Interconnect - Jackfile (Part 1)
Jackfile 1: a look at jack plugs and sockets.
In the last issue, we finished looking at the American contribution to interconnection - the XLR. This month, we move onto the great British invention - the jack plug. Developed in the early years of this century for patching telephone calls, the jack now comes in a variety of sizes and configurations. As we shall now see, matters aren't helped by a profusion of nomenclature (names) for each type. However, the name we feel to be most representative for each style of jack, heads each section; hopefully, this will help to standardise the terminology.
Quarter inch jack, mono jack, 2 pole jack (UK). Single-circuit cord plug, tip and sleeve plug (USA).
This one is ubiquitous. It's found on unbalanced line and mic inputs, line and speaker outputs, sends, returns, FX switching, and speaker inputs on most low cost PA, FX, control room equipment, not to mention backline gear (instrument amps).
If you're designing an interconnection system, this is the plug to use whenever a simple, single, unbalanced connection is required.
Quarter inch stereo jack, standard stereo jack, 3 pole jack, 'A' gauge jack, 'A' gauge stereo jack (UK). Tip, ring and sleeve plug, 'A' gauge cord plug, 'A' gauge tip, ring and sleeve cord plug, RTS cord plug, stereo cord plug (USA).
Almost universally found on domestic, low impedance stereo headphones. Also used in conjunction with combined send/return sockets and FX footswitching on backline equipment, and on low cost mixers. Another use is for universal balanced/unbalanced inputs - we'll discuss this aspect shortly.
In a system, you can design in this jack to replace XLRs, to offer mic or line level balanced connections at lower cost, providing there's no need for a latching facility, and no heavy usage is anticipated. If not, you'll save more money by sticking to the more rugged XLRs. For patching, use 'B' gauge jacks; they're the universal standard for this application, and cost about the same. Don't use 'A' gauge jacks for line level outputs, unless the connection of headphones is intended, because sooner or later, somebody will doubtless plug in and destroy a pair of high impedance cans.
This plug differs from the 'A' gauge variety - it has a smaller tip diameter.
GPO jack, PO pattern jack, BPO type 316 jack, 'B' gauge stereo jack, RTS jack (UK). Phone plug, PO pattern cord plug, 'B' gauge cord plug. Tip, ring and sleeve cord plug, stereo cord plug (USA).
This is the universal professional patchcord plug. It's also used for balanced line-level connections, and for headphones.
The 'B' gauge plug is the one to use for patching (ie. temporary connections) throughout the control room, if you're a professional outfit. 'B' gauge patch cords are also bound to be needed if you hire in, or borrow up-market equipment.
In Britain, we often carelessly use the term jack whether we're referring to the plug or the socket, although it's strictly the socket. In the USA, the plug is a cord plug, and jack invariably means that the socket is being discussed.
Quarter Inch compatibility
All the above plugs have ¼" diameter barrels. This means it's possible to push plugs into any of the 3 varieties of socket. 'B' gauge sockets have a more closely sprung tip contact compared to the mono and stereo sockets, so attempting to push 'A' gauge plugs into one of these will cause mechanical damage. At the same time, it's easily done by accident, particularly with 'A' and 'B' gauge sockets on the same panel. If you're designing a system from scratch, you can of course delineate the varieties by spacing them into well defined groups. To the same end, Rendar 'B' gauge sockets come with a red or a grey nut - which makes them stand out clearly in a field of 'A' gauge and mono sockets; these latter types have the usual black plastic nuts.
Pushing 'B' gauge plugs into 'A' gauge sockets is harmless - it just gives odd (sonic) results. A special advantage of the 'A' gauge stereo sockets is that they can be used for both unbalanced or balanced input signals, without modifying the wiring of any plugs. For balanced connections, we use a stereo jack, in the normal fashion. Unbalanced signals frequently arrive via mono jack, and again, you simply plug it in, and presto, the input is set up correctly for an unbalanced termination. This trick relies on the fact that the socket's 'ring' contact (normally wired as 'cold') is grounded by the mono plug.
The same applies to transformer balanced outputs, although there is strictly no need to 'unbalance the line' at this end. Also, it's dicey to 'unbalance' so-called electronically balanced outputs by shorting cold to ground. In this instance, modern circuitry isn't likely to expire, but the shorted line-amp may oscillate or overheat.
1) Latching is available, but is rarely seen; XLRs are the norm when this facility is required. Neutrik do a latching plug to mate with a special socket, whilst conversion kits are available to retro-fit latching to most Rendar jacks.
2) Jacks are unipolar: there's no such animal as a chassis socket, although 'line' sockets are available (ie. cable mounted females). So leads are exclusively 'male cable plug to male cable plug'. Leads cannot, therefore, be joined together for easy and tidy storage, nor can they be readily extended, unlike XLR cables. You could use a short 'female line socket to line socket' adaptor lead to lengthen cables, but this is clumsy. It also adds lots of extra contacts in the line - contacts that are prone to go noisy, or intermittent! The alternative, and no less expensive route is to simply possess leads of varied length. This in turn entails lots of cables. You'll then have to concentrate energies into devising an efficient 'anti spaghetti' storage system!
