Rupert bounced me an email from the Avid L requesting a bit of info on tech mains, balanced/unbalanced audio etc. Here is the thread;

I've been having an ongoing discussion with a co-worker this last week about audio wiring. More specifically about lifting the ground. To define, it's when you don't connect the ground wire in a balanced, hot/cold/ground, connection. Now I personally have firm opinions on wiring based on what I have been taught and what has worked for me for years, but I am no expert by any leniency of definition. And I thought it would be informative for all on the list to hear a discussion on some of these issues so that they can evaluate what work is done when they hire an installer. I'm going to ask a few questions that will hopefully not display my personal opinions but open the topic up for discussion. This discussion is more than welcome to cover not only Avid editing bays, but also portable field systems and audio mix stages. I'm also interested in knowing if a clean electrical supply for the building makes a difference in how you decided to wire.

Nine times out of ten you "get away with it" using the same supply throughout a building. The BBC really set the standard in the UK for segregating domestic (aka "cooking" mains) from technical power supply and the reasons have now largely changed because of the move from linear power supplies to switch-mode supplies in pro-equipment. Pre the eighties pretty much everything had a linear supply and so a clean mains supply was essential and so it was common to segregate the mains for the production chain from the sockets used to plug in vacuum cleaners etc. Since then we see a lot more switch-mode supplies which tend to be more immune to mains-bourne noise but the concern now is the large potential difference that can occur between different earth feeds. The worst case is plugging a bit of kit in one room on the technical supply (and hence the tech earth) and connecting it via tie lines to another device that is powered by the domestic supply. Since both devices will (probably) have a common signal and chassis earth an potential difference between the earths will be effectively dumped onto the signal screen - depending on how the receiving bit of kit terminates the feed you get mains hum of the audio and hum-bars on any analogue video. Digital signals are effected in that you're more likely to get transmission errors - splats on the AES audio.

When would you, or would you ever, lift the ground on an analogue audio signal in a deck and mixer configuration? At what point would you lift the ground if you were too (deck or mixer)?

As a "get me out of gaol quick" I'd lift the earth in situation I'd described - I'd make sure I fixed the mains supply problem first though. Loosing the electrostatic shielding that the screen gives you probably looses you six dBs of noise performance as well.

When would you, or would you ever, lift the ground on an analogue audio signal in a configuration where all of the audio passes through a half or full norm patch bay? At what point would you lift the ground if you were too (patchbay or device)? Would you tie/strap/buss (the practice of using a wire to connect all the grounds of all the jacks to one another and then connect to the rack) the grounds together on either the half or full norm patchbay?

The modern practise is to take signal-earth through the jackfield on each position and terminate in ABS krone blocks. If you view your jackfields as a re-assignable resource then you should make them AES capable and that pretty much dictates earth integrity. By having individual screens you also avoid the problem of dumping mains hum onto every feed on a jackfield if the problem I described (above) happens.

When would you, or would you ever, lift the ground on an AES audio signal in a deck and mixer configuration? At what point would you lift the ground if you were too (deck or mixer)?

I'd never lift AES earths! ISTR that when AES was starting to hit in the late eighties the spec document said you should take the shield all the way to the krone block and segregate it with a 10pF capacitor if you had earth mismatch problems - again, you really should get your tech earth sorted! AES is so unreliable an interface (1.4mBits over 110ohm twisted pair cable!) that I tend to put it over 75ohm co-ax whenever possible - see an article on my blog

When would you, or would you ever, lift the ground on an AES audio signal in a configuration where all of the audio passes through a full norm patch bay? At what point would you lift the ground if you were too (patchbay or device)?

Previous answer

When building a cable for analogue audio that goes from balanced to unbalanced do you wire [T/T R/S S/S], [T/T R/T S/S], [T/T R/x S/S], [T/T R/S S/x]? And why that way? (and yes, I do know that one should actually use a matchbox or something, but indulge me please) (T=tip, R=ring, S=sleeve, x=not connected) (balanced is TRS, unbalanced is TS)

With the proviso that you DON'T mix balanced and unbalanced on a jackfield - you run the risk of unbalancing the whole facility and hence destroy any noise immunity that common-mode rejection is bringing you - I take the view that tieing the cold and screen together is no longer a good way to go as most kit now terminates feeds in op-amps rather than a rep-coil. Loosing six dBs of level seems a reasonable trade-off compared to slowly burning out the receiving stage in the DA, etc.

I've designed/supervised the install of many small and large facilities/studios/OB trucks over the last decade and without exception I've never had to deal with induced mains hum or noisy audio because of balanced/unbalanced problems by following the principles above. Rep-coils (passive match-packs) are a superb way of isolating audio feeds that may have induced mains hum.