Why your vintage turntable could kill you
Accompanying the recent "vinyl revival"
-- the resurgence of interest in vinyl records -- is a fascination with
vintage equipment to play these records on. There are
plenty of contemporary turntables,
but there's a certain authenticity that comes from playing your
records on a fifty year-old machine.
There's a brisk trade in 70s and 80s turntables on eBay and similar sites and, in fact, some of these old machines perform at least as well as the best modern turntables (usually after a careful service). Unfortunately, most vintage turntables and, I'm sorry to say, some modern ones do not comply with contemporary electrical safety standards. Although the risk of electrocution is small, it's not zero.
Doubly unfortunate is the fact that a lot of dangerous nonsense is written about this subject on websites. It's crucial to understand the distinct roles of safety earthing and signal earthing and, it seems, many people do not.
Why is there a problem?
Many vintage turntables (and some modern ones) are connected to the electrical mains as shown in the diagram below. They have only a two-wire electrical supply, along with an earth post or a trailing earth wire. If there is an earth wire, it's usually terminated in some kind of spade connection, for attachment to some other piece of equipment.
The problem with this arrangement is that there may be no path for fault currents if a live electrical connection makes contact with a metal part of the turntable (as suggested by the red line in the diagram above). No fuse or breaker will trip, and the live parts will be a hazard to anybody who touches them.
To be fair, earth-leakage protection in modern electrical installations will disconnect the supply fairly quickly if somebody does actually touch the part of the turntable that has inadvertently become live. Still, it's not considered good practice to rely on this -- a live-to-earth fault ought to result in an immediate (well, milliseconds) disconnection. Earth-leakage protection should prevent a fatal electrocution, but you could still get a nasty jolt.
Given this hazard, it's worth asking why this was such a popular way of wiring turntables. The answer lies in that old bugbear, the earth loop. Phono cartridges produce a very tiny signal, which needs a lot of amplification. The amplification will magnify the tiny electrical currents that flow between devices when their signal (not safety) earths are at different potentials. In most consumer-grade amplifiers, the roles of safety earth and signal earth are mixed: amplifiers have metal cabinets both to provide a path for fault currents (safety), and to absorb electrical interference (signal).
The conventional solution
Given this dual role of the earth, the conventional way to wire up a vintage turntable is as shown below.
This arrangement (mostly) avoids earth loops because the turntable does not have its own earth (either safety or signal). The turntable is earthed by either, or both, of the wire between the earth posts, and the screens of the audio connectors.
In the event of a live-to-earth fault in the turntable, the path for fault current is through the turntable chassis to the amplifier chassis, and thus to the mains earth. All being well, this fault current will be sufficient to blow a fuse or trip a breaker.
Why the conventional solution is inadequate
The conventional earthing strategy does not conform to modern safety standards, for two reasons.
First, the wire between the earth posts will almost certainly not be matched to the tripping characteristics of the mains circuit breaker or plug fuse. Remember that, in the event of a live-to-earth fault, this earthing wire will carry the entire fault current, until the breaker trips. As a minimum, it's got to have a higher current rating that the breaker or fuse. Suppose the turntable is fitted with a 5-amp plug fuse. In the event of a fault, which will fail first -- the plug fuse or the earth wire? If it's the latter, it could start a fire. Many vintage turntables have a captive, trailing earth wire, which is probably capable of standing 5 amps until the fuse blows. But would you bet your house on it? At least if you're providing your own earth wire, you can fit a heavy one with a substantial current rating.
The second risk factor is more prosaic -- the earth wire can become disconnected. The safety earth wire in a three-pin plug is not only rated to handle the expected fault current, it's impossible to disconnect it without tools. If you're providing your own earth connection between pieces of audio equipment, you've got to remember to reconnect it when, for example, you move the equipment around. Since this earth connection is probably behind a stack of equipment, you won't see if, for example, it just works loose with vibration.
A digression: self-contained pre-amps
Modern amplifiers often do not have phono-level inputs for turntables. Contemporary turntables often have line-level outputs suitable for modern amplifiers, but vintage models rarely did. In this situation, you'll need some kind of self-contained pre-amp, to convert the phono signal to line level.
