone jack. An extremely wide variety of adapters is available; many are incorporated into highly reliable molded cable assemblies.

Adapter Kit -- An assortment of adapters designed to anticipate the needs of audiophiles or recordists to make a variety of connections quickly and conveniently.


Alligator Clips -- Spring-loaded metal jawed clips with serrated inside edges for grasping a single bare wire or terminal; generally used for temporary test connections. They may be bare or insulated with a flexible boot; some designs have a pin for piercing insulation.

Attenuation -- The decrease in amplitude of a signal during its transmission from one point to another. Usually expressed in decibels (dB).

Banana Plugs and Jacks -- A female receptacle and male plug capable of reliably handling rather large currents.

The plug has a single solid center post and four spring -like outer pieces which compress on insertion into the plug and form a large, high-pressure contact area. Some banana plugs "nest" for multiple connections. Some banana jacks also have a screw collar for bare wires or spade lugs. Their most frequent audio use is for loudspeaker connections.

Battery Connector -- Electrical connector provides power supply connections for battery operated devices.

For use with 9-volt transistor batteries, the connector contains a male and female, connector that snaps onto mating battery terminals.

Binding Posts -- Screw-down connectors which accommodate bare wires or spade lugs, frequently for speaker connections. They are sometimes combined with banana jacks as a "universal" connector.

Bushing -- The mechanical element of a jack usually attached to a panel. It is typically threaded, and held in place by a threaded nut or trim ring. The bushing also serves electrically as part of the sleeve circuit.

Cable -- An assembly of one or more conductors, usually within a protective jacket. Conductors are insulated from each other, and one conductor is often a shield formed of metal braid which encloses one or more other insulated conductors. Size of conductors (single or stranded) and size of insulation, as well as method of cable construction vary widely according to application.

Capacitance -- The property of storing electrically separated charges when potential differences exist in conductors separated by an insulator. Low-capacitance tonearm and patch cord wiring is required in CD-4 phono systems, where very high audio frequencies are used.

Clamp, Cable -- A mechanical device which holds a cable and prevents twisting, pulling and straining from damaging or separating internal connections of conductors from connector terminals.

Closed Circuit Phone Jack -- Phone jack with a shunt spring connected to tip. The shunt opens the circuit when a plug is inserted, and the shunt closes when the plug is removed. Shunts are used to switch audio connections, such as to divert the audio signal from speakers to a set of earphones when the earphone plug is inserted in the earphone jack. When the earphone plug is removed, audio is automatically reconnected to the speakers. Three -conductor jacks may also have a shunt on the ring spring.

Connector, Multi-Pin -- This audio connecting device has two or more pins/contacts inside a metal shield body. Insertion is snap -lock, with a release clip on the female. Both male and female halves are available for in line and panel -mount use. Most frequent application is for microphone use in professional applications.

Connector, Single-Contact Microphone -- A coaxial connector with external threads for tight screw -on connection, which contains a single center contact. It is usually used with microphones, and the entire body acts as a shield when attached to coaxial cable.


Dielectric -- The insulating material that separates the conducting parts of a plug or jack.

Crimp -- A method of securing a wire within a hollow termination by compressing the terminal around the wire. Crimping makes an excellent mechanical and electrical connection.


No solder is used for crimping.

DIN Plugs and Jacks -- Miniature (Deutsche Industrie Norm) European plugs and jacks with a number of pins and contacts. The number of circuits are from two through six.


Fahnestock Clips -- Spring-metal clips used mainly for breadboarding or experimentation. They can accommodate a variety of single conductor terminations, including bare wires, and hold conductors securely yet temporarily.

Finger -- The portion of a plug that protrudes from the handle and is inserted into the mating jack. Contains the tip and sleeve elements (and the ring element in a 3 -conductor plug), plus insulators for separating these conductors. Standard phone plugs have Y, -in. dia. fingers; miniature phone plugs have %6/ in. dia. fingers; and subminiature plugs have 0.097-in. dia. fingers.


