What are phono plugs and what is the problem with them?

Phono plugs are probably one of  the most common types of connectors in use today for connecting audio and video signals. They have been around for many, many years... since the earliest record player pickups needed to be connected to their amplifiers. In fact that's how they got their name, it's an abbreviation of "Phonogram". I don't know the exact history of this next name "RCA plug", but it is likely that they were named after the Radio Corporation of America (or RCA) who, no doubt, used them in phonograms that they used to make in those days long ago. Another name I have seen in Asian catalogues is "Pin Jack" and there are bound to be others.

Phono plugs were designed to interconnect a single, shielded, unbalanced audio signal. You need two of them for stereo and you need to get them the right way round (left-right), so they are usually marked white or black for left and red for right. These connectors have also found their way to being used for connecting composite video signals too , and yellow is usually the colour for that purpose. Their use for video is restricted to domestic and some entry-level semi-pro video gear. They are not suitable for critcal video or RF applications because they do not have a charcteristic impedance of  75 ohms as required for video cabling, and most of them have no locking mechanism, so it is too easy to pull them out.

One of the main problems with these connectors (and the reason for this article) is that the outer connection which is used for the screen or earth side makes a pretty poor connection in many instances. With an audio signal you may get some hum and/or some RF pickup, but with video the signal can look really terrible because a good screen connection is essential for the 75 ohm impedance to be maintained. The visible effects of a bad screen connection include multiple images (ghosting), smearing of fine details, incorrect colour rendition, unstable synchronisation, video signal present in audio (buzzy sound), etc.,  etc.

Why does this happen? Well, the outer shell of a phono socket (the part on the equipment) is mainly manufactured these days with parallel sides. They look nice and straight, but they make no better or firmer connection as the plug is pushed home. The phono plugs are affected too. The nice looking ones with a machined / turned appearance have parallel sides inside and make no better contact as they are pushed home. Because these plugs are so rigid, it is very hard to squeeze the sides in a bit without cracking the brass they are made from. I have measured many problematic phono plug and socket combinations with  an ohm-meter and found that the shield connection is completely open-circuit!

Changing the cable to a good quality mass-produced type with the pressed metal type of shield contact (which is really easy to squeeze without damage) has cured many audio and video problems over the years. It is also possible to buy many types of very high-quality phono plugs which have spring-loaded or other clever internal mechanisms which make very positive contact. If you are having audio and video problems which are intermittent and disappear as soon as you move the equipment to have a look, it might just be due to the type of phono plugs you have.

(c)2000 Quest Electronics Pty Limited