Ben Duncan offers a firm grounding on the abolition of the GLC (Ground Loop Connections).

Let's begin by identifying two distinct reasons for having 'earth' or ground connections in the first place. One is to do with defining a common point, or 'reference' between your equipment necessary to make it work. The other isn't essential to the equipment's operation, but is all about safety. Good grounding practice is little more than keeping the signals in each ground-path separated physically as well as conceptually. On this tack, you might find it helpful to use the word ground for common connections in general, keeping earth as an alternative term for the occasions where actual soil is involved, namely when dealing with the mains safety connection, the green/yellow wire.

Types Of Ground

Signal ground is the earth or screened side of your cables, when they're unbalanced. In transformer balanced systems, there may be no through signal ground connection (ie. between equipment), but some transformer-balanced set-ups, treat the cable screen as the signal ground, as do actively balanced inputs and outputs. And in all systems (balanced or not), signal ground is the common or '0V' (zero volts) connection to the circuitry inside the enclosure.

Mains ground is the stake outside the green/yellow wire on a 3 core mains lead, and the big pin on your mains plug (UK only). Professional equipment is generally enclosed in aluminium and steel, and this is normally connected to mains ground, viz 'earthed', for safety. Another method, beloved by 'consumer' equipment manufacturers is 'double insulated' construction. Originally, this was meant to relieve manufacturers of hairdriers and such like (ie. goods in plastic cases), from the legal requirement to provide a mains ground connection, enabling them to cut costs.

To spot this category of gear then, you need only look out for a two core mains lead. Oddly, the 'double insulated' category now appears to extend beyond bona fide plastic enclosures; many oriental tape machines with acres of exposed aluminium can be seen sporting 2 core mains leads.

Hum-Loop

When two currents flowing in a ground (ie. common) connector are vastly different in size, the bigger one modulates the smaller, and if we get our signal and mains grounding paths mixed up, this is exactly what happens. Mains related 'hash' impinges on our music. Hum caused by loops varies between a 'rasping hum' and an 'edgy buzz', depending on how 'filthy' your mains supply is.

Hands up anyone who's spent hours tracking down a phantom hum? After unplugging every lead, the hum turned out to be incurable, because it wasn't caused by a hum-loop in the first place! Valve gear usually hums of its own accord, and power supply faults in any equipment or naff connections can give rise to a variety of hums and buzzes. Beware also of acoustic hum from badly made transformers, especially if the offending equipment is next to the monitors. Putting your ear right against the speaker will confirm that the hum isn't acoustic in origin.

Returning to hum-loop hum, whenever you track down a loop, listen to the characteristic harmonic structure, and try to learn to recognise it by ear alone - ground loop buzzes have a distinctive sound, and knowing what to listen for may help if you encounter problems at a future date, especially in a situation where there's no time to be spared.

Now to prevent hum-loops, we'd ideally like to keep both types of ground electricity separated (Figure 1). But electronics technology abhors a vacuum, and in the next section, we'll see two reasons why they should be connected; one good, the other a nuisance. Why? Well it's handy to know that whilst one low impedance (ie. direct) link between the two grounds is hunky-dory, any additional connections will set up the ideal conditions for a hum-loop. Our problem then, is to take control of connections between the grounds and place one link in sole command.

Two Earths Collide

Ideally our equipment's circuitry shouldn't need to be tied to mains ground. It's isolated from the mains power and the absence of a mains earth connection won't prevent it from working (think of portable, battery-operated gear, Walkman to you). Trouble is, the mains transformer that's supposed to isolate electronics from the mains has imperfections, in particular interwinding capacitance. It's rather as if a very small value capacitor were connected between live mains and signal ground. With a low power transformer (ie. gear other than power amps) the current passed by this capacitance is small, circa 10μA, and just enough to transmit a mild tingling sensation if you've got sweaty paws. At least, it tickles if the electronics inside the box 'floats'; in other words, if it's not tied to the mains ground (Figure 3).

The level of leakage current available when a lone item of equipment floats isn't particularly dangerous, although it wouldn't be pleasant if you were 20 feet up scaffolding. But if we hook together the usual home studio's complement of gear, lo and behold, the capacitances multiply. Pretty soon, the sum of our interwinding leakage currents adds up to produce a fairly nasty bite. The remedy, of course, is to connect the signal ground to the safety electrical earth. This swamps out the leakage, so you can touch signal ground - or indeed, any part of the audio electronics - in complete safety.

Moving over to the enclosure per se, with the notable exception of oriental cassette machines (which masquerade as 'doubly-insulated') the majority of equipment with an extensive area of exposed metalwork has this tied to the mains earth for safety. The head-scratching begins when we encounter a processor (say) where the signal ground and the metalwork are tied together, or worse, are one and the same.

Examples are when metal-bodied jacks are mounted directly on the metalwork, or when the area of copper on the PCBs dedicated to signal ground is screwed down in contact with the chassis. These practices are especially naughty when the system employs unbalanced connections. You can test for a suspected connection beween signal and mains ground by looking for a low ohms reading with a meter going between the signal ground terminal on any other connectors (ie. where the cable's screen would be connected), and the earth pin on the mains plug. Do this with the gear unplugged of course!

If you get a low reading - below 40 ohms (some meters won't accurately read this low), you'll need to sort out the grounding inside. Above 40 ohms and up to 1k or a little higher, indicates that the processor (or whatever) has been groundlifted. Higher - or infinite - resistance readings mean that there's no immediate hum problem, but you might want to add groundlift. We'll come to this in a moment.

Grounding Point

The first stage to proper grounding relationships is to decide on the nerve centre of your system. Normally it's the mixing console, but the choice is yours. Anyhow, this is the ONLY point where we tie the mains earth to signal ground - at least directly. Let's call it the central grounding point (Figure 2).

If your test meter shows no such connection (ie. an infinite reading, see above), you'll have to make a simple internal modification, taking a wire from the chassis (or at least, from the mains earth) to a suitable signal ground point, typically the 'dirty' 0V busbar (Figure 3). Ask a boffin if you need help identifying the correct connections.

For the remainder of the equipment, we need to ensure that the opposite is true. Essentially, this means checking out with an ohmeter, seeking out direct 'mains ground' to 'signal ground' connections, and removing them wherever possible. However, gear which might be used on its own, and therefore without the benefit of the mixing console's central ground connection, shouldn't be wholly isolated in this way. Instead, we groundlift it. Rather than separate the two grounds 100%, we link them together via a resistor. Any value under 10kohms will swamp out the leakage interwinding current, but better safe than sorry. So we'll pick a much lower value to be double safe. "But surely, hum-loops..." NO! The hum arises out of large currents flowing in the loop. With a groundlift, we're essentially placing a resistor in series with the loop. And any resistor in excess of 40 ohms or so will reduce the current level to such an extent that it's effect is no longer audible. But careful: if ten equipments have 40 ohm groundlifts, the overall loop resistance falls to 40/10 = 4 ohms. This is because the groundlift resistors appear in parallel when all the gear is connected together. With this in mind, a 1k, ½ watt groundlift resistor has become something of an industry standard (Figure 2). This lets us connect large quantities of equipment without risk of a low impedance developing.

Groundlifting will only normally apply to units with a 3 core mains cable. In theory, anything double-insulated (ie. with a two core mains lead) shouldn't exhibit any significant leakage reactance. We hope so, because there's no mains ground to tie down to!

Back to the 'lifting' as it's known. The resistor is normally connected between the chassis or enclosure, and the signal ground appearing on an adjacent input connector - or any other convenient point. You can also wire a choke in series while you're at it. A 10μH inductor will preserve the 50/60Hz groundlift impedance, but decouple your audio ground from the whine of electric drills and other RF trash appearing on the mains ground. It's particularly worthwhile if your setup is already beset by RF (radio) interference nasties. The choke should be rated at 1Amp; a suitable part is available from RS Components' suppliers.

As time goes by, an increasing number of leading UK manufacturers are making groundlifting and ground isolation easy. One of the first was Chas Brooke: his BSS crossovers have featured a groundlift switch (plus dinky green LED!) since 1978. Other manufacturers offering similar contrivances are Klark-Teknik and HH. There's either a switch, or a link-bar at which two grounds are tied - and can be untied. With the link bar, you simply screw your resistor - with or without choke - across the terminals. These facilities are hardly an earth-shattering high-tech development... but let's hope that more manufacturers adopt these features in future, because if they did, correct grounding would be made a lot easier for everyone.

Defects

The next stage is to sort out any units (other than the mixing console of course) with defective standards. For clarity, we'll refer to it as bonded-ground gear. As I implied above, enemy number one in this class is the cassette machine or amp with a direct connection between the case and the signal ground. This sorry situation frequently relates to uninsulated panel connectors, particularly jacks and phono sockets.

Sometimes we can remedy this by putting insulated connectors in their place, but often this sort of modification isn't feasible, for a variety of reasons. Even if you're not borrowing or hiring the gear, the scale of the modifications required may be in the major surgery league. For instance, the signal ground may be bonded to the case at dozens of places on the PCB, not to mention the control pot brackets. I have had to hack away large chunks of PCB track on more than a few dbx 165s in my time! The best way to do this is with a very hot soldering iron, and a very sharp chisel. It's a job for the confident though; if my chisel had slipped, I would have had to explain to the band's manager why I'd destroyed all their compressor/limiters..!

So if you're not up to modifying equipment bought from a shop, we suggest you ask the supplier, dealer, manufacturer or importer if they can make the necessary grounding mods. If you request this at the time of purchase, the charge should be small, because you can truthfully insist that the work is necessary for you to use the equipment properly ie. without him. In fact, there's probably a good case for having the modification done on the house, because without it, the offending article isn't "fit for its intended purpose", namely high quality audio. But I'll leave you to argue about the Sale of Goods Act in the county court...

Bonded-Ground

We'll finish this section by highlighting the problems that bonded-grounds entail:

1) With the signal ground linked to chassis, any attempt to connect the case to mains earth (for safety) will set up a hum-loop (because we've already made the sole direct link at the console).

2) And as soon as we mount-up the unit in a rack, the metal fixing strip will bring the offending article into electrical contact with the other equipment enclosures, some with metalwork which is tied directly (and quite legitimately) to the mains earth (Figure 4). This has elements of Catch 22, of course: the very connection we're trying to avoid is inherent in the mechanics of the situation.

Alternative Solutions

Let's assume that the actual physical amendments to the grounding discussed in the preceding section aren't practical. Now if you don't use a rack, problem no. 2 (above) is easily remedied by keeping incompatible enclosures separated. Of course, you'll want to stack up your gear, and this is okay so long as sensitive equipment is insulated from adjacent units with rubber feet or any suitable insulating material, in the past, I have used scraps of cork tile for this purpose, as it also damps down tinny lids which are apt to rattle in response to loud bass notes.

To solve the no. 1 objection, you'll also have to disconnect the mains earth cable. This is strictly very naughty, but there are mitigating circumstances. First, most of the gear we're discussing at this point has a two-core mains lead. But even if not, and we end up unhooking the green/yellow wire at the plug, safety can be maintained, providing you fit a 30 milli Amp current-balance circuit-breaker in line with your master mains socket.

If you do decide to disconnect any mains earth connections, be sure to chop back the redundant wire to the cable sheath, so it can't float around and touch anything nasty. Also, mark the plug accordingly: U/G in big red letters will remind you that it's UNGROUNDED.

Returning to the trail, what happens to our rack-mounted gear? Well, here, we need to keep units with bonded-grounds isolated from the rack strips (these latter are tied to mains ground in the usual fashion). To isolate, removable metal rack-tabs can be replaced by plastic members. These are often available as accessories from Pro-audio and Hi-Fi dealers, and can be readily adapted to fit equipment other than whatever they're aimed at specifically. All you need do is drill out 4 fixing holes in the enclosure's side panels.

Next month, we'll look at an altogether different way around hum-loop elimination - that of balancing.