Sound system interconnections and grounding

Rane Sound System Interconnection note,¬†originally written in 1985, continues to be a very useful reference. It’s popularity stems from the continual and perpetual difficulty of hooking up audio equipment without suffering through all sorts of bizarre noises, hums, buzzes, whistles, etc.– not to mention the extreme financial, physical and psychological price. Many things have improved in the audio industry since 1985, but unfortunately wiring of analog signals isn’t one that has progressed much.

In 2005 Audio Engineering Society (AES) issued a standards document for interconnection of pro audio equipment in AES48 standard: “AES48-2005: AES standard on interconnections — Grounding and EMC practices — Shields of connectors in audio equipment containing active circuitry.” The standard was reaffirmed in 2010 and stabilized in 2015. This standard specifies requirements for the termination, within audio equipment, of the shields of cables supporting interconnections with other equipment, taking into account measures commonly necessary for the preservation of EMC (electromagnetic compatibility) at both audio and radio frequencies. The shielding (or screening) of audio equipment, cables, and microphones can be critical for EMC. The improper connection of these shields can cause common-impedance coupling in equipment.

Somewhat related standard is CENELEC – EN 61938: MULTIMEDIA SYSTEMS – GUIDE TO THE RECOMMENDED CHARACTERISTICS OF ANALOGUE INTERFACES TO ACHIEVE INTEROPERABILITY. EN61938 is an International Standard that gives guidance on current practice for the characteristics of multimedia analogue interfaces to achieve interoperability between equipment from different manufacturers. For more details check EN61938 standard index overview.

IEC 60958 is standard for the interconnection of digital signals. The AES3 standard parallels part 4 of the international standard IEC 60958. An AES3 signal can carry two channels of PCM audio over several transmission media including balanced lines, unbalanced lines, and optical fiber. AES3 has been incorporated into the International Electrotechnical Commission‘s standard IEC 60958, and is available in a consumer-grade variant known as S/PDIF.

Read also Shields, grounds and microphone cables (pdf) document that describes the complexities of shielding and balanced cabling explained by one of the acknowledged masters of the subject Bill Whitlock.

 

16 Comments

  1. Tomi Engdahl says:

    Audio RF interference and ground loops
    https://www.youtube.com/watch?v=ezBLZjwpops

    A video tutorial on the causes and remedies of radio frequency interference and ground loop hum in the project and home recording studio.

    Reply
  2. Tomi Engdahl says:

    AES made this standard
    https://www.aes.org/publications/standards/search.cfm?docID=44

    the cable shield of all balanced audio connections is tied to the chasis immediately at the connector;
    all grounds (i.e. earth shunt, PSU ground & audio grounds) are tied together and to the chassis at a single point (star ground scheme).

    Reply
  3. Tomi Engdahl says:

    HOW DOES CLAMP ON EARTH TESTER WORK ESS
    https://www.youtube.com/watch?v=4LdTEE7FB-A

    stake-less method for earth resistance measurement.

    Amazing. First clamper that i ever seen to measure the resistance.

    Reply
  4. Tomi Engdahl says:

    If you plugged your mixer in a building outlet and your sound equipment in 100 feet away at the stage, then the building wiring itself would function as an antenna, picking up EMF noise, RF noise, and any leakage currents occurring in the building. Although usually only fractions of a volt, these would be superimposed on your audio lines and amplified through your system. Conversely, if you ran a power cable from your stage rig back to your mixer, then you would be grounded at a single point, your stage rack. This is what is being referred to when audio guys talk about single point ground.

    IG does suffer from some of the same problems, but you eliminate other equipment in the building from superimposing noise on your ground line, provided the ground conductors are each “home run” to the panel. IG and “single point ground” are two different concepts. In commercial buildings, the conduit and building frame become paths that can introduce noise into the ground conductor as the receptacles bridge the ground to the mounting screws. IG receptacles do not connect the ground lug to the mounting screws.

    I guess I’m just having a hard time seeing how even with voltage drop you are getting multiple voltages considering faraday’s law. Unless of course the buildings ground system is just poorly installed to begin with

    That would be a valid situation. I guess I’m coming at the problem from a construction perspective of how to create the most quiet audio room

    Well, there should be no voltage drop on the ground conductor as it is not a current carrying conductor, it is just there for safety. However, ground leakage is a fact of life so you end up with noise in the ground circuit, in addition to any noise that may be induced by induction. Again, these are fractions of a volt, but when these small voltages end up superimposed on your audio lines they get amplified. Most often this occurs because the mixer is grounded at a different location then your amp rack grounded.

    I 100% agree stray voltage happens on the egc.

    However, still can’t wrap my mind around how the voltages could be different, even by a fraction. Assume bonding terminations are correctly installed. All grounds are bonded at the first means of disconnect. This makes them essentially the same wire now

    One wire has one voltage, no matter which two parts of it I put my meter long. It could be a wire a light year long. If I get 10v at one point, I’ll get 10v at any other point

    Once again, I’m thinking about this not as someone dealing with poorly constructed electrical systems. I’m saying if there was none of that to overcome, would these things help?

    Michael Gilleran The point is that your wires are not perfect conductors – far (in terms of the currents and voltages in question) from it. If you have 10mA of current (from equipment EMI filter capacitors) through an earth conductor with 0.1 ohms of impedance, you get 0.001V, which doesn’t sound a lot. But if full audio power is 1V, then your ground voltage is at -60dB signal level – which is going to be obvious as hum if the volume is turned up. It will certainly be well above the digital noise floor.
    Then at high frequencies you get inductance – so your meter really does read different voltages at different points on a wire (ask any antenna designer!).

    That said, the solution should be balanced signal wiring and differential inputs (so that the fact that the equipment disagrees about ground doesn’t matter, and the screen of the cables doesn’t carry audio signal at all). This solves most problems because the equipment can bounce around on the grounds as much as it likes, and the signal still flow cleanly across the divide. Digital differential inputs are much cheaper than analogue ones and so nearly universal (even a SPDIF connector has a transformer behind it).

    John Dziel they aren’t talking about the wires on the same circuit, they are talking about separate circuits, some of which might take vastly different paths including separate sub panels and the like. It is then possible to have slight impedeence mismatches on the two circuits which can lead to current flow to the path of least resistance: the ground.

    John Dziel, that sounds nasty! Of course, the main reason for independent distribution, which we all know is the primary method for arena shows. Unfortunately most mid size shows and venues don’t have the budget for such luxury and hence all the buzzing, ground lifts and iso transformers.

    Requirements for audio:
    Voltage should be between 108 and 126 volts.
    Everything should be grounded, on the same ground
    Sufficient amperage should be provided for the gear being used. Some amplifiers use 20amp plugs. Some ampracks use L21-30 connectors. Some audio companies bring in their own power distribution boxes that often take cam locks.

    Do they have one amp on one side of the house and one on the other that are cross connected with signal wire (not speaker wire or video or network) ? If not, then one or two standard 15-20 amp circuits are all they need. If so, then they might benifit from an isolation transformer. The difference in length of ground wire is what causes inducted noise, unless you are looking at a ground loop. For a ground loop situation, that is mostly solved with line level transformers on the signal side, not on the power side.

    Remember that signal wires are very low voltage and amperage, so much more susceptible to RF. That is why they are shielded. The amount of interference that can be ignored as below the noise floor is much lower in signal wire than in mains. Look at ham radio antennas, for instance. They are at their heart a piece of wire, that is set up in such a way as to induce a signal from very very low levels of signal and then filter out the noise. Much (maybe most) of analog signal management is aimed at keeping your wires from behaving like an antenna.

    Venues with IG have caused me less headaches than those without.

    Perhaps more about not having lots of other equipment with functional earth’s causing transient interference? Become less of an issue these days for ground loop humm as more and more audio runs are digital anyway.

    Isolated ground is always nice if you can get it.

    Line power can be very dirty (especially from large pumps and fans). A conditioner does wonders.

    Reply
  5. Tomi Engdahl says:

    The problem comes when the grounding conductor carries energy. It’s never supposed to but in certain cases or if something down line is wired wrong the grounding conductor will carry a current as a neutral and if sensitive audio equipment is tied in along that path the harmonics in the line may cause that 60hz buzz we all dread. Isolating the ground all the way back to the building bond point will ensure that doesn’t happen.

    Reply
  6. Tomi Engdahl says:

    If you plugged your mixer in a building outlet and your sound equipment in 100 feet away at the stage, then the building wiring itself would function as an antenna, picking up EMF noise, RF noise, and any leakage currents occurring in the building. Although usually only fractions of a volt, these would be superimposed on your audio lines and amplified through your system. Conversely, if you ran a power cable from your stage rig back to your mixer, then you would be grounded at a single point, your stage rack. This is what is being referred to when audio guys talk about single point ground.

    Reply
  7. Tomi Engdahl says:

    Ground loops can be an issue. Single point grounding usually solves that. Beyond that, as long as the equipment has a well designed power supply, and is supplied the proper voltage and sufficient current, there should be no issue with the sound. As for “audio grade” outlets…..

    Reply
  8. Tomi Engdahl says:

    The point is that your wires are not perfect conductors – far (in terms of the currents and voltages in question) from it. If you have 10mA of current (from equipment EMI filter capacitors) through an earth conductor with 0.1 ohms of impedance, you get 0.001V, which doesn’t sound a lot. But if full audio power is 1V, then your ground voltage is at -60dB signal level – which is going to be obvious as hum if the volume is turned up. It will certainly be well above the digital noise floor.
    Then at high frequencies you get inductance – so your meter really does read different voltages at different points on a wire (ask any antenna designer!).

    That said, the solution should be balanced signal wiring and differential inputs (so that the fact that the equipment disagrees about ground doesn’t matter, and the screen of the cables doesn’t carry audio signal at all). This solves most problems because the equipment can bounce around on the grounds as much as it likes, and the signal still flow cleanly across the divide. Digital differential inputs are much cheaper than analogue ones and so nearly universal (even a SPDIF connector has a transformer behind it).

    Reply

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