From Brian Willoughby Sent Sun, Jun 17th 2018, 23:42
From Brian Willoughby Sent Sun, Jun 17th 2018, 23:42
Hi Quincas, I found that schematic right after you first mentioned Befacto on this = list, and your message has been sitting in my inbox, due a reply, ever = since. The Befacto employs the first and most effective aspect of a balanced = line, even though it is not fully balanced. Many people focus on the = identical but opposite polarity signals in a fully balanced line without = realizing that it=E2=80=99s also very important that the impedance to = ground for each of the (+) and (-) lines be equal. If the impedance is = different, then the noise picked up by the cable and other parts of the = interface circuit will have a different amplitude in each of the (+) and = (-) lines, and then it won=E2=80=99t cancel out so well at the balanced = receiver. Analyzing a circuit to make sure that the DC and AC impedance = is completely matched on each line can be quite difficult - many = products from well-known brands have failed at this, as surprising as = that may seem. The Befacto circuit saves money by not actually bothering to put the = inverted signal on the (-) line. Instead, they focus on making the = impedance of both (+) and (-) match, at least at the first order, by = using identical resistor values. I think that the Rane link that someone = else shared might actually mention this circuit setup as a sort of poor = man=E2=80=99s balanced. It=E2=80=99s not the best. So, the Befacto outputs are set up so that any noise you might pick up = on the cable will get canceled out by the balanced input that you = connect it to. The signal will be 6 dB lower in amplitude, because the = (-) is 0 V, and the S/N will therefore be 6 dB worse than it could be, = but you get noise cancelation for the cost of a resistor and whatever = extra cost for the balanced connector. An unbalanced setup would not = have any noise cancellation. I haven=E2=80=99t had time to completely review the circuit to see = whether there are some other impedances contributing to the total in an = unbalanced way. A quick guess is that the PCB layout might ruin its = potential if the ground paths are shared between the (+) and (-), but = probably only when significant current flows. The best descriptions of true balanced audio come from Bill Whitlock, of = Jensen Transformer fame. Somewhere, he has a paper that diagrams the = various popular audio circuits for unbalanced, fake balanced, poor = balanced, and fully balanced topologies. Each diagram has a depiction of = how the current flows for each of the signal and noise currents, and = shows how the noise gets canceled (or not, as the case may be). I tried = looking for his paper, but haven=E2=80=99t found it yet. Thanks to the = AES (Audio Engineering Society) for supporting people who research all = of this stuff, find ways to make audio better, and then teach the = industry how to avoid common mistakes. I feel like I=E2=80=99ve received = a free continuing education thanks to the PNW Chapter of the AES. Good eye, Quincas! Brian p.s. The Befacto uses two op-amps in parallel, but with resistors to mix = the outputs properly, all so that the headphone output has enough = current to drive headphones directly. I haven=E2=80=99t heard the = Befacto in person, but it probably sounds better than a lot of systems = I=E2=80=99ve suffered through which use specialized =E2=80=9Cheadphone = amplifier=E2=80=9D chips with way too much noise for my tastes. If this = dual op-amp design is loud enough, it probably has better S/N = performance. I=E2=80=99m just guessing, though. It can be tough to get = enough gain for headphones to be audible on stage, what with all the = competing sounds. On Jun 17, 2018, at 3:25 PM, Quincas Moreira wrote: > And actually, I was just looking at the schematics, and the output is = TRS and reportedly balanced, but I don't see an inverted signal at the = ring, only a 1k resistor to ground... Is this legit balanced? Or just a = way to send unbalanced out through TRS? Brian? >=20 > https://www.befaco.org/docs/Output/Output_V3_schematic.pdf >=20 > On Sun, Jun 17, 2018 at 4:43 PM Quincas Moreira wrote: >> Hey, kind of on topic here, I just did a review for a module that is = basically a dedicated DI box for modulars, with the added bonus of a = Headphone output with a switchable cue input for auditioning sounds you = don't want your audience to hear yet... >>=20 >> https://youtu.be/SN1BNx3amM4 >>=20