From Brian Willoughby Sent Sun, Jun 17th 2018, 21:19
From Brian Willoughby Sent Sun, Jun 17th 2018, 21:19
Yes, what you describe below would be very easy to create. You could = also just find in an existing product that has basically the circuit you = described. There are example circuits everywhere for this sort of thing. = There might even be schematics for some of the DI products that would = show the same, basic circuit. I recently designed and built an adaptor board to convert Roland 13-pin = to 6 balanced outputs. Basically, it=E2=80=99s a DI for my Godin = SynthAccess guitar to feed the balanced inputs of my Metric Halo Labs = MobileIO interface (which then feeds my wave shaping guitar synth = plugin). Since the 13-pin standard requires a +/-7V power supply to = =E2=80=9Cturn on=E2=80=9D the guitar electronics, I just put in a dual = op-amp for each string. One op-amp buffers the positive signal and the = second op-amp creates the inverted or negative signal. Both outputs have = matching resistors to maximize noise cancelation. It wasn=E2=80=99t = really necessary that this circuit be balanced, but by the time the = required power supply was added, a few op-amps meant that I wasn=E2=80=99t= wastefully grounding the (-) pin on the TRS connectors. I designed around the standard 8-pin dual op-amp pinout, so that I can = try different models of chips. If this had really been a critical piece = of electronics, I might have designed around the OutSmarts balanced line = driver chips from THAT Corp. They claim that their chip performs like a = floating balanced transformer, and provide example circuits that would = probably work for a DI. About the only challenge in what you describe is avoiding clipping. You = might want to research the power supply voltages for your synths and = make sure that the high level unbalanced to balanced circuit has at = least as much voltage. Even with matching power supplies, though, some = op-amps have more headroom than others, so designing with more voltage = in the active DI will allow more flexibility in choosing op-amp models = without clipping. If you decide that you want to DIY something, let me know. Most of the = stuff is cheap to make. The only real challenge is the enclosure. My = SynthAccess DI just has a 19=E2=80=9D rack face plate and nothing else. = Not quite road worthy, but fine for the studio. Brian Willoughby On Jun 14, 2018, at 5:05 AM, DJ Maytag <xxxxxxxx@xxxxxxxx.xxx> wrote: > So this has me thinking about designing/building what we called a = =E2=80=9Cline driver=E2=80=9D in the car audio world 20 years ago (often = with a matching =E2=80=9Cline receiver=E2=80=9D at the end of the = chain). >=20 > I guess all I=E2=80=99d really want is a box that would accept an = unbalanced input, route it to an audio grade op-amp with inverting and = non-inverting outputs, to create a balanced output. >=20 > I=E2=80=99m no circuit designer, but it seems like this is essentially = all that is needed. I can=E2=80=99t think of a single synth I=E2=80=99ve = ever owned that had an issue with a really weak output signal, so 10/20 = dB pad wouldn=E2=80=99t really be needed. Matching typical output to = input impedances would be the only think off the top of my head that = would be critical (and I=E2=80=99ve given this about as much thought as = it=E2=80=99s taken me to write this email). >=20 > So basically a high level unbalanced to high level balanced = converter/driver to send to a balanced line input on a mixer (or = whatever desired destination). >=20 > On Thu, Jun 14, 2018 at 12:03 AM Brian Willoughby wrote: >> The signal on the =E2=80=98+=E2=80=99 wire in a balanced connection = is identical to the only signal in an unbalanced connection. The signal = on the =E2=80=98-=E2=80=98 wire is also identical except that it is = inverted in polarity. When one is a positive voltage, the other is the = same magnitude but negative voltage. For this reason, feeding a balanced = output to an unbalanced input with a carefully wired cable results in a = signal that is 6 dB quieter than a true balanced signal. The balanced = input combines the =E2=80=98+=E2=80=99 and =E2=80=98-=E2=80=98 signals = to create a difference voltage that is potentially twice that of either = wire alone, while also canceling out nearly all noise picked up on the = wire. >=20