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PI, DAC, Amp earth issue (not Moode)
#1
Hello

I'm not sure where else to ask this question as it's not a Moode problem.
I have a PI 3B (running latest Moode) connected to an old NAD amplifier via a USB DAC. The pi has, connected via GPIO, an Adafruit OLED screen (SPI) and a on/off button that includes an LED.

LED setup - https://howchoo.com/g/ytzjyzy4m2e/build-...-indicator
Button setup - https://howchoo.com/g/mwnlytk3zmm/how-to...spberry-pi

My problem is that the amp turns the pi on/ off. I turn the amp on the pi turns off. I turn the amp off the pi turns on. I think the pi is receiving a power spike from the amp.
I cant figure this out. I think the pi and/ or the DAC is 'floating' but i don't know how to weigh them down.

Any help with this would be much appreciated.
Thank you

Paul
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#2
I had a related issue with my set-up - using the Pi to turn my amp on and off (intentionally) via a GPIO pin + relay. During testing I powered the Pi with its own DC supply (isolated) and connected the GPIO to a relay inside the amp (also isolated). This worked.

However, when connecting the audio things went downhill. The amp ground (earthed) and the floating Pi ground were connected. Lots of noise brown-outs etc etc.

My situation was complicated as I wanted the amp to power the Pi and have a single umbilical carrying audio and power from amp to Pi. Attached is the topology.

The only way to fix this, that I could see, was to isolate the audio signal using a pair of transformers. I made my own PCB for this, but the function is the same as: https://alexnld.com/product/car-rca-ampl...uppressor/

So either your ground loop is causing issues, or when the amp is on the power supply can't supply the necessary voltage and current to the Pi.

What I learnt was that not all USB cables are equal and not all power supplies actually deliver. I had to make my own umbilical with thicker gauge wire to avoid additional voltage drop and made it as short as possible.

Tristan


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#3
(04-06-2020, 09:52 PM)Baisey Wrote: Hello

I'm not sure where else to ask this question as it's not a Moode problem.
I have a PI 3B (running latest Moode) connected to an old NAD amplifier via a USB DAC. The pi has, connected via GPIO, an Adafruit OLED screen (SPI) and a on/off button that includes an LED.

LED setup - https://howchoo.com/g/ytