When I use my Novation Controllers (Launch Control + XL) I usually use a usb splitter cable to avoid noise from the controllers LEDs. While this works relatively well I'm annoyed by the extra clunky cables, my cable mess is big enough anyway.
I was wondering if it was possible to cut the 5V usb line somewhere on the board and inject the 5V from a different source.
Since you know the board the best, you got any Ideas @Johannes?
We have discussed this one before. You might find some related posts etc on a search, one spot we know that works, are the connections you use when you remove the Midi part from the Axoloti, one of these is a 5v supply and another is a ground. Can't remember which ones though, and it can be used to provide power to USB. I think TB did something similar recently. 
Thx for the reply but I don't mean tapping from Axoloti's 5v supply but cutting the 5v supply that axoloti feeds to a usb midi controller and replacing it by an external 5V wallwart or battery.
I may be wrong so don't test it until you are sure, but I believe it is just a matter of disconnecting the power line from the usb cable and adding the power supply to that, the bit I am most unsure about is the ground, resource states it needs to stay connected at both ends of the USB cable otherwise the communication wont work, and you need to tap into the USB ground line from your power supply, if I understand it correctly, this is what powered USB Hubs do, but not entirely sure and have not tested. Maybe some others can confirm.
Yes that is exactly what I'm doing right now with a usb splitter cable that splits into one connector for data and one for power. But I'm really annoyed by the extra bulky cables and eventually I want to put axoloti in an enclosure with preamps and mixer integrated so I thought it would be more practical to somehow feed the external 5v to axolotis usb host connector.
So if you want to hack the board to inject power to the USB host port, you have to cut one track of the circuit board, and connect ground and +5V of the power supply to these points. Best to use little wires that connect those points to free holes on the circuit board, and attach your power supply wires to free holes, in order to get better mechanical stability, as wires soldered to the bottom of the USB connector will break when stressed.
thank you so much johannes, this is exactly what I need!
So what are the possible dangers if I do this hack?
One more question, can hook up the GND to a different pin on the board, gpio for example?
melting usb cable and fire in case of a short circuit, depending on the current capability and protection of the power supply used. Not much different than when using a USB power injector cable, unless such would have an internal fuse or some other protection.
The interference noise that you want to reduce, comes from a current through a (non-zero but near zero) ground resistance, causing a (current multiplied by resistance equals...) voltage difference between two "ground" points. The biggest contribution to non-zero ground resistance is likely power supply wiring and connectors rather than the Axoloti Core circuit board itself, the ground track in the Axoloti Core circuit board is a copper plane inside the circuit board rather than a tiny track. So you can try using a different point to connect ground to the external power supply than the ground pin on the back of the USB host connector. That's less ideal, but I expect only a small difference in reality.
Ok thx again.
Usually I use a 5V powerpank with a 5V ->12V step up converter feeding axoloti, 2 guitar preamps and a mixer in a daisy chain. The controller is hooked up to the second 5V power bank.
Usually when I do this with the battery it's a lot clean than with a psu. The workaround with the splitter makes the led noise suprisingly lower although eventually all power comes from the powerbank.
If you have a different maybe more effective solution I'd be more than happy to hear!
Little Update here:
I got one of these little boxes to power all elements of the setup independently:
Result is that the LED noise is completely gone! Unfortunately there is still quite an amount of other kind of noise that I'm trying to get rid off.
Maybe tell something about what other devices are connected, and to which power port each of them is connected?
thx Johannes.
I got a guitar, with 3 pickups, one going to a tube preamp one to a pitchshifter and there is a piezo going to a piezo preamp. the pickups are summed in a little mixer that goes to axoloti's right input.
on the left input there is a dynamic mic.
I'm right now checking each device for noise. I realized that the mic input is quite noisy (gain of that channel is set to 27db). So propably using a proper mic preamp might be better?
Also the mixer seems to have a little hiss. Generally speaking I'm getting to point where it sounds a lot quieter. The power brick thing might have improved the whole setup quite a lot.
I also realized that the reverb in my patch makes weird noises. Somehow when the allpasses are set to anything lower as x 1,0 I get a weird sound.
p.s. pickup hum / interference seems to be quite low.
Hi @Johannes
unfortunately I lost my axoloti suitcase in the metro wich means I don't have this isolated power out thing anymore and I'm too broke to buy a new one.
I'm now rebuilding my setup and want to take it as a chance to make a portable/travel friendly version that I want to take to africa next week.
The biggest problem I have is again this led noise! So the hack you described could be a cheap and easy solution but I would like to know is there is a way to reverse it if I find a better solution in the future? Like a little solder bridge that replaces the cut, is that realistic?
You have other suggestions of how to get rid of the led noise? You explained the source of the problem a couple of times but apart from this expensive commercial isolated power things I couldn't find a good solution.
For testing purpose I tried with simple setup:
Guitar ---> Guitar Preamp Pedal ---> Axoloti
Axoloti + Preamp on same power supply. Even with no guitar connected led noise is strong, so the guitar pickup is out as a noise source.
Do you think an isolated dc converter could solve it? like this thing but then with more watt? http://www.ti.com/lit/ds/symlink/dcp010512b.pdf
