Many consumer HIFI equipment and other audio signal sources offer only unbalanced outputs. Unbalanced interconnections pick up easily all kinds of noise (especially ground loop noise), so you might want to avoid them everywhere you can.

You can convert the simple unbalanced preamp output to balanced circuit with one of the  following tricks:
- Use a DI box to convert unbalanced signal to balanced microphone level signal
- An audio transformer is a classic way to convert unbalanced to balanced
- Balanced opamp output circuit can convert unbalanced to balanced (more modern approach but more components)

In addition to those there is not so widely mentioned impedance-balanced output option:

1. Figure out the output impedance of your unbalanced signal source. Usually looking at the circuit diagram of the device will tell you that easily. If you don’t have that, you can always measure the output impedance.
2. Pick a resistor that has same resistance as the output impedance of your unbalanced output (as close as possible… preferably within 1% accuracy).
3. Wire the unbalanced output signal to XLR pin 2 (+).
4. Wire ground to XLR pin 1 (ground).
5. Wire that resistor you just selected between XLR pins3 (-) and pin 1 (ground).

Now you have a impedance-balanced output. It is not exactly as good as a real balanced output, but performs pretty close a real balanced output in normal applications. You can use the same idea also with 6.3 mm jacks: signal goes to tip and the resistor to ring. An impedance balanced output with 6.3 mm jacks works as well as an unbalanced output if that is what is needed (just plug in a cable with mono plug).

Impedance-balanced principle has been used some professional electret mics and on outputs of some “budget” mixers! Just by adding one resistor an unbalanced output is converted to impedance balanced output that works very well with all equipment that has balanced inputs.

More information on line balancing and theory can be found at great The Self Site Balanced Line Technology document.

The first video and the picture below (from that video) shows a downward-propagating negatively charged, stepped leader. The lightning branches out in many different directions, causing one leader to make a connection with the ground, creating a bright return stroke.

After watching the videos it is a good idea to also read how to protect buildings and electronics against lightning damages. K1TTT on grounding and lightning protection highlights the need for single point grounding system. Check also surge suppression links and documents on ePanorama.net.

Differential video amplifiers have a limitation on their input voltage range which gives some limitations how much common mode signal those circuits can tolerate. If the ground potential difference is more than few volts, then operational amplifier based isolators don’t work effectively. Too high voltage difference can cause problems from very distorted video signal to damaged differential video amplifier. If the voltage difference is a substantial proportion of the DC supply voltage of the amplifier, you will probably have trouble using an amplifier alone.

It is a good idea to measure the voltage difference before using differential video amplifiers to be sure not to damage them. Measuring can be done using a multi-meter (check using both AC and DC ranges) or better using a scope earthed to the mains supply, and put the probe on the earth connection of the incoming video cable. If you many potential difference which are many volts, then you have quite probably something wrong in the grounding of the building and you should consult a qualified electrician to check and correct this potentially a dangerous problem

Image source: http://www.edn.com/archives/1997/050897/10di_06.htm#Figure%201

Look also: http://pdf1.alldatasheet.co.kr/datasheet-pdf/view/136144/MAXIM/MAX9546.html

National Instruments tutorial Ground Loops and Returns teaches you to select the right configuration to use. Here is the material from the document related to ground loop problems in condensed format:

A grounded signal source is one in which the voltage signals are referenced to a system ground, such as earth or building ground. The most common examples of grounded signal sources are devices, such as power supplies, oscilloscopes, and signal generators that plug into the building ground through a wall outlet. The difference in ground potential between two instruments connected to the same building ground system is typically 10mV to 200mV, or even more (up to several volts at normal use and tens of volts during short circuit surges).

Single ended is the “default” configuration for most data acquisition devices, modular instruments, and stand-alone devices. Single-ended systems are very susceptible to ground loops. There are essentially two main types of Single-Ended measurement systems: Ground Referenced Single Ended (GRSE) and Non-Referenced Single-Ended (NRSE).

An ideal differential measurement system reads only the potential difference between the positive and negative terminals of the amplifier and thus it completely rejects common-mode voltages. However, practical devices are limited in their ability to reject common-mode voltage.

A grounded signal source is best measured with a differential or non-referenced measurement system.

The pitfall of using a ground-referenced measurement system to measure a grounded signal source is that grounding potential difference between signal source and measuring system causes a current called ground loop current to flow in the interconnection which can greatly affect measurements causing offset errors, especially when measuring low level signals from sensors.

Image source: http://zone.ni.com/devzone/cda/tut/p/id/3394

Although the instrumentation amplifier is usually shown schematically identical to a standard op-amp, the electronic instrumentation amp is almost always internally composed of 3 op-amps. The three-op-amp instrumentation amplifier is seemingly a simple configuration in that it uses a basic, decades-old operational amplifier to gain a differential input signal. Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are also available in integrated circuit form from several manufacturers.

EDN Magazine article Understanding CMR and instrumentation amplifiers tells that from the CMR (common-mode rejection) perspective, instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. his situation is not so mysterious when you think about the inside of this device. And the inside operation of the instrumentation amplifier is well presented on this article.

Earth-ground symbol represents a real connection to earth. That connection could be for example 10-foot-high copper-clad steel bar driven into the earth (at your premises or nearby provided by power company) or metal water lines. That earth ground is wired to the neutral of your house wiring at the breaker panel. You can reasonably use the earth-ground symbol for the ground pin on the electrical outlet. Your electronics equipment schematic should use the earth-ground symbol to indicate connection to the electrical outlet ground pin on equipment plugged to wall outlet.

It is bad practice to use earth-ground symbols for chassis common. You an use the chassis common wherever a power supply or circuit card connects to the chassis. In a circuit board schematic you can use chassis symbol when a standoff screws the PCB to the chassis.

Signal-ground symbols are most suitable for most circuitry on a PCB. A design can have several of these symbols, with notations to identify them.

Information source: EDN magazine article Draw the line: Isolation shields systems from shocking surprises

ELFA catalogue product 56-550-55 “PE65612 Trafo dig.siirt.” is a tranformer suitable for isolating S/PDIF digital audio signal. The manufacturer for this product is Pulse Engineering and their product code is PE-65612.

Ratio: 1:1
Bandwidth: 100 kHz-55 MHz ±3 dB

Here is a picture of the S/PDIF signal isoltor I built (box open):

More information on S/PDIF and related circuits can be found on my SPDIF document.

Disconnecting and then reconnecting the cables take a lot of work. This caused that ground loops are frustrating to troubleshoot in large systems where there are lots of cables.

I have found that a clamp type multimeter can help to troubleshoot ground loop problems. The ground loop noise is normally caused by the extra noise current flowing on the shields and ground wires of the cables. That noise current is normally mains voltage frequency (50 Hz / 60Hz) or it’s harmonics. Normally the signal cables should not carry any (or very little) mains frequency current in them, so by measuring this kind of current flowing on the cable it is possible to determine where the noise current flows. A clamp type multimeter is a very good tool for doing the measurements, because you can easily measure the noise current flowing on the cable with it without need to disconnect the cable or disturb the signal inside the cable. This means that you can troubleshoot a live system with clamp multimeter in AC range.

The wires which have considerable current on them are part of ground loop. The wires with most current on them are pushing most noise current to the whole system. So first locate the wires that have highest or otherwise very high current flowing in them. Then you can try to disconnect them and test if that stops the noise. Usually there is one or few cables that cause all or most ground loop current on the system. That noise current gets typically flowing around in different cables on the system, causing more or less noise problem here and there around the system. Then the real noise source or sources are disconnected, suddenly the whole system becomes noise free. When you have found out the problem source then just add suitable cure to that connection (typically signal isolation transformer or similar device).

Clamp on multimeter allows you to easily measure the current on cables. Just clamp the meter over the audio cable and get the AC current reading. If you want you can clamp several audio/video cables inside the clamp and get the reading of the sum of their noise currents (remember that there is possibility that if there are two cables with exactly same noise signal but different direction you get zero reading). Clamping the meter around a number of signal cables speeds up the troubleshooting process where there are lots of cables, for example near audio mixer. If the group of cables you measured with clamp meter shows a considerable noise current, then measure the cables individually to see which one has the most current flowing. If there was no considerable current on the cable group, continue measuring next cable group. Besides audio cables you can do the measurement with video cables, mains power cables and other signal cables.

There are few things to consider the selection of the clamp type multimeter. First the multimeter needs to measure the AC currents with the clamp. You don’t need the DC current measuring capability, although getting a clamp meter with also DC capability can make the meter more useful for other applications (usually the DC capable clamp meters are more expensive than AC only). The second thing to consider is the resolution of the meter. The ground loop currents you normally want to measure are in few mA to 1A range (in some severe case the current can be considerably more). It is preferred to have a clamp meter that can measure currents down to few mA. Unfortunately many meters with this good resolution are usually quite expensive.

Usually the cheap clamp type multimeters have 10 mA or 100 mA resolution, meaning that they can’t detect anything lower than 10 mA or 100 mA. A multimeter with 100 mA resolution is practically useless in ground loop problem solving, because over 100 mA ground loop current are not seen often. A multimeter with 10 mA resolution is already useful to troubleshoot ground loop problems, but it will not reveal you all the details in most cases. Usually 100 mA-1A current on cable means very serious noise problem on audio and video systems, currents in 10-100 mA range cause some noise problems. Usually when the current is well below 10 mA there are no considerable noise problems.

I have used a clamp meter with 10 mA successfully for troubleshooting ground loop problems, but when used that I wished I had a meter that can show even lower currents down to 1 mA or less. So if you are buying a clamp meter, consider trying to get as good resolution as possible with the money you are willing to spend it. When looking for multimeter for this application the actual measuring accuracy (measurement error percents promised) is not important, we are merely making checks if there is current flowing or not and approximately how much (just some approximation around how much current is enough).

This picture shows the cheapest I know well working clamp multimeter that can measure currents AC down to few mA currents. The meter has 1 mA resolution at 2A measurement range (the display started showing current higher than 2 mA). You can get this 1.3″ LCD Clamp Style Digital Multimeter with Pouch from Dealextreme for around 20 US dollars.