Of Equal and Opposites
Of the dizzying array of questions that the expansive and all-knowing Sound Devices Technical Support Department fields, the one which has the most people confused, bamboozled, and flummoxed is: "What is this 'impedance' balanced output stage that you use on on early MixPre's and the TA3 outputs of the 442?" Here is the answer:
The common misperception is that a balanced output stage needs to have equal and opposite signals on pins 2 and 3 of the XLR connector (or tip and ring of a 1/4" connector). What defines a system as balanced is that the impedances from pin-2 to ground and from pin-3 to ground are both equal. The signal(s) appearing at the output pins has nothing to do with whether the output stage is balanced or not. Bill Whitlock, President of Jensen Transformers and a noted authority on this topic, offers an excellent overview on balanced and unbalanced systems in his paper, Interconnection of Balanced and Unbalanced Equipment.
The best type of topology for inputs or outputs (but especially inputs) is one which use a high-quality transformer, as transformers provide galvanic isolation and typically provide the superior common-mode and RF rejection compared to non-transformer balanced inputs or outputs. However, when a transformer-balanced output driver is not practical, an "impedance" balanced output is quite suitable. ("Impedance" balanced is really a misnomer - it is simply a truly "balanced" output stage.) This topology has equal impedances from pin-2 to ground and from pin-3 to ground; however, only pin-2 is driven with signal voltage. It provides common-mode rejection of unwanted noise into a balanced output just like an "active" balanced or a transformer balanced output stage. This topology is not new; it has been used for years in audio equipment. It works equally well with line-level or mic-level signals - Neumann microphones are one of many examples of equipment utilizing this topology.
The advantages of the "impedance" balanced output topology are:
- Stability issues that exist with "cross-coupled" balanced outputs are eliminated.
- Unbalancing the output is easy, pin-3 can be either grounded or ungrounded, and it will work fine.
- There is no 6 dB loss of signal going from balanced to unbalanced output as there is with a standard active balanced output.
- It uses less parts and is therefore more reliable.
The only real caveat with the "impedance" balanced output is that it cannot provide inversion of the signal polarity, as pin-3 is not driven.
Hopefully, this explanation has been lucid and edifying. If none of this is new to you, perhaps a job as Senior- or even Chief-Guru in the Sound Devices Technical Support Department is in your future. In the extremely remote case that one of the several Sound Devices Gurus who wrote this made a mistake, then please send corrections here, and the Guru(s) in question will be summarily terminated.