3) Plastic jack plugs offer no screening, but do come at temptingly low prices. Avoid using these if possible, as they're generally of inferior quality (patch plugs with hard plastic bodies are excepted), and you'll end up throwing them away after hassle with bad connections.
But if you have to use plastic plugs, keep them for applications where they'll not be bashed, leant against, or trodden on. Also, avoid using them in mic level circuits, and also at line level wherever high-impedance, budget gear is involved; otherwise you cannot entirely discount future problems with crosstalk, hum and general interference.
4) Never use jacks for power amplifier and loudspeaker connections over 100 watts: their contacts can't handle high currents without undesirable side effects. For instance, arcing upon insertion seriously increases the contact resistance.
5) Pin 1 on XLRs is the ground pin. It's also longer, and so makes contact before the others. Jacks don't have this asset: plugging an input cable into a piece of 'floating' gear, like a guitar, is a good example. The result is a very loud hum/screech until the sleeve of the plug reaches its target and grounds the source circuitry. You won't often encounter this difficulty, but if so, consider using a different type of connector.
6) The best jack plugs are more rugged, and mechanically more reliable than XLRs, or indeed, virtually any other audio connector. Generally, best means ones with solid brass bodies. Switchcraft 'B' gauge and Whirl-Wind mono jacks (as found on decent guitar leads) are examples. Close behind are the UK made Rendar plugs, which are often bought under the RS brandname. Whilst not indestructible, these ones are sufficiently rugged for studio work; their pitfall (and it's not a serious one) is that the centre contact is clamped to the tip of the plug by means of a screw. By comparison, the solid brass plugs discussed above are monolithic assemblies, and unless the soldered joint falls apart, they can't fail.
The screws in the Rendar plugs, on the other hand, have a habit of undoing themselves. Of course, the problem is easily put right by undoing the plug and retightening the screw, but this isn't any fun if it happens often. Make a mental note, then, that when wiring up Rendar plugs, you should check the assembly screw is fully tightened.
Always avoid plugs with screw contacts, if given a choice. If not, ignore the screws and solder the connections. Low cost jacks from Taiwan Inc. with sleek metal covers are especially attractive when you simply need a lot of plugs. Whilst individual specimens can work well, quality varies widely, so plugs should be lovingly scrutinised for loose rivets and other defects before wiring.
Plugs of this calibre don't take kindly to lots of use, so keep them for out-of-the-way, semi-permanent connections, such as monitor amp inputs.
7) How often do you polish your jack plugs? Jacks should be self-cleansing because of the heavy wiping action upon insertion or removal, though the cleansing action on the sleeve contact will obviously be less good than that on the tip and ring. In practice, all the contact surfaces tarnish, and the layers can build up to become an intermittent contact/diode/noise generator surprisingly quickly in the studio environment's typically polluted atmosphere.
Cleaning first in alcohol, followed by mildly soapy water, is perhaps the best way of cleaning jacks. As you won't have to perform this ritual more than twice a year, it may be worth finding a friendly local factory with an ultrasonic cleaning bath, and taking all your leads down there for a spring clean one afternoon. Failing this, brass contacts may be lightly abraded - with fibreglass say. Avoid abrasion absolutely with lightly plated contacts, typically chrome or gold. For these you'll have to use meths, and lots of elbow grease.
Sockets are another matter. For a start, to clean them ultrasonically, you'll have to remove them, so it's a job that's best left until gear requires a major service. Mid-term cleaning can be with a pipe brush, soaked in meths. On frame jacks, which utilise tougher, thermosetting plastics, much stronger solvents may be used without fear of melting the guts - 1.1.1.tricholoroethane for instance. For heavy soiling, begin with an ammonia/water-based foam cleanser. And remember, don't use heavy abrasives on any jack.
8) Most readers will be familiar with black plastic panel jack sockets, coming from Rendar, Cliff and various oriental sources. Spring quality varies, and some varieties have gold-plated contacts; others are distinctly 'Mickey Mouse'. Over one to five years, depending on quality (which varies greatly) and the frequency of (ab)use, these jacks develop spring and contact problems, to the extent of causing intermittent connections.
For patch panels, where sockets will be worked hard, rugged 'frame jacks' are used. These can be mounted in rows on a metal or wooden panel, but more often, a pre-assembled row of sockets is used as a jack-field. Note that the ring connections may be connected to the frame. With unbalanced terminations, this can lead to earth-loop problems, so pre-assembled jackfields are largely restricted to handling balanced terminations. For unbalanced signals, you can still use frame jacks, but they'll need to be assembled on a plywood frame to keep the ground (sleeve) connections mutually isolated.
Feature by Ben Duncan
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