Cables & the Case for Solder

When you graduate from mass-market hi-fi to high-end audio, one of the first things you notice is that audiophile gear dispenses with the gadgets and gimmicks that clutter up the front panels of most stereo receivers and integrated amplifiers. The dominant philosophy in mass-market design is that features sell products: the more functions a product has, the more desirable it will seem in the store. High-end designers, on the other hand, prefer the KISS approach: Keep It Simple, Stupid!

Every added feature means that the audio signal must go through additional wires, plugs, switch contacts, capacitors, and amplifying circuits—every one of which may potentially degrade the quality of the sound in some way (footnote 1). In the last dozen years a lot has been learned about how these losses of quality can occur.

For example, an ordinary copper wire is not perfectly continuous; on a submicroscopic scale it actually has a crystalline structure, often with copper-oxide impurities at crystal boundaries. Copper oxide is well known to be a poor conductor of electric current; it tends to distort signals by causing partial rectification. Manufacturers of high-quality cables avoid this subtle distortion in three ways: 1) by using high-purity ("oxygen-free") copper (I use cables made of Nimico "six-nines" copper, said to be 99.99997% pure); 2) by forming the wire with long, stretched crystals having fewer boundaries (the best-known example is LC-OFC—linear-crystal oxygen-free copper); and 3) by using a completely different conductive metal, such as silver.

Very subtle changes in sound may also occur at the junctions between different kinds of metal. Plugs and sockets affect sound, as do wires. The quality of a cable may depend on how the wire is attached to the plug at each end. For example, after the insulation was cut away, was the exposed wire scraped to remove the copper oxide from its surface before it was attached to the connector? Was the wire physically crimped onto the connector under high pressure to form a real metal-to-metal connection before soldering? (If not, the solder—itself an alloy of tin and lead—may become the conductive path.) In cheap patch cords the connection may not be soldered at all; the wire may simply be held on the connector by melted plastic. It's a shame that high-end cable manufacturers don't discuss these construction-quality issues in their ads, instead of promoting silly mythologies about sub-nanosecond time delays, so-called "phase noise," etc.

The plug or socket typically has a core of brass (an alloy of copper and zinc) with a shiny nickel exterior. In some connectors the nickel is covered with a molecule-thick "flashing" of gold, to produce an appearance of costly quality and perhaps also to prevent the slow corrosion that occurs in most metals when they are exposed to air.

Consider all of the metal-to-metal junctions that an audio signal's small electrical current may have to flow through when you plug a cable into a socket: beginning in a copper wire, emerging through the wire's unscraped copper-oxide surface, then passing through a layer of solder (tin and lead), through the nickel plating on the metal tag where the wire is attached, into the brass core of the plug, out through the layer of nickel (and perhaps gold) on the plug's contact surface, then into the mating gold surface of the socket, through the nickel plating into the brass core, out through the nickel plating on the socket's wire tag, through more solder to another layer of copper oxide, and finally into the copper wire that will carry the signal to the next stage in its journey. No wonder some designers prefer to have fewer plug-in connections in the signal path!

Most electrically conductive metals undergo slow corrosion on surfaces that are exposed to air, humidity, pollution, and fingerprint oil. This applies not only to exposed wires, plugs, and sockets, but also to the electrical contacts in mechanical switches. Two metals are exceptions to the rule: silver and gold. (Some of the best switches use solid silver contacts, but they're costly. Solid gold might be even better but would be ridiculously expensive. Gold plating, or flashing, doesn't help in switches because in a well-made switch the contact surfaces scrape against each other under pressure each time the switch is operated. This wiping action would soon scrape off the plating, exposing the brass contact surfaces you started with.)

The wiping behavior of switch contacts is an important part of their design. Each time a switch is used the accumulating corrosion on its contact surfaces is scraped away. Result: a switch that is used every day is likely to be trouble-free. But a switch that is used only once a month may develop a significant film of corrosion on its contacts. Someday this will become obvious when your left channel disappears into the void. Long before the signal vanished it was slowly being corrupted by low-level rectification at the contact points.

