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The Five of Jacks

Ben Duncan guides us through the mystic world of jack plugs and tells us why not to take them for granted.

Ben Duncan here turns his creative energies towards examination of the merits and deficiencies of five jack plugs.

Originally conceived for telephone exchanges in about 1906, the Edwardian designer of the jack plug unwittingly figured in the most crucial elements of male anatomy, and from these, fashioned an audio connector which remains universal after 80 years. In this epoch, jacks have evolved into just three related types for the majority of sound system tasks.

Let's begin with the basic 'mono' or 'two pole' plug, the original telephone's tool. Up until the late 60s, this was used to connect just about everything electrical a musician owned, from Strat to PA stack. Today, it's still needed for interconnection between instruments and their respective amps and speaker cabs, and for budget PAs and effects connections.

The Mono Jack

Working from the catalogues of two audio/studio spares suppliers, we ordered three mono-jacks.

Plug 1 is a special, heavy-duty model from the US manufacturer Switchcraft. It's the type of plug seen on the end of a good, prewired curly lead. It's barrel was unusually large (which allows lots of room inside for hamfisted wiring), and judging by its brassy hunkiness, it would easily withstand a heavy impact, such as when a 100kg speaker cab comes crashing down from on high. The spigot is well supported by the chunky headstock. It's pointless having an armour-plated barrel if the plug snaps off at the base!

Plug 1's tip is somewhat broader at the base than average. This makes it less likely to slip out from sockets with warped or twisted springs. In the unique features department, the Switchcraft exhibits a sensible idea: a sprung-steel pin senses whether the plug's inserted or not. If not, the internal switch shorts across the terminals, so cutting out loud bangs or bursts, should the guitar or amp input lead go flying out. Naturally, shorting the terminals is a bad idea for speaker outputs, but this particular Switchcraft plug is available without the 'Silent Plug TM' feature, as model 170.

Now to the terminals: these are presumably made in Texas, judging by their capacity. Equally, the terminals and insulator are doubly riveted to the headstock: once for a good electrical connection, and a second time to stop the terminal from turning or being twisted to one side, and coming loose, when the mic lead's next used as a lassoo. In turn, plug 1's cable is gripped by a 'grommet' (a rubber ring), which is squashed up tight around the sheath by pressure from the screw-on cap. Given some solidity in the rubber, so it can withstand massive axial forces when the cap is fully tightened on a broad cable, full compression can't be expected to crush the hapless grommet to zero. Therefore, there's a minimum cable diameter, thinner than which the grommet won't grip securely. In this instance, it's 3.5mm; a point worth checking if you're in the habit of using skimpy hair-wires in your studio. As for the maximum (reviewer whips out 12" ruler), it's a disappointing 6.5mm OD (Outside Diameter), which means it isn't any good for the thicker speaker leads (Figure 2).

To summarise: this plug is clearly aimed at medium-sized leads on stage, and will survive a lot of abuse, provided it's properly tightened up. But for a HM band, or something of the sort, I'd take the precaution of taping up the end-cap with self amalgamating tape.

Plug No.2

Plug No.2 is the cheapest of the group, comes from the general Hong Kong/Taiwan area, and it's the one commonly seen in shops under the RS brand name, or on the Panda-pack carousel. Once again, the base of the tip is elongated, for a good tight fit in a sloppy socket. Up at the headstock, the absence of a protruding flange cancels that score. Without the flange, we'll need to grip the spigot itself, to tighten up fully, after wiring. With any sort of use, hand tightening soon means an unscrewing act. Using pliers and/or molegrips guarantees that plugs won't come unscrewed for a long time, but without the gripping flange, we inevitably end up flattening the spigot, or imprinting it with monster 'grip-prints'. None of this is good for sustaining a reliable interconnect (Bang! Bang! Crackle... thssssssszz etc.) With a plug like this, you'll need to wrap the spigot in corrugated cardboard, to avoid marking it. How boring.