This potentially creates an additional hazard because, with such an arrangement, it's not very clear what to connect the turntable's earth post to. The pre-amp may have an earth post but, if it does, is it suitable for use as a safety earth? If your pre-amp is powered by a wall-wart (or a battery), any earth post it provides will not be suitable for a safety earth. There is almost certainly not an earth connection between the pre-amp and the wall-wart and, even if there is, it probably won't be heavy enough to carry a fault current. A mains-powered pre-amp might have an earth post that is suitable for a safety earth, but it's best not to rely on this, unless the manufacturer clearly states that it is suitable.
In general, you'll need to (safety) earth your turntable to something that you are certain is a proper safety earth point. There's really only is one way to be 100% sure of this -- more on this in a while.
What about double-insulated turntables?
Many contemporary turntables, and some vintage ones, were designed to be double-insulated. This means that the live parts are completely enclosed in some highly insulated enclosure withing the appliance. Even in the event that a live connection goes astray, there's no possibility of it contacting a metal part that a user may touch.
Modern safety standards for double-insulated appliances require careful testing, to ensure that the insulating housing really does provide the level of insulation required to ensure safety. Double-insulation was rare in vintage turntables. If it is used, and if it is expected to conform to modern standards, there will probably be a prominent marking on the cabinet (in the UK, it's a symbol consisting of one square inside another).
In a vintage device, however, that claims to be double-insulated, how far will you trust the insulating properties after forty or fifty years? I'm not sure it's something I'd want to rely on.
The solution
Despite what is often stated on web forums, there is only one solution to this problem that complies with modern electrical safety standards: throw away the two-core cable, and replace it with a three-core cable, with the earth soundly bonded to all exposed metal parts. The metal parts that need to be bonded might all be connected to the existing earth post or flying earth wire, but it's not safe to assume that. You'll need to use a meter to ensure that there is a low resistance between any metal part a user can touch, and your new earth connection.
Will this create an earth loop? Perhaps. If it does, you may be able to break the loop by disconnecting the screens of one or both audio leads, at one end or the other. This is a fiddly process, requiring a certain amount of trial-and-error. With your turntable earthed properly, you may no longer need the secondary earth connection between turntable and amplifier. If all else fails, you might need a pre-amplifier that has electrical isolation in the signal path. Many good-quality amplifiers have this already, and are more-or-less immune to earth loops.
In the end, though, safety has to override other concerns. Even if you're prepared to take the risk of electrocution or fire yourself, is it a risk you want to impose on other people in your household? If you earth your turntable properly (from a safety perspective) then all you risk is poor audio quality. If you don't, the only music you'll be hearing could be that of harps.
A better solution
I'm interested in retro electrical and computer equipment, and I have (I hope) the skills and time needed to get this old machinery working safely. But it's often not easy.
You can buy a vintage turntable advertised as "working" on eBay for, perhaps, £50. But is it worth it?
Quite apart from the safety concerns, unless the vintage appliance has been professional restored and serviced, it's probably a money pit. An old turntable will probably have internal linkages that are gummed up with decades-old grease, that will have to be stripped and replaced. Electrical contacts might be corroded -- if they're not actually loose. Weight-bearing springs will probably have been under compression for so long that they have lost their springiness. Drive belts will be rotten; styluses will be worn. It's easy to get replacement parts for popular vintage turntables, but they can be expensive, and installing them is not always straightforward.
With enough time and money, you can make a vintage turntable as good as new. But, let's face it, if you got it for fifty quid from eBay, it probably wasn't a top-notch design even when it was new.
On the other hand, for about £200 you can buy a perfectly decent moden turntable which is not only likely to be electrically safe, but it will work out-of-the-box. For a little more -- perhaps £250 -- you can buy a modern turntable with built-in pre-amp, removing a bunch of wiring problems. You can even buy turntables that transmit wirelessly using Bluetooth, although I'm not sure I'd want to go that far. In short, using a genuine, vintage turntable creates safety problems, but those aren't the only problems you're likely to encounter. A modern turntable will almost certainly be safer, and in better order, it will probably sound better as well.
Of course, for the vintage vinyl enthusiast, there's no substitute for a turntable of the same era, even considering all the problems it brings.
