Thanks for breaking this out into its own thread. I’m also super interested in what other folks think about this, especially since I have somewhat limited real-world experience to draw on.
I’d definitely second the recommendation of reading some of Bill Whitlock’s writing on the matter, as that’s one of the things that triggered this discussion in the first place. Whitlock’s writing (and possibly some of Rod Elliott’s related discussion) do touch on the ground loop issues as well…
After a bit more reflection, and some research into the specs on the gear that I have now or have had in the past, it does indeed look like most prosumer-y things that I’ve encountered don’t actually have balanced outputs, but a surprising number of things do have balanced inputs.
Two questions stand out to me right now:
- How hot is the actual signal?
- What role does the device play in the overall system?
With microphones, the signal is usually pretty low voltage, and usually the balanced termination is followed by some pretty substantial gain, making the SNR of the channel extremely relevant to the noise floor, so having a balanced output connection seems of paramount importance.
With a synthesizer or other type of electronic instrument, you usually have a pretty reasonable output signal, and perhaps the assumption is that in professional environments, one would use a DI box before connecting to outboard gear, or a mixer, etc.
None of the synths or samplers that I have access to now, or have had access to in the past, have balanced outputs, but, interestingly, most of the line inputs in mixers I’ve seen are balanced. Even the line inputs on the cheap Behringer Xenyx series are balanced. (Though the outputs are unbalanced, of course
This leads into the “role in the system” thought for me: you could use a mixer in the local context of a single person’s rig whose instruments all have unbalanced outputs, making the balanced-ness of the line inputs somewhat useless, or you could use it in the front of house, where you’re getting long-run balanced inputs from DI boxes on stage. Balanced inputs enable the same hardware to play both roles.
So, what’s the role of the device in question? Does it need to be flexible enough to use in a FOH-type situation (or, for example, a gallery installation) where balanced inputs would be helpful? Or is it instead reasonable to expect whoever’s using it in that situation will stick a local mixer in front of it to terminate any necessary balanced lines? Similarly, is it reasonable to expect that whoever’s using it will have DI boxes available to balance the outputs if/when needed?
Obviously these aren’t necessarily questions that warrant forum responses, but they seem to be relevant design thoughts.
The Whitlock writing touches on this a lot, but to summarize some of it: Balancing is effectively just matching the impedance of the two signal lines. What makes it good or bad is how closely the impedances are matched. Transformers do this very well inherently, and the THAT ICs do this well because they laser-etch the resistive substrate of each channel to match them. Doing this well in a discrete circuit is time consuming, since you’ll want to manually match your output resistors, and expensive, since you’ll need to buy lots of “high-precision” (although not high-enough precision) resistors so that you have enough choices to make the best matches.
From a user’s perspective, a “good” balanced circuit “just works”, doesn’t introduce noise, and/or doesn’t color the signal in undesirable ways. From a design perspective, you basically just want it to be cheap and simple enough to be economically practical, generally stable, and easy to troubleshoot (and possibly also small, if moving to 1/8" connectors is the thought).
From what I’ve read, it seems like the answer to this is “maybe”… My understanding is that an unbalanced connection in a high-noise environment just impart a higher noise floor (and possibly introduce artifacts, like audible clicking from your cell phone or wifi RF). A poorly balanced connection may color the signal or behave as a filter in an undesirable way. Which one of these is better than the other, I’m not really sure, and probably depends on the context.