Preventive maintenance is as easy and important as brushing your teeth: periodically, and immediately before every important listening session, exercise every switch in the signal path a few times to wipe its contacts clean. Indeed, one of the unwritten rules of good engineering design is that the only mechanical switches in the signal path should be ones that will be used often; eg, the input selector. Switches that will be used only occasionally, such as a tape monitor or tone-control bypass, should be designed so that in normal use the signal need not pass through a set of contacts.

This advice applies to mechanical switches that actually conduct the audio signal, not to electronic switches. In the latter, pressing a button simply sends a DC voltage to turn on a transistor, which conducts the signal. Most of the switching in CD players, VCRs, and modern TVs is electronic, but mechanical switches are still common in preamps.

Wiping action was also built into the design of the RCA phono plug. The male plug's protruding central finger is scraped by the collar that it fits into, within the phono socket. And the plug's outer skirt should scrape the exterior of the socket when plugged in. The skirt usually is not a solid cylinder but is split into several metal leaves. If the skirt doesn't tightly grip the exterior of the socket, gently bend its leaves inward until you get a snug fit.

Wiping action provides no benefit if it isn't used. If you leave the cables from your turntable or CD player plugged into your preamp for three years, you aren't doing anything to prevent low-level rectification at the socket. To ensure a clean signal path, every few months you should unplug and replug every cable in your system a few times to wipe the contact surfaces clean. This is so beneficial that when somebody marvels at the sonic benefit of a new interconnect cable, I often wonder how much of the improvement was due simply to the wiping of the plug contacts that occurred when the new cable was connected.

Reviewers who are constantly hooking up and disconnecting new components may forget how important this procedure is for people who use their equipment for years at a time. Plug contacts may be a less compelling problem in the high-desert environment around Santa Fe than in the salty air of Boston and Miami or the smog-filled atmosphere of Los Angeles. In these hostile environments a contact-enhancing cleaner (Tweek, Cramolin, or Kontact) may provide benefits far greater than their modest cost, both by improving signal flow and by deterring the re-accumulation of corrosion on the contact surfaces.

The RCA phono plug has some important drawbacks as a connector, but it has two profound virtues: its wiping action (assuming that the plug is correctly sized to make a tight fit) and its relatively large metal-to-metal contact area. Another connector used for audio, the phone plug, is terrible by comparison. (I'm referring both to the ¼" phone plug used for full-size headphones and amateur microphones, and also to the 3.5mm mini-phone plug used for walkabout products—lightweight headphones, small cassette recorders, and portable players.)

When a phone plug of either size is inserted into its socket, there is virtually no wiping action and no tight fit between mating metal surfaces. Within a phone socket, the round shaft of the plug makes contact only with V-shaped tips at the ends of spring-metal fingers. The area of contact between each finger and the plug shaft is tiny—a fraction of a thousandth of a square inch. Phone plugs are fine for their original use—temporary connections in old-style telephone switchboards, where they are plugged and unplugged many times every day. But they are lousy for audio applications where the plug must remain in its socket and provide continuously reliable service, such as a microphone input used for a long recording job, or the line inputs of a portable DAT. The XLR connectors used in pro audio are much better, with wiping action and ample contact area.

Everything I've spoken of so far is minor compared to the real disaster area in audio: the use of bare-wire connections for speakers. If we could survey all of the households in the country, we would find that in the majority of stereo systems there are no connectors on the speaker wires. A half-inch of insulation was stripped off each end of the wire, and the bare copper wire was connected to the terminal. In mass-market hi-fi this approach is popular because it costs nothing and is very easy—especially if the amplifier and speakers are equipped with spring-tab terminals. (Just press the red or black tab and stick the wire in the hole.) But if you're serious about sound, you should abandon both the spring-tab terminal and the bare-wire connection.