Ground -- A common reference connection point, such as a chassis of an electronic device, a shield of a coaxial cable, or a metal body of a connector.

Handle -- The metal or plastic shell which fastens to the main body to form a completed plug. Protects internal terminals and cable clamp.


Impedance -- Opposition to the flow of changing current. Values are specified for some components (such as microphones) and their cables when the value is critical. Also sometimes known as the "load" on a circuit or audio device. Expressed in ohms.

Insulator -- Any part of a plug, jack, or cable designed to prevent the flow of an electric current. Insulation must be sufficient to isolate the highest current level that will pass through the conductor (see Dielectric).

Inductance--The property of a component or circuit which opposes any change in an existing current. Inductance is expressed in Henrys.

Jack--The female component of a plug/jack combination, which is generally panel mounted.

Jack, Phone--A two-or three-conductor connector, originally developed for audio connection. It may be open -circuit or closed-circuit, and may also have isolated switching circuits as well. Because of its long insertion/withdrawal life, the phone jack is usually used for headphones in audio applications, as well as high impedance microphones. Sometimes called a commercial phone jack.

Jack, Miniature Phone -- A scaled down version of the phone jack, the bushing I.D. is 9/6, in. as opposed to'/. in. for the standard phone jack/plug. It is typically used in portable, battery powered equipment. These jacks are usually two -conductor types.

Jack, Subminiature Phone -- This tiny jack mates with a plug with 0.101 -in. finger diameter, and is used in applications where space is at a premium. Usually a two-conductor type.

Jack, Phono -- Also known as an RCA jack, this two-conductor connector has a 1/8. in. -dia. bushing I.D. (nom.), and is a favorite for connections to and from turntables. Its design is coaxial, providing shielding for low-level signals when connected to coaxial type cables and phono plugs.

Complete body handle

Molded in place

Molded Cable Assembly -- An integral patch cord with connectors and cable fused together in one piece for added reliability, strength, and durability.

Assemblies are available in a variety of lengths, colors, and terminations to fit particular applications.

Plug, Phone -- A two- or three-conductor audio connector, used to mate with a phone jack. Comprises a body, handle, and finger assembly. (Some types have a cable clamp inside the handle.) Finger normally has 1/4-in. dia. Sometimes called a commercial phone plug. Phone plugs are normally used as terminations on the end of cables.

Plug, Miniature Phone -- A scaled down version of the phone plug, usually a two-conductor type, which mates with a miniature phone jack. Finger dia. is 9/64 in.

Plug, Subminiature Phone -- An even smaller version of the phone plug, usually a two-conductor type, which mates with a subminiature phone jack. Finger dia. is 0.097 in.


Open-Circuit Phone Jack -- A phone jack where the tip (and ring, if used) does not have shunt springs.

Resistance -- The non-reactive opposition to the flow of direct or alternating current. Heat is produced when current flows through a resistance. Conductors have a low resistance; insulators a very high one. Expressed in ohms.

Shielding -- A metal covering on a connector, or wire braid or wrap around a cable used to prevent undesired radiation, hum, or signal pickup.

Spade-Lug -- A U-shaped terminal which is usually crimped on a bare wire and attached to a screw terminal or binding post. It is superior to bare wire connections because it prevents damage of the wire; most frequently used for speaker connections.

Spacer -- A non -conducting (insulating) jack element used to separate and electrically isolate the conducting elements of a phone jack.

Strain Relief -- A cable clamp or other device used to prevent mechanical twisting and pulling from disturbing the electrical connections within a plug.

Stripped-and-Tinned Leads -- Wires with insulation removed around the conductor at the end. Wire is then solder coated (this makes stranded wires stiff). Stripped-and-tinned leads are ideal for making wire connections to screw terminals, and solder lugs on phone plugs, phone jacks, alligator clips, banana plugs and jacks, etc.

(Source: Audio magazine, March. 1978 )

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