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

[rsdio at audiobanshee.com]

On Sep 10, 2015, at 3:15 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> Interestingly, the standard shows a single 220R resistor in the input
> circuit, although I have seen several manufacturers (e.g., Roland)
> split that into a 100R and a 120R resistor, one each side of the
> opto-isolator.

I believe someone else mentioned this, but having two resistors allows protection on both pins of the opto-isolator.


> For EMC/RFI/noise reasons I would recommend have equal source
> impedances on the two lines to maintain a semblence of balanced
> impedances.

Since neither LED pin is referenced to ground, does it even make sense to think in terms of balanced impedances?

The 6N138 has a teraohm of resistance from input to output. That's greater than the conductivity of the PCB itself. In other words, there's more isolation in an opto-isolator than on a PCB.

I also think it's invalid to treat a current loop as if balanced noise rejection is possible. Current loops are already rather immune to noise.

Brian