Connecting Axoloti directly to 5V (Eurorack rail)

[scitoast]

Hi, new Axoloti user but pretty advanced electronics nut here. I have built my own small Eurorack (everything-- case, modules some finished and some in progress, including very smooth custom linear power supply), basically for me it is cheapest to go all DIY with Eurorack!

Before you tell me to look at Euxoloti, I already read up. But my power limitations are not the same. Any/all digital modules I install must come from the 5V rail which I have as a regulated 5V with 1A capacity, much more than the other rails. This also limits digital noise in the analog modules.

So I cannot follow the Euxoloti pattern which I believe draws power from a presumably much beefier +12V rail than I have, and downregulates.

My 5V rail is a linear regulated supply, gives 5.11 V which is within USB spec. Questions:

• Is this OK for Axoloti's tolerance?

• To power axoloti from 5V is this possible, can I wire it in directly (and where?)

I am aware the barrel connector requires >=7V.

Other options: Install a filtered outlet inside my Eurorack with a wall wart to power axoloti, inelegant. Or alternatively I can instead turn the un-utilized "Internal CV" bus of the Eurorack into a non standard 7 or 9V supply rail and just consider never to install a module that uses internal CV! (I could never sell that system to anyone lol!) What do you think? Then I would wire into the barrel connector, but it would mean YAPSIME (Yet Another Power Supply In My Eurorack).

Edit: Unlike Euxoloti, I do not plan to cut off the midi section of the Axoloti, why get rid of perfectly good functionality?

Thanks!

Ken

[scitoast]

A follow up. Let me take a moment to aggregate the topics I have found that seem most relevant to this question:

1. https://sebiik.github.io/community.axoloti.com.backup/t/ultranoob-questions-cv-out-shared-power-supply/703/2

Quote from Johannes:

I think you can solder a header connector on the free pads near the din midi/power supply part. I think it is best to feed axoloti from the +12V/ground modular power supply rails - to the X2 pads, so using the switching power supply on the axoloti core board. I think the on-board switching power supply is a better option than using the +5V rail from eurorack, lots of eurorack power supplies use a linear regulator to derive 5V from +12V and that 'd get hot if you 'd use the usb host port with high current devices.

1. https://sebiik.github.io/community.axoloti.com.backup/t/can-i-connect-5v-supply-direct-to-the-axo/1734

However this user's question was not actually answered; it was only pointed out that the barrel jack does not accept voltages as low as 5V.

So to bring the force of consensus to this question: It appears @Johannes is of the view that direct power to the board from 5V is not a good idea. Edit: But I think mainly this is because of the assumption that a cheap 7805 is powering the 5V rail? In which case if we have a good 5V source is this no problem? Edit: From what I have seen in other topics, most people using 5V power have accomplished this by powering from USB connector. And it would appear that the community has rarely or never encountered a situation where direct 5V power to the board would be necessary in an application?

On the Axoloti board I see several points that might be places to accept 5V power directly, but it is not clear to me looking at the Axoloti schematic which would be the best place. This and the fact that others have not apparently tried direct 5V power leads me to contemplate a kludge, basically installing a 9V wall wart internally in my Eurorack to provide barrel jack power to the Axoloti. It would work, it's just not so elegant.

Cheers

[Blindsmyth]

You can feed the 5V at the pins described in the Image:

Breaking apart the boards

Here's a legend for the 5 pin patch points JP2 and JP3, top view: [image] Merging two midi streams with a connection or simple logic is not possible, I mean, connecting an Arduino to MIDI in will not work in combination with the on-board MIDI input circuit.

https://sebiik.github.io/community.axoloti.com.backup/uploads/default/original/2X/d/d3caa27d2e2236f06b06ae7589ea7c6cd868dd7c.png

I've been doing so succesfully (with a cut off midi/dc part) feeding in 5v from a smartphone powerbank.

I don't know if 5.11V would be a problem sorry!

[scitoast]

@Blindsmyth Thank you, so with a cut off MIDI/power section it appears a direct 5V wired in supply can work. However it's a possibly different situation when the board is intact. It's unclear to me what might happen with the board intact (not cut) because you have now power regulation circuitry which will be floating on the input side and powered on the output side.

Looking at the power regulation / supply schematics you can see a few obvious places to think about. Disclaimer: I'm no expert and my opinion is probably not so trustworthy! But here is what I see:

• It appears the barrel jack feeds into an AOZ1280 buck (switching) regulator programmed by the external resistors to provide 4.96 V. This feeds the +5V supply rail on the Axoloti board.

• The only other risk point I see is the STMPS2161 power switch chip which provides the power feed from micro USB socket. Looks like a nice little chip!

• Of course maybe I missed something!

In both cases, direct-wiring 5V power would supply potential to the output side of the chips while the input side is floating or maybe low. Looking at the data sheet, I think the STMPS2161 would be safe: it says,

When the switch is OFF (disabled through the EN pin), or when the STMPS device is unpowered (VIN = 0 V) the switch behaves as an Hi-Z at the output pin, ensuring that no reverse current will flow into the device when VIN< VOUT

So unless this chip receives an erroneous / incorrect EN signal there is no problem.

What about AOZ1280? Another really nice chip. Soft-start/inrush limiting; overcurrent protection, etc. This too -- if I am reading correctly -- has an enable (EN) pin which in the Axoloti schematic is linked to the barrel jack input via a 47K resistor, so that power input automatically enables the chip and begins soft-start process. The IC should be in disabled mode then when no input is applied there. But the datasheet does not specify what exactly the "disabled" state means except that the chip is not operating. I do not think you can trust that the output pin would enter a high Z state; in other words reverse biasing the chip by feeding 5V power directly to the 5V rail on the board has unclear consequences, from what I can see.

So in other words it might not be a wise idea to direct-power with 5V unless as Blindsmyth has done, you have broken the Midi/Power part of the board off. Have I answered my own question?

But the AOZ1280 is such a beautiful chip, I can't let it go to waste. I think I will just work a 9V supply into my system and use it!!

So thanks.

[Blindsmyth]

Hey there any chance you managed to power the board with 5v? I put mine into an enclosure together with smartphone power bank and would like to feed it 5v from the battery to some pins.

[thetechnobear]

Im intrigued if anyone has tried powering from 5v rail too.... and if you did, did you encounter any (switching) noise issues?

I ask, as recently I connected a BelaMini to my AE modular, and powered it off of the 5v supplyand found this introduced noise on the power line (heard as a high pitch noise in other modules).
… this is a small blow, as id planned to do the same for Axoloti, but now fear id have the same issue.

Im discussing with both tangiblewaves and bela a solution, which involves using an inductor (+ capacitor) on the 5v supply, buts its complex for a beginner like me, so not fixed it yet,
so Im interested if others have got around this...

---

side note:
im not sure if running of the +12v has the same issue, as im guess this goes thru some additional circuitry which might smooth out the supply?
similarly I think designs with take the 12v line, down to 9v or 5v using a regulator, don't have the issue due to the regulator?

note... all the above take with a pinch of salt, given this is all beyond my knowledge of electronics, so its what Ive tried to interpret from a variety of sources

[lokki]

yes, if you use the 12v supply and regulate that down to 5v you should be fine and have no additional noise. on bela you just have to buy the regulator yourself and add it to your modular to get 1amp of 5v (and some heat)

[scitoast]

Hi,

I do have some interesting findings for you. A year ago I built a Eurorack module out of the axoloti sparing no inputs or outputs to the extent I could. I ended up powering axoloti from the 12V rail which worked fine but actually the high pitched whine / noise was still present in my audio pathway. This turned out to be hard to pin down but I knew the Axoloti was the origin. Solution was two fold: add a thicker, better and shorter ground path for axoloti and all circuits connected; second, add a big fat poly-film 10 uF decoupling capacitor in parallel to 20 uF electrolytic, between my 5V rail and GND as well as another between my 12V rail and GND attached straight on the power connector immediately next to Axoloti. Having improved ground path and decouling locally, problem vanished! See pics for module.

This is actually not finished, I have since added several buttons and 4 gate outputs which act as a midi to gate and clock to gate/divider output. And this, this is magic. Axoloti is the holy grail of digital oscillators and eurorack paraphony.

cheers