If you own high-quality equipment you probably don't have any spring-tab connectors; they're used mainly in low-power mass-market gear. But I've seen bare-wire connections in systems at all price and quality levels. The problem is that, unlike nickel-plated phono sockets, where corrosion is slow and subtle, the corrosion of exposed copper is rapid and serious. In the short run this is okay, but in the long run a bare-wire connection is a cancer.

Perhaps my awareness of this problem is related to the fact that I've lived within 10 miles of a seacoast all my adult life. This has been my experience: When you strip off the insulation from a wire, the freshly exposed copper has a brilliant reddish-gold luster like that of a new-minted penny. But the copper surface immediately starts oxidizing, and within a week the luster has lost its brilliance as the surface of each wire becomes covered with copper oxide—which, as I said earlier, is a poor conductor of electrical current. Within a month the thickening film of copper oxide visibly darkens the wire. After a year the darkened wire starts turning green (from sulfate and chloride contamination) and bits of solid copper oxide flake off. If a copper wire is fastened to a dissimilar metal, such as a steel screw or brass binding post, electrochemical reactions at the metal-to-metal interface may accelerate the buildup of corrosion.

There's a simple way to provide a secure, corrosion-free, high-current connection from amplifier to speaker: install a connector on each end of the wire, one chosen to mate well with the amplifier or speaker terminal that it will be attached to. The connector may be a U-shaped "spade" lug, a hook-shaped lug, a pin-shaft, or a banana plug. In any case the wire should be crimped onto the connector with high pressure to make a good metal-to-metal connection, then soldered.

Contrary to popular impression, the object of soldering is not just to secure the electrical connection. By flooding the wire-to-connector joint with a heavy liquid metal that solidifies as it cools, soldering seals out air and guarantees a perpetually corrosion-free joint (footnote 2). As a final step, any exposed length of bare wire should be wrapped in heat-shrink tubing. As its name implies, when this stuff is heated it shrinks tightly around the wire and joint, sealing out air in order to prevent the copper from oxidizing.

If you don't want to do all this yourself, now you know what to look for in store-bought cables. As I suggested earlier, when you buy high-quality cables you're paying for good connections as well as good wire.—Peter W. Mitchell



Footnote 1: It seems to me to be an obvious if rarely appreciated philosophical point that, in absolute terms, doing anything to the signal can only degrade it. The Second Law of Thermodynamics implies that while you may achieve a certain desired improvement in one particular area, the overall effect will still be negative.—John Atkinson

Footnote 2: Almost 50 years ago as of the time of posting this article to our website, I was working for an aerospace company. I learned from the engineers that the optimal way to join two metals in cables was with a cold weld, which a high-enough pressure is applied to literally melt the two at the interface between them. This is achieved, for example, by using a wire-wrap machine, and I note that some high-end cable manufacturers do use cold-welding rather than solder.—John Atkinson

COMMENTS
georgehifi's picture

And that's the humble volume control ( potentiometer ) Doesn't matter what quality, it has:

"two different materials" metal wiper, against carbon or resistive plastic track

"very small light contact area" so it doesn't wear out the carbon or plastic track

"current running through them". between two different materials? with light small contact area?

Cheers George

ajcrock's picture

A couple clarifications
While lead based solder is still in use most solder is lead free and high end solder uses silver. In Europe lead based solder has been outlawed for a number of years

Most connectors are brass on the inside but there are copper based connectors that are either nickel or gold plated

Bare wire connections are fine if the wire is silver. Oxidized silver is conductive unlike oxidized copper. If connectors are used, consider insisting on plated copper for the best results.

GLADYS ZYBYSKO's picture

was nearly universal when the article was written.

GLADYS ZYBYSKO's picture

Every record made since the advent of electrical recording utilized a mechanical wiper pot ... or up to 128 of them.

ajcrock's picture

Thanks did not catch the date. The article today is still valid. Just needs a few updates