The terminals are skimpy; look at this springy, bendy metal. The small size also makes them harder for the occasional weekend solderer to get a good joint, but for someone who solders a lot, and with professional hand-tools, it won't be any problem. The riveting is also sad: in fact, it's pathetic, because with just one skimpy rivet, the centre tag can turn, and will quickly loosen up, if the transverse (turning) forces are potent enough to twist the cable inside the plug. This in turn depends on the cable clamp. In this instance, it's crude, being a pair of metal 'wings', which are supposed to be crimped over and around the cable.

Even if the appropriate crimping tool were available (they're rarely seen outside of 3rd World factories), the perfectly crimped cable clamp is both non-rewirable, and not very strong.

This is the cheapest of all the jacks, plug 2 is the one most likely to be bought by people who're on a tight budget, and can't afford a course on precision soldering, nor the specialised tools needed to make the most of a bad job.

Put another way, a professional wireman will get good results from these plugs by careful wiring and choosing 'back of the rack-type' locations. When 200 odd plugs are needed for some non-critical behind-the-scenes hookup, this is the 'Renault 4' model.

Plug No.3

Plug 3 is made by Rendar in the UK, and is also available under the RS brand name. This one has a standard width at the base of the tip. This could mean the occasional 'Whoops! It's fallen out' escapades. I've noticed this in conjunction with some plastic nutted sockets in particular. The barrel looks chunky, but is in fact a little thinner than plug 2. However, with it comes the vital flange for tightening-up. The signal passing up the spigot depends on a nut in the headstock. If this comes undone, the plug's innards fall apart, and vital bits will very likely go walkies. As the nut is invariably fully-tightened on purchase, a spot of nail varnish applied to the top of the nut will prevent this happening, later. Equally, it obviates the development of loose, crackly connections.

Like plug 2, the space inside is limited; there's no room for big bulbous joints, and the terminals are small. Clamping is with a four-sided doubrie, akin to medieval vaulting, moulded in plastic. When the cap's screwed down to it's utmost, the hole at the apex shuts down to a tiny 1.8mm OD. At the other end, the maximum is like plug 1, at 6.5mm/¼"; whereas plug 2 does at least allow an 8mm (5/16") ODs, ie. speaker cables. In practice, the Rendar will take chunkier lengths if you're prepared to chop back the tip of the 'vaultoid' clamping doubrie.

All in all, the Rendar plug is a fair general purpose model provided the spigot nut is checked to be tight on assembly, it'll withstand a degree of hammer, but excels in studios, where thin cables need to be gripped securely.

Stereo Funk

Stereo jacks are a development of the basic 2 pole model, by the country that invented stereo: USA. There are three contacts, and the extra contact is called the Ring, and appears between the tip and sleeve. In the event, stereo jacks didn't catch on for anything more than headphones. Twenty years later, musos and engineers found other uses: for simple FX send/returns, for low-cost balanced mic inputs, and even talkback cans.

There's plenty of scope for confusion with three pole jacks, because this is just one of two common types. To set the record straight, the plug discussed here comes under the following aliases: ¼" stereo jack, 'A' gauge stereo jack, and stereo cord plug (US).

Plug 4 is a new model in the UK, Neutrik's first foray into jack plugs. Like plug 2, the base of the tip is well elongated, for a snap fit. The remainder of the construction is different to the preceding models: The spigot and headstock aren't screwed to the barrel, but drop down into it. Ultimately, they're held in place by the cap at the other end. This in turn means there's only one threaded joint to come undone, instead of the usual two. In turn, there's no need for a flange. The barrel itself is made from rugged diecast alloy, of the usual (nickel-plated) brass. This has the advantage that the shell will shatter outright if run over by a 40 tonner, rather than just being deformed, like brass. Overall, this plug won't pretend to be fit and healthy, when it's really mortally wounded. Just not noticing a bad plug is far more frustrating in the long run.

Plug 4's terminals are good because they're chunky and there's plenty of space for heavy cables and weekend-soldering. Cable clamping is with a split plastic sleeve; the split means you can slip it over the cable sideways, after wiring, should you forget to thread it over at the outset. Another advantage of this plastic doubrie, is that along with a length of reasonably heavy cable, it wholly fills up the space inside the barrel, to provide a very solid assembly, and moreover, one that cushions the diecast metal on the exterior.

