[sdiy] MIDI phantom power...over 5 pin MIDI connector ?

[rsdio at audiobanshee.com]

Sorry, Roman, but I did not paint the full picture. A ground loop appears when two audio devices are connected by MIDI cables with a MIDI-line-powered device in the middle under certain, admittedly rare conditions.

Normally, a MIDI cable or a MIDI-standards-compliant device will not make a ground loop between two audio devices because - while the outputs are grounded - the MIDI input jack has No Connection to ground (pin 2). This is part of the design because it's far too easy for two audio devices to hum like mad when linked by a cable or two.

In contrast, a MIDI-line-powered device has a connection to ground (pin 2) on it's input jack, contrary to the MIDI spec. Of course, if your only other connection is to a MIDI-standards-compliant audio device, then that should break the ground loop because that audio device will have No Connection to its ground (pin 2). However, if your MIDI-line-powered device is in a metal case that is grounded, and you place it such that it's touching to something else, then there's potential for a ground loop. I'm sure we've all tracked down ground loop problems in a rack where devices had an unintentional ground link through their rack rails.

Granted, what I've described seems unlikely, since there's probably no easy way for a MIDI-line-powered device to make that second ground link. But it's not quite as error-proof as the official MIDI specification.

I should have said "ground link between input and output" not "ground loop" - there isn't a loop until something else happens.

Brian


On Sep 14, 2015, at 1:00 AM, Roman Sowa <modular at go2.pl> wrote:
> There is no ground loop in MIDI-line-powered device! There's only one input and one output. No 3rd lead that will cause ground loop. It's just like the cable - there's shield on pin 2 going from one end of the cable to the other. Both sides of transmission don't care if there is a cable in between DIN plugs, or a black box. There's no current path anywhere outside one end of the cable or the other.
> So using one IN/OUT MIDI-powered black box is exactly like using a cable. No need for optocouplers or any other isolation methods in the black box.
> 
> Roman
> 
> W dniu 2015-09-11 o 21:55, rsdio at audiobanshee.com pisze:
>> By the way, the second caveat of this approach is that you will have
>> a ground loop between input and output, making your MIDI device fail
>> the usual MIDI guarantee that there won't be any ground loops.
>> 
>> You can solve this by filtering your pin 2 output so that it's
>> connected to ground through a high impedance, but that means you
>> cannot chain more than one MIDI phantom-powered device.
>> 
>> Brian