Noise Currents Inside Equipment
I’ve said repeatedly that well‑designed equipment using balanced audio connections isn’t affected by circulating ground‑loop currents, but in poorly designed equipment ground‑loop currents can find their way into the audio reference ground to create audible hums and buzzes — hums because the noise current is derived from the 50 or 60 Hz mains supply, and buzzes because of the related harmonics that accrue on mains supplies.
Diagram 6: Grounding inside equipment is also important. Here, despite the star‑grounding arrangement between channels, a loop is created because the input cable shields pass through the audio reference grounds of each channel’s amplifier on their way to that star‑ground point.
Diagrams 6 and 7 illustrate part of a mixing console or audio interface. I’ve shown several transformer‑balanced inputs feeding high‑gain amplifiers (the yellow triangles). The amplifier outputs might feed output transformers (as here), or be mixed together, or be converted to digital — it doesn’t matter for the purposes of this example. What does matter conceptually for this example, though, is that the input transformers convert between the balanced external connections and the unbalanced internal amplifier circuitry, which measures audio signal voltages against the reference audio ground (shown in green). The red loop represents the audio signal current path around the amplifier circuitry, while the blue line shows the external noise current path from the input connection going to earth. We’ve met this situation before...
As you can see in Diagram 6, the reference grounds of each input channel are connected to a ‘star ground’, which is itself earthed via the mains cable’s protective earth connection. However, in this ‘bad’ design, the ground‑loop or RFI currents carried on the input cable shields pass through the audio reference grounds of each channel’s amplifier on their way to that star‑ground point. Consequently, unwanted ground noise is inherently added to the wanted audio signal within each channel. A lot of traditional audio mixers were built in exactly this way, with pin 1 of the channel’s XLR wired straight into the mic preamp’s ground (simply because it was convenient). This has become known as the ‘pin 1 problem’ and much has been written about it in professional audio circles!
Avoiding this catastrophic problem is remarkably easy, as shown in Diagram 7: noise currents on the cable shields simply require a separate route to earth, bypassing the audio channel reference grounds. This is shown in the diagram below, in which the ground‑loop and interference currents from the cable shields at the inputs are kept completely separate from the audio reference grounds, so can’t get into the audio amplifier circuitry at all. This arrangement is typically achieved simply by bonding the shield terminals of the input (and output) connectors directly to the equipment’s metal enclosure, maintaining the Faraday Cage concept discussed earlier.
Diagram 7: Thankfully, curing the problem in Diagram 6 is trivially easy!
So, by using balanced connections and constructing equipment sensibly, noisy ground currents are kept well away from the audio circuitry, and thus ground loops and external interference aren’t problems at all. But what do we do with equipment that isn’t well‑designed, or when using unbalanced connections? In these situations, the best approach is to prevent the noise currents from entering the equipment in the first place, for example by breaking the ground loop.
The protective earth cannot be disconnected in equipment power cables or their mains plugs — such outrageous ‘solutions’ have killed a lot of guitarists over the years!
Clearly, there are several places where a ground loop could be broken — but many of them are fundamentally unsafe! For example, disconnecting the protective earth at the mains wall sockets is obviously not acceptable, since it would defeat a primary safety provision. For the same reason, the protective earth cannot be disconnected in equipment power cables or their mains plugs — such outrageous ‘solutions’ have killed a lot of guitarists over the years!
There are many safe and simple alternatives and the easiest, often provided by the manufacturers of Class I equipment, is a ‘ground‑lift’ switch. Class I equipment normally has a direct connection between the internal audio reference ground, the equipment’s metal chassis, and the mains cable’s protective earth — usually at a star‑ground point. A ground‑lift switch simply disconnects the audio reference ground from that star‑ground, allowing it to ‘float’. In this way noise current trying to get to the protective earth via the audio reference ground (as in the ‘bad’ equipment example above) no longer has anywhere to go — the ground loop is broken. Note, though, that the equipment’s metalwork is still safely earthed via the protective earth connection in the mains cable.
If a ground‑lift switch isn’t available on a Class I device, the next easiest option is to interrupt the audio cable’s shield connection in some way, and this approach also works if the noise currents are due to RFI. (This won’t work for phantom‑powered microphones, as the shield connection forms the 48V power supply’s return path.)
The cable shield connection can be severed simply by opening the connector, cutting the screen connection inside the plug, and isolating it with insulating tape. This only needs to be performed at one end of the cable (usually the destination/input end) and it’s an arrangement often referred to as ‘one‑end‑only’, which was a backbone of much professional studio installation wiring. If you do cut the shield connection, do remember to mark the cable in some clear way, to avoid it being re‑deployed later in a situation where the shield connection is required!
With balanced interconnections, the shield plays no role in transferring the audio signal, so breaking that connection doesn’t affect the audio, just unwanted shield noise currents. The same cannot be said of unbalanced connections, though, and alternative solutions are required for solving ground loops with unbalanced equipment (see below). The one‑end‑only solution is also not applicable with battery‑powered and Class II equipment, as the cable shield is used to share the signal ground reference.
Diagram 8: even if not left permanently in the circuit, isolation transformer boxes such as the ART DTI can be very helpful in diagnosing or ruling out the possible causes of hum.
If you’d rather not hack your cables, a more versatile though more expensive solution is to employ an audio isolation transformer box. The 1:1 transformer passes the audio signal as a varying magnetic field across the transformer’s windings without any direct electrical connections (including the shield) between the box’s input and output sockets. This breaks a ground loop, and prevents RFI currents from getting into the receiving device (Diagram 8). Audio isolation transformer boxes are available in various formats and prices from numerous manufacturers, and although ultra‑low‑distortion transformers are quite expensive, modestly priced options are often perfectly acceptable. For instance, I routinely use ART’s DTI isolation box for identifying and resolving ground loops and related problems. It offers two channels with a plethora of connector formats, making it very convenient and versatile to integrate quickly into any system with balanced or unbalanced connections.
In general, transformer isolation boxes work best when installed at the destination end of a balanced cable run, but at the source end for unbalanced connections (where it can also convert the output to a balanced format, if helpful). Transformers are susceptible to external magnetic fields, of course, so do make sure that the isolation box is kept well away from mains cables and, especially, any mains transformers in nearby equipment — it’s quite irritating if a ground‑loop hum is replaced with magnetically induced hum!
When I suspect a ground loop or shield interference problem I usually employ a transformer isolation box to test my theory. If the unwanted noises go away when the box is inserted, I know they were due to noise current on the shield connections. If practical, I would usually then remove the isolation box and cut the original cable shield(s), marking them accordingly, because it’s a much less expensive solution than permanently installing a transformer box, while also avoiding any possibility of a transformer degrading the sound quality. If the noises don’t go away with the isolator box connected, I know the problem lies elsewhere!
Unbalanced Sources
The transformer isolation box is particularly helpful in resolving ground loops with unbalanced Class I sources (like some vintage synths). But if the unbalanced source is connected to a balanced input (like in a mixer or interface), another option is a ‘pseudo‑balanced’ cable. Unfortunately, and despite their immense usefulness, I’m not aware of any commercial cables wired to this format, but bespoke cable‑making services can construct them to order, and they’re an easy DIY build for anyone familiar with a soldering iron.
A standard balanced cable (two‑core plus shield) is used, with the balanced destination end wired as normal with a male XLR or TRS plug. At the unbalanced (source) end, the hot signal wire is connected to the unbalanced signal output (the tip of a jack plug, for example), while the cold signal wire is connected to the plug’s shield (sleeve) terminal. In this way the source’s signal voltage is passed directly to the balanced input, but there’s no connection between the source’s ground and the destination ground — hence no ground loop. The cable’s shield is connected at the balanced destination end but left isolated (and insulated) at the unbalanced source end.
Many of my synth cables are built in exactly this way, and work perfectly well. However, the unterminated cable shield is effectively acting as an aerial to pick up local interference — if that’s a problem, then better RFI protection can be achieved by connecting the cable shield at the source end’s plug sleeve terminal via a small capacitor. The value isn’t critical; anything under 100nF that fits inside the plug body will be fine.
Alternatively, a low‑value resistor can be used to maintain some RF screening while reducing any ground‑loop currents to inaudibility. 100Ω is a typical value in this application, but again it’s not critical. If you have a large enough connector body, the resistor and capacitor can be used together in parallel.
The correct wiring for a pseudo‑balanced cable.
It’s important to note that a transformer isolation box and/or pseudo‑balanced cable are only appropriate if the unbalanced source is a Class I device. If these options are employed with Class II devices, or those powered via wall‑wart PSUs, they are more likely to create problems than to fix them! This is because such devices rely on receiving a ground reference directly from the destination device via the connecting cable’s shield. Intentionally interrupting that ground connection leaves the source floating and acting as an aerial to collect and inject unwanted RFI.
Plan It: Earth!
So, I’ve described some practical solutions for a variety of ‘grounding problems’ but how should they be deployed? I mentioned earlier the importance having a clear and detailed plan of how the entire system is connected together. I like to sketch it out on paper, with a note on each device identifying whether it’s Class I (safety‑earthed), Class II (double‑insulated), or battery‑powered and, ideally, where each unit is powered from (which wall socket or mains distribution board).
The plan should also show all audio connections between devices (multiple channels between two devices can be shown as a single thick connection for the purposes of ground loops), and specifying which connections are balanced or unbalanced. Include digital audio, digital clocking, USB, FW and TB connections, too. Although it is helpful to include ADAT, optical SPDIF and MIDI connections as part of the system overview, these are all optically isolated and can’t contribute to ground loops or RFI. All the others (bar a few specialist optical alternatives) involve screened cables that are normally grounded at both ends. With a good ‘paper plan’, you can see if the system as a whole is earthed, as well as anticipate likely ground loops and any unbalanced/balanced connection issues.
If fault‑finding an existing system, I start by disconnecting absolutely everything apart from the computer, interface, powered monitors and monitor controller (if there is one). I then play some reference material to set a sensible listening volume and check there are no hums of buzzes. If this minimal rig is noisy and involves Class I devices I’d try an isolation transformer between the interface and monitors. If it’s all Class II, I’d try an earthing plug.
Once this core system is quiet, I add the other equipment on the plan, piece by piece, checking again at every stage and correcting any problem introduced by that device using the techniques described above. With good equipment and balanced connections there probably won’t be any problems at all, but by building the system up slowly, testing devices and connections one by one, it’s usually straightforward to find and solve any issues. In contrast, diving into a complex, fully wired system that hums or buzzes can be a confusing and frustrating nightmare.
Ground‑loop problems will always be worse with unbalanced Class I equipment, so try and plug the mains cables from those devices into the same plug board powering the computer/interface/mixer to minimise the loop area. Well‑designed equipment with balanced connections can be plugged into more distant sockets as they won’t mind circulating ground‑loop currents!
Remember, nobody designs their equipment to hum or buzz, and there’s usually a way of connecting them to be completely noise‑free. It just talks patience, logical thinking and educated experimentation. Good luck!
Terminology
The terms ‘earthing’ and ‘grounding’ are often used interchangeably, which can lead to confusion. In this article, I’m generally using ‘ground’ to refer to the electronic circuitry’s ‘zero Volt’ reference and ‘earth’ when discussing a safety earth connection — the green and yellow striped wire in the mains cable which ‘bonds’ the equipment’s metallic chassis to the incoming mains supply’s ‘protective earth’ (PE). The protective earth connection is essential to the safety of some types of equipment (see the ‘Classy Equipment’ box) and ensures that, in the event of a serious internal fault, the mains power to that unit is disconnected promptly by a fuse or breaker. Regulations vary in different countries, but most require automatic ‘breakers’ to switch off the mains power supply if the current draw is excessive and/or if the currents on the Line and Neutral feeds are dissimilar — implying current is being diverted to ground somewhere (possibly via a human!). These automatic breakers can be fuses but are more often resettable mechanical switches known by initials such as RCBO, RCD, RCCB or GFCI.
Classy Equipment
The identifying symbol for a Class I device.Most studio equipment is mains‑powered, but there are different ‘classes’ of mains powering, which determine how grounding issues can be resolved.
Traditional devices with internal mains power supplies are built into a metal chassis connected to the protective earth via the mains cable. Products built this way are called Class I devices, and include most desktop computers, power amplifiers, large mixing consoles, professional tape recorders, vintage outboard equipment, and so on. They can be usually identified by an ‘earthed’ symbol on the identification plate near the mains inlet.
The electrical safety of a Class I product relies on two things: insulation around the live conductors, and exposed conductive parts being connected to a protective earth. If the insulation breaks down AND the live conductor makes contact with exposed metalwork, a very large current flows directly from the Line (Live) connection to the protective earth, instantly activating legally‑required protection devices within the mains supply (fuse, RCBO, RCD, RCCB, GFCI etc).
An IEC C14 connector.Consequently, all Class I products have a three‑terminal power connector — most commonly the IEC C14 inlet connector — and they MUST be connected with a three‑core mains cable from a three‑terminal wall outlet to maintain the protective earth connection. NEVER disconnect or circumvent the protective earth; it’s extremely unsafe, quite unhelpful in resolving grounding issues, may be unlawful, and it could kill you in the event of a fault!
The symbol for a Class II device.An increasingly common alternative arrangement is called, logically enough, Class II or ‘double insulated’, and is identified by a ‘box‑within‑a‑box’ symbol.
An IEC C8 connector.Class II equipment includes many musical keyboards, outboard processors, some active loudspeakers, hi‑fi equipment, and much more besides. Electrical safety is ensured in a Class II product by using two independent forms of insulation around live components so that no single fault can result in the user becoming exposed to the Line voltage via exposed conductive parts. Consequently, Class II devices don’t need a protective earth connection and therefore normally use a two‑core cable and two‑pin connector such as the IEC C8 (fig‑8) or C18 style IEC inlets.
An IEC C18 connector.A subset of Class II is identified as Class IIFE, where ‘FE’ stands for ‘Functional Earth’. These devices fully comply with the double‑insulation requirements of Class II and are marked with the same box‑within‑a‑box symbol. However, they have a three‑core mains connection — most commonly using the IEC C6 (Mickey Mouse or cloverleaf) inlet.
An IEC C6 ‘cloverleaf’ connector.This Functional Earth is connected via a three‑core cable to the wall outlet, as with Class I, but the Functional Earth’s role is not to provide shock protection in fault conditions (which can’t occur because of the double insulation). Instead, the Functional Earth helps mitigate electromagnetic noise and interference by providing a direct path to Earth from Faraday shielding. Class IIFE devices include some laptop power supplies and lots of computer‑related and digital audio devices.
Class III devices are powered (or charged) via an external low‑voltage mains adaptor (‘wall‑warts’ and ‘line lumps’), such as laptops, some effects units, keyboards and so on — anything with a low‑voltage power inlet, basically. However, the salient point about these devices is that their mains power units and chargers are always Class II or Class IIFE, so we can consider them as being part of that category when resolving grounding issues.
How To Wire XLRs
Linking pin 1 of an XLR to the shell via a small capacitor ensures effective grounding at RF, while also preventing ground loops between pin 1 and the case/chassis.
XLRs are commonly employed for balanced audio connections, with the audio signal normally passed over pins 2 (hot) and 3 (cold). Of more interest in the context of this article is the cable shield, which is wired to pin 1 (except in one‑end‑only cables where it is isolated at the destination end). Some commercial cables link pin 1 to the connector shell. While this makes no difference with modern equipment, it can cause issues in badly designed equipment since it joins the chassis (via the shell) to the audio reference ground (via pin 1), potentially creating a ground loop. So, as a rule — and especially if working with legacy or poorly designed equipment — it’s best not to link the shell to pin 1. Some argue this omission impacts RF screening at the connection if two XLR cables are joined together (since their shells are floating). While that is technically true, leaving the shells floating avoids the potential risk if they touch some other conductive surface and pick up hostile ground currents! If RF screening is a concern and XLR cables are routinely linked together, consider fitting Neutrik EMC‑series XLR connectors (see below) instead of standard XLRs.
However, in some situations linking the shell to pin 1 can be very helpful, such as when using some vintage capacitor microphones. There was far less RF floating around in the ’50s and ’60s than there is today, and mics from that era often lack the RFI protection that modern designs include as standard. In some, the microphone’s metal case is connected to the XLR pin 1 via a short wire, which, at radio frequencies, has significant inductance greatly reducing the effectiveness of the Faraday Cage screening. By linking the cable shield to the connector shell (and thus directly to the mic’s case), the shielding can be improved dramatically and RFI greatly reduced.
A better technical solution might be to link pin 1 to the shell in the cable XLR via a small capacitor, which would ensure effective grounding at RF, while also preventing ground loops between pin 1 and the case/chassis. Better still, Neutrik’s EMC‑series range of XLRs incorporate a ring of SMD capacitors linking the shell to pin 1 to significantly enhance its RFI screening properties. Although these are more expensive than standard XLRs, they are a very good solution where RF is expected to be a problem.