Author Topic: Power supply via USB instead of the DC barrel  (Read 1169 times)

Capjona

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Power supply via USB instead of the DC barrel
« on: January 03, 2022, 08:04:57 AM »
Hi all!

I am planning on using the Pi-Stomp together with my Rockboard ISO Power Block V16. It has a 5V 1A (5 watt) USB output and I was wondering if i could use it to power the Pi Stomp. I know that tests showes that the Rpi 3A+ uses something allong the lines of 3.5/4.5 watt (or 400 ma) at full load.

So my question is; is it possible to power the Pi-Stomp via the micro USB port on the pi instead of the DC barrel jack on the PCB?

Thanks in advance :)

Randall (Admin)

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Re: Power supply via USB instead of the DC barrel
« Reply #1 on: January 05, 2022, 04:15:02 PM »
Yes, you can definitely power it via the micro USB which is rated at at least 3A.  No changes required, but if that's your plan for powering it, I'd suggest not even including the barrel jack.  Very bad things could happen if it was powered from both simultaneously.

Regarding how much pi-Stomp requires, I've never seen it draw more than about 250mA.  Thus the fuse I selected for the barrel jack is 1.2Amp hold, 2A trip. That's way more conservative than the polyfuse onboard the RPi 3A+ which holds at 2.6A and trips at 5.2A.   

Venya

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Re: Power supply via USB instead of the DC barrel
« Reply #2 on: January 13, 2022, 05:07:12 PM »
One caveat for people who may try this: make sure you have a very clean source of USB power, or else you may get all kinds of interesting noise in your signal chain. The difference between using the included power supply and using the USB hub I had handy because I was impatient and there were no convenient outlets nearby was... profound.

Randall (Admin)

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Re: Power supply via USB instead of the DC barrel
« Reply #3 on: January 13, 2022, 06:02:46 PM »
Good point Venya.

Yes, please use the included power supply unless you absolutely can't. 

Although it pains me that purchase of a pi-Stomp means adding another wall wart to your collection, the supply choice was mainly based on it's noise specs.  USB power and switch mode power is just inherently noisy.  And the Raspberry Pi itself leaks digital noise onto the power rail.  It's not the ideal power to use for audio, but with the included power supply and the extra regulation and filtering, the noise floor should be tolerable.  If not, please let me know what you're hearing.

ReneMadeira

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Re: Power supply via USB instead of the DC barrel
« Reply #4 on: February 08, 2022, 02:37:00 PM »
Hello!

I'd like to add something to this thread.
I still haven't assembled my kit in an enclosure, it is on top of a table + breadboard.

Despite having the psu provided in the kit, I don't really use it as it would require an extra adapter to plug into the wall socket which I'd rather avoid (I believe that is the case for almost anyone outside the US).

I'm still thinking about the psu because I'm also building a mic preamp to install inside the pi-stomp, and the power requirements will be different.

I will buy a reliable psu according to my needs, but at the moment I'm using a usb type C psu from the raspberry pi 4B, connected directly to the pi.

Connected to the raspberry pi 4B power input, it behaves normally, without any noise, until a random moment when a whining starts.
The noise doesn't stop even if I reboot or unplug power and wait to turn it back on. However, it's not often, it only happened a few times.

After trying everything, the only solution was to power it from the pi-stomp power input. Note that it's the same psu, only plugged in the pi-stomp board instead of the pi power input. (I'm using a pi 4b, so I didn't solder the power input jack since it would be misaligned with the enclosure, therefore I can use any psu)

After powering from the pi-stomp board, I can disconnect and connect to the pi again, and the noise is gone.

I can't explain why this happens, but thought it was important to share with you so you may think twice about powering from the pi if the same happens to you.
« Last Edit: February 08, 2022, 02:39:03 PM by ReneMadeira »

Randall (Admin)

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Re: Power supply via USB instead of the DC barrel
« Reply #5 on: February 08, 2022, 03:14:00 PM »
Astute observation.  I had noticed that behavior as well during v1 development.  That's one of the reasons the kits include a power supply and attach via the barrel connector.

Although the 5v ultimately gets fed to the same places via the barrel vs the pi-USB, the path taken to get to each component is different.


USB > Pi > Audio Card
         > Pi-Stomp board > regulator/filter > input buffer

Barrel > Pi
       > Audio Card
       > pi-Stomp board > regulator/filter > input buffer



See the difference?  From USB, the power goes through the Pi before it forks off to the audio components.  From the barrel, each component gets it's own fork of the power line.  Much less likely to pass noise to another component that way.

If you do go with a different supply, I'd highly recommend that it have noise specs equal to the supplied one:
https://www.mouser.com/datasheet/2/75/SPEC_TRG15_VI_V19-1947990.pdf

50mVpp ripple, 1% Line Regulation, 4% Load regulation.  Supplies that don't publish noise specs are usually fairly noisy.  Low noise doesn't happen by accident, it's designed in.

I also wouldn't recommend higher voltage (eg. 9v) then bucked or regulated down to 5v.  A buck converter will almost for sure add noise.  9v to 5v regulation at 2Amps will waste a lot of power to heat.  I'd instead use a rail-to-rail op-amp like the buffer on the pi-Stomp, or if you've gotta have the headroom of higher voltage, use a 9v battery to power your mic pre.

ReneMadeira

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Re: Power supply via USB instead of the DC barrel
« Reply #6 on: February 09, 2022, 10:26:53 AM »
I also wouldn't recommend higher voltage (eg. 9v) then bucked or regulated down to 5v.

That's exactly was I was planning to do. :-[

I have an adjustable buck down regulator board based in MP1584, with specs of 30mVpp ripple and 92% efficiency.
I'm very concerned about noise and heat, and even though the specs tell me not to worry about it, I'll test everything before putting it in an enclosure.
My preamp design is based on the INA 217 "famous $5 preamp", so the power requirements are simmetrical +/-12 to 18V. Powering with batteries is the original design, but it would require 4x 9V batteries, which I would not initially consider in my design.
The preamp will also suffer from noise so I have to be very cautious in that direction. However, I don't plan to use it in recordings, so I will tolerate some noise as long as it doesn't hurt my ears when amplified.

Let''s see how it ends.

I'm all ears to your suggestions.

Thank you!

Randall (Admin)

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Re: Power supply via USB instead of the DC barrel
« Reply #7 on: February 09, 2022, 11:06:19 PM »
Well, 30mVpp of ripple, and 1.3nV noise density on the op-amp seem impressive so maybe you'll be fine.  Only other suggestion I have is to pay attention to your grounding.  Specifically, I found using the pi ground for the pi-Stomp buffer introduced significant noise.  So I changed the buffer to use the audio card input ground as the ground reference and that reduced noise to a decent level.  Yes, the pi ground and the audio card ground are at the same potential, but just like where you tap the power can affect noise, so can where you tap your ground.

I'll be very curious to hear how it works out for you.  I know other pi-Stompers have wanted mic inputs as well.  I've just had to tell them to use an external mic pre.