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Phono Guide

A step-by-step guide to wiring phono connectors.


Phono connectors are the norm when it comes to interconnecting budget recording equipment. Ben Duncan outlines the phono's development and provides a helpful guide to phono wiring techniques for readers to follow.


Phonos are a legacy of the Forties, when RCA was a major innovator on the audio scene. Today, they're made by a variety of manufacturers (eg. Switchcraft) but you'll still hear phonos being called 'RCA plugs' (or 'RCA jacks') in the US.

Phono plugs differ from all the other common audio connectors in being truly coaxial - the outer screen tightly encircles a (single) inner connection. This makes them good for RF (radio) and video connections. Significantly though, for audio purposes, this translates as small size. In fact, the coaxial concept in general packs interconnection into the smallest possible area (consistent with any given plug size) coincident with excellent screening and ruggedness.

Coaxial connectors in the phono style are also easier and cheaper to make than most types of plugs, so small wonder phonos are the number one choice for manufacturers who need to jam dozens of connections onto the rear of budget-priced equipment. And joy of joys, they're single circuit connectors: there are never more than two connections. That rules out balanced lines, but it also abolishes the myriad of permutations common to jacks, DINs and XLRs. So prosaically, all the 100 million or so phonos around this planet are wired identically - unless of course, someone has squeezed a 20dB pad inside one of them... But regardless, phonos are very much an international connector - ever tried buying a ready wired DIN lead or a 'B' gauge jack in mainland China!

Another attribute, a negative one this time, is that the plug's hot (centre) pin connects the socket hole before the ground (outer) makes contact. Typically, this means a short burst of hum or some bent meter-needles whenever a phono input is patched. This is especially true with regard to high sensitivity, high impedance inputs. After a while, you may develop the habit of instinctively taking down the gain beforehand.

Styles



Phonos come in 3 basic formats:

1. Pre-wired, with crimped contacts and a moulded-plastic cover.

2. Rewirable, with a clip-on or screw-on plastic cap. Usually, the body is of pressed-steel, which is relatively weak.

3. Rewirable, with a metal cap. These types are usually nickel-plated with solid brass bodies; this is the type shown in the wiring-up photos. Gold-plated versions are readily available, if you want to pay the extra.

Crimp connections can, in theory, be superior to soldered joints. Practice is a different matter. If a moulded lead works, use it, but the moment you suspect its integrity, immediate disposal is the best move.

The second type - with a plastic cap - are essentially okay providing they're not going to be trodden on, viz. for patchbay duties. However, in view of the fact that these types are low in cost, you're likely to find skimpy materials inside, loose riveting, and that special joy from the orient - plated steel that's nearly impossible to solder! So keep a special eye on the quality.

This leaves metal phonos as the sole serious choice, with the better types being nearly as rugged as jacks. Incidentally, metal phonos are available with coloured bands, so the advantage of identification by colour code (as with types 1 & 3) isn't lost. You may want to colour signal sends and returns (blue and green) and left and right channels (black and red), for example.

Gold Standard



The good news about phonos is that so long as you select quality types, wire them carefully and maintain them, the basic connection is very good, owing to the sheer friction involved in the connector mounting. Gold-plated phonos are doubtless superior, as they don't corrode or tarnish, and the value of the plating means that they tend to be built to higher standards overall, to justify the prices asked!

But, like any other electrical conductor used in plugs and sockets, they still become coated in every studio's unique organic-gunge cocktail! Major ingredients include sweat, tar, sugar, and silicones (from aerosols), none of which are particularly renowned for conducting electricity. Providing part of the molecular layer of corroded metal/gunge is scraped away from both surfaces each time a plug is inserted, and the cleaned areas are pressed tightly together, all will be well. Of course, this assumes that the corrosion doesn't set in (literally) too deeply. Given all these conditions, gold-plated phonos offer no intrinsic advantage.

The major exception applies to permanent connections - plugs which may remain in one place for a year or more. Without the regular/occasional cleaning action of withdrawal/insertion, the connections behind the monitoring amp (say) will become very dicey, and after a time, hiss, crackles, distortion - or even a dead silence - may set in. So this is one place where you can use gold-plated phonos without feeling guilty about wasting money. Are musicians ever guilty of this?

Whenever a cheap phono plug fails to make a tight, solid connection, you can easily beef-up the contacts: For the outer (ground) connection, bend the split-skirt slightly inwards. For the inner, a thin layer of solder over the plug's tip will usefully increase the contact pressure. These rituals apply to plugs only, and you MUST make sure that it's a specific plug that's loose. If not, the culprit is probably the socket, and by beefing up a plug that's 'in tolerance', you'll only stress the other sockets.

In view of the organic contaminants just discussed, it makes sense to clean your plugs and sockets at least once a year - especially the inner ('hot') connector. The plug tip can be burnished with a cloth soaked in alcohol, whilst for sockets, a matchstick, again soaked in a polar solvent like isopropyl alcohol (available from any chemist and handy for cleaning tape heads too) can be used. With gold-plated plugs, you'll also need to avoid cleaning with anything abrasive for fear of wearing away the gold, which is usually very thin. For the rest of the time, you can gain improved metal-to-metal contact by twisting phono plugs 180° around as you push them into the socket; this corkscrew action will tend to wipe loose nascent corrosion, preventing a thick layer from ever building up.

Grounding



The chassis of a phono socket is also its ground (or earth) connection. So, to prevent hum loops on mains powered gear, or to groundlift the equipment, it's necessary to isolate the screw-thread and nut from the phono chassis, which can then be earthed separately. The most basic phono sockets are mounted on bakelite strips, but failing this, isolation is achieved either by (a) adding insulating bushes, or (b), by mounting the sockets on a fibreglass - or any other convenient insulating strip. Both of these modifications involve opening out the panel hole to around 9.5mm.

As implied, the better quality sockets feature screw-threads, and from time-to-time, the back-nuts may come loose. Aside from locking the nuts, it's wise to make a routine check on their tightness whenever you have to take the lid off an item of gear.

Phono Wiring



Wiring for all phonos is basically identical, and considering the size of the barrel, the terminal size is not at all unreasonable. Indeed, with a little care, phonos are much easier to wire than DINs - and only a little more fiddly than soldering up ¼" jack plugs.

Cable clamping is poor on phono plugs, although easier to crimp tightly than on DINs. Also, the cable access is limited: cables much over 4mm OD (outside diameter) won't fit. In any case, a major part of avoiding phono leads which disintegrate is to use a thin, light, and flexible cable - one which won't overstress the clamping. For best results, Phonoflex is recommended because, as it's name suggests, it's built especially for phonos. A cheaper, less durable alternative - for emergencies only please - is an ordinary lap-screened cable eg. RS 'Medium Screened'.

Procedure



1) Begin by stripping back the cable's outer sheath about 40mm (2"). With Phonoflex, place the drain wire to one side of the wire stripper's blades to avoid nicking, then unscrew the phono plug. The barrel goes over the cable end, as shown, whilst the body will need clamping for the next operation. In the photo, a cardboard shim has been used to prevent the lever-wrench's jaws from making (harmful) indentations in the centre pin, which is made from brass (the best phonos) or hollow, pressed-steel (poor quality plugs).



2) Strip back the inner core, leaving 6mm (¼") of insulation at the cable end. Then firmly and neatly twist the freshly exposed wire (Phonoflex is ready-dressed in this fashion), and tin the end, as shown in the photo.



3) At this juncture, chop off a short (4mm) length of Hellerman rubber sleeve - the H30 size is best. This is depicted in the picture. The 'sleevelet' can now be threaded over the cable's inner core. Some lubricant may help with this. Next, thread the cable into the plug. The rubber sleeve is intended to prevent the plug's inner terminal from coming into contact with the screen terminal or wire, so make sure it's pushed over firmly.

In the photo, the Phonoflex insulation is still showing, and for the sheath to be gripped securely by the cable clamp, the cable will need to be squeezed about another 2mm into the phono; for long, trouble-free results, this sort of finesse is well worthwhile.

4) Once you've pushed the centre core fully into the phono, the sheath should overlap the cable clamp. Then, using long-nosed pliers, gently squeeze the lugs together, perhaps coming in at several angles, so the clamps close around the sheath, rather than just squeezing it flat. With the cable securely in place, we can then solder the inner core without risk of it falling out of position.

For best results hold the iron on the tip of the plug for about 15 seconds to allow the metal to attain full temperature before applying any solder. At the same time, beware of excess heat, as this will damage the insulation of the inner core at the point where it's in contact with the hot metal. A considerable quantity of solder will be sucked up the tube by capillary action, so you can expect to feed in a fair quantity before the end is seen to 'fill out'.

The picture shows the plug immediately after soldering: note the excess solder forms a cone-shaped nozzle. This is the sign of a healthy joint. If not, it's most likely that the screened cable is dirty or tarnished. Perhaps the solder didn't take too easily to the wires when tinning? If so, I'm afraid you'll have to start again, with a fresh section of the cable, or maybe a new cable outright.

5) The drain wire on the Phonoflex is simply hooked over the screen terminal, given a slight downwards tug, and soldered. For lap-screened cables, you'll have to twist the wires neatly, and look out for stray whiskers, which may cause annoying short-circuits at a later date. (Phonoflex cores come ready twisted, so whiskering needn't arise). The picture shows the drain wire just prior to soldering. Soldering itself is easy; the key point being to keep the solder's profile as low as possible, otherwise you'll have difficulty when screwing up the barrel.

6) Likewise, soldering the inner core may result in a build-up of solder on the plug's tip, making subsequent connections too tight. The photo shows the screen terminal's solder profile being tidied up. Here, it's often possible to make the barrel fit by carefully bending the terminal downwards into the plug. The rubber sleeve we've just applied should also prevent short circuits from occurring.

After screwing on the barrel, you can place a Hellerman sleeve over the plug for identification. Once the lubricant dries, the sleeve will prevent the barrel coming undone. Also included in the photo (alongside the red, rubber-covered phono) are moulded, prewired phonos just for comparison's sake.



Previous Article in this issue

Yamaha RM804 Mixer

Next article in this issue

ACES 27 Band Graphic Equaliser


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

 

Home & Studio Recording - May 1984

Donated & scanned by: Mike Gorman, Gab

Feature by Ben Duncan

Previous article in this issue:

> Yamaha RM804 Mixer

Next article in this issue:

> ACES 27 Band Graphic Equalis...


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