Holding the whole lot together is the end cap, which is a deceptively tough plastic. To test for this, we can unscrew the cap and try crushing it underfoot. This jack cap's also unique in that it comes with a strain relief bush, a synthetic rubber bit which saves the cable being bent at too sharp an angle, as it leaves the plug. In turn, this means the cable stays in one piece longer, for a given level of abuse.

So barring any hidden snags, plug 4 comes closer to the ideal rock 'n' roll connector than any jack hitherto. It's recommended for all purposes on stage and in the studio.

'B' gauge jacks have three contacts, like the preceding stereo version. The vital difference is the smaller tip, meaning a 'B' gauge plug won't make contact in an 'A' gauge socket. Surprisingly, the 'B' gauge pre-dates the 'A' version; it's been the standard jack in telephone exchanges since the 20s. Today, it's the universal patch cord plug in traditional studios, and is occasionally spotted for balanced line level connections on the rear of consoles. To be really confusing, it's also used on headphones in some long established studios (as hi-fi headphone outlets, which are always 'A' gauge) and has a string of aliases: GPO plug, BPO type 316 phone-plugs, RTS jack, PO pattern cord-plug etc.

Plug No.5

Plug 5 is the superjack, specially produced in Newcastle, by Canford Audio, presumably out of frustration of getting supplies of a decent 'B' gauge plug for studio installations. In fact, they've married up a Neutrik XLR's shell, bush and cable-clamping, with a high quality spigot. In essence, the result is nearly as good as Neutrik's own jack, but with the 'B' gauge format (which Neutrik don't make at the time of writing). A notable detail is the gold plating on the tip and ring contacts. For balanced connections, these are the two which actually convey the signal; the sleeve is only a ground, for shielding. The difference will become apparent when the bright, silvery nickel plate has all worn off, and the brass underneath has developed a dull crud, while the gold stays bright and clean. The upshot is that gold-plating helps to guarantee a good contact in the long-run. This is especially valuable for complex patch set-ups, where a crackly lead can be a real pain to trace.


It's a good idea in the studio to have a guitar lead made up with Switchcraft 172 plugs, for the peace of mind when problems occur.

Otherwise, for myself, I'll be fitting-up all new jack leads with Neutrik's NP series; at just a few pence more than the cheapest 'n' nastiest in jacks. Their new design outweighs all the jack plugs I've seen in the past 20 years. This recommendation also embraces Canford Audio's Superjack, where space permits, and where a bulk purchase more than cuts the price in half.

Jacks 1, 2, 3 were kindly supplied by: Future Film Developments, (Contact Details)

Jacks 4 & 5 were supplied by: Canford Audio, (Contact Details).

Figure 1.

Jack Data

No Maker Model Retail cost (1 off inc. VAT) Mono Availability (* = Yes)
A gauge B
1 Switchcraft 172 £6.20 * X X
2 (RS Components brand name) 478-425 £1.00 * * X
3 Rendar R22609000 £2.16 * * X
4 Neutrik NP2C £1.08 * X X
NP3C £1.24 X * X
5 Canford Superjack 43/150 £2.59 X X *

Figure 2: Some common cable diameters

Line Level & Mic Connections O.D.
West Penn/Belden foil-screened pair 3.8mm
Miniature twin-screened 3.9mm
Phonoflex 4.6mm
Foil screened, 2 pair 5.6mm
Rockflex 6.5mm
Musiflex 6.5mm
Foil-screened, 10 pair 9.9mm

Speaker connections
0.5mm2 2 core (3 Amp) 5 to 6mm
0.75mm2 2 core (6Amp) 6 to 7.6mm
1.0mm2 2 core (10 Amp) 6.4 to 8mm
1.5mm2 2 core (15 Amp) 8.5 to 10mm
2.5mm2 2 core (20 Amp) 10 to 12mm

OD = 'Outside Diameter'

Note also that the OD. of the mains type cables used for speaker leads varies considerably; rubber is generally the thicker, cf. PVC.

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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Jun 1986

Donated & scanned by: Mike Gorman

Feature by Ben Duncan

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