MIDI phantom power from 9vac

[trixdropd]

I'm looking to make an arduino based Midi receiver.

I plan on coming in with 9vac, and then I need to copy that to the midi jack to power a ground control pro. I assume since this is AC power that I can't just simply parallel it. From there I will go through a diode bridge and regulate down to 5vdc.

Any tips?

[ElectricDruid]

I'd be cautious and definitely don't assume. I don't know what power the GCX switcher puts out to phantom-power the Ground Control Pro, but you need to find out. They might have just paralleled its 9VAC power input to the spare pins on the MIDI jack (yes, you can parallel AC), but they might have sent a regulated 5VDC to the jack instead. The first option is probably more likely, but "probably" won't help much when smoke comes out, so be sure to check as much as you can.

It's all a bit dubious, because phantom power is totally against the text and spirit of the MIDI spec. Whilst it's true that there are two pins unused on the standard 5-pin DIN, it's not true that all MIDI cables connect those pins up, so you might find some MIDI leads don't work to power up the ground control. And you'd have to be very careful what else you plug into your MIDI receiver, since you'll have a non-standard MIDI input which actually *outputs* power. Maybe add a switch so you can turn the phantom power off when not required, just to be safe? Or will you only be using this with your ground control? (That might be true now, but for future-proofing?) With those caveats in place though, it should be fine.

Once you've got power, a 5V power supply is easy to do with the diode bridge you mentioned and a 7805 or 78L05 (if you don't need more than 100mA) and a few caps.

HTH,
Tom

[trixdropd]

I'd be cautious and definitely don't assume. I don't know what power the GCX switcher puts out to phantom-power the Ground Control Pro, but you need to find out. They might have just paralleled its 9VAC power input to the spare pins on the MIDI jack (yes, you can parallel AC), but they might have sent a regulated 5VDC to the jack instead. The first option is probably more likely, but "probably" won't help much when smoke comes out, so be sure to check as much as you can.

They are coming into the GCX with 9vac. So perhaps they are just paralleling it to the midi jack. Many devices on the market like the GCX guitar switcher and RJM effects gizmo put out phantom power that works with a bunch of different midi controllers like the liquid foot, the ground control and the mastermind. I just want to replicate that function in my unit.

It's all a bit dubious, because phantom power is totally against the text and spirit of the MIDI spec. Whilst it's true that there are two pins unused on the standard 5-pin DIN, it's not true that all MIDI cables connect those pins up, so you might find some MIDI leads don't work to power up the ground control. And you'd have to be very careful what else you plug into your MIDI receiver, since you'll have a non-standard MIDI input which actually *outputs* power. Maybe add a switch so you can turn the phantom power off when not required, just to be safe? Or will you only be using this with your ground control? (That might be true now, but for future-proofing?) With those caveats in place though, it should be fine.

I have thought all those things through. I will be using a 7 pin midi jack and the phantom power will be on a switch.

[ElectricDruid]

In that case, it sounds like you've worked out all the gotchas already! Go for it!

T.

[anotherjim]

A lot of midi phantom power doesn't require any special or separate power wiring. The downstream device derives it's power from the "hot" data pin which is an unswitched connection to the host +5v supply via a resistor. This does mean the phantom powered device gets less than 5v supply, and usually runs at 3v3 (or may use a DC-DC converter to obtain 5v or more). Because midi output is current loop, a 3v3 device can still drive it's own midi output using lower value resistors in the data wires than the 5v devices have.

If you have a commercial product that can run purely off a 5pin DIN midi input, the above is how it's powered.