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Q. What is a ‘fully floating’ balanced output?

The popular Sennheiser MK416 is described as having a balanced output that is ‘fully floating’, but that term can mean different things to different people!The popular Sennheiser MK416 is described as having a balanced output that is ‘fully floating’, but that term can mean different things to different people!

I’d like a clearer understanding of what a fully floating balanced output is and how I can take advantage of it. My Sennheiser MK416 mic uses this at its output. Do I require a specific XLR cable wiring arrangement (some of my cables tie pin 1 to the plug chassis) to get the best possible signal out from my mic?

SOS Forum post

SOS Technical Editor Hugh Robjohns replies: A balanced output means an output designed to feed a differential receiver, with the signal conveyed between two signal wires, usually (but not always) protected by an overall grounded screen or shield. The specific aspect that makes it ‘balanced’ is that the two signal wires have exactly equal (balanced) impedances to ground at each end, and this is critical for the interface’s ability to reject common‑mode interference.

The ‘floating’ part means different things to different people. Most probably associate floating with a transformer‑coupled output, but it’s a bit more complicated than that. For example, for some a floating output will imply galvanic isolation between the source equipment and whatever it’s connected to — something that can only be done practically with output transformers. For others, the term relates to the interface’s behaviour if one side of the balanced output is grounded (to feed an unbalanced destination); a floating output will deliver the full signal voltage, whereas a non‑floating output may only deliver half the signal voltage (‑6dB). This balanced/unbalanced level consistency applies to transformer‑coupled outputs (provided the transformer isn’t a grounded centre‑tap type), but some types of cross‑coupled electronic (active) outputs manage the same feat.

A further aspect to consider is what happens if a large common‑mode voltage is applied between the output wires (ie. external interference). A floating output presents a very high (>10MΩ) impedance to common‑mode signals, so there’s negligible current flow into the output circuitry. This characteristic is inherent in transformer‑coupled outputs, but also in some specialised forms of active electronic output. A non‑floating output typically presents a much lower (<200Ω) impedance to common‑mode signals, so considerable current can flow into the output circuitry. This unwanted current can potentially cause damage, and is likely if working in electrically hostile environments — for example, in areas of strong RF interference, or where the ground potentials are very different at the source and destination.

Finally, some people confuse the term ‘floating’ with ‘symmetrical’, using it to describe the nature of signal voltages conveyed on the two wires. The audio signal voltage can be passed in a symmetrical format (half the signal voltage on each wire, one polarity inverted, each referencing the other), or in the so‑called ‘impedance‑balanced’ format (all of the signal voltage carried on one wire, referenced to the other).

I believe the MK416 provides a symmetrical output, but it doesn’t have an output transformer and, since all modern versions are phantom‑powered, a ground connection is essential — so there is no galvanic isolation (not that this would be desirable in a mic anyway!). I don’t know if the signal voltage is maintained if one output is grounded, but if you tried it you’d probably starve the mic of phantom ‘juice’ anyway. So, in this context, I would surmise the ‘floating’ term is referencing the mic’s ability to handle large common‑mode signals, though I should stress that this is informed guesswork!

Taking Advantage?

How can you take advantage of it? Just use a good‑quality XLR cable, and a decent mic preamp! Standard XLR wiring is fine; it doesn’t matter at the mic end whether pin 1 is linked to the shell or not; the AES recommend that it shouldn't be, although most commercial XLR cables are wired that way. It can make a difference, in terms of hum and interference, at the preamp end, but that depends entirely on how well the preamp has been designed/ built. If done right, a link between the shell and pin 1 will make no difference. At the end of the day, the floating balanced output of your MK416 is no different from that of any other standard mic, and will work just fine with any standard XLR cable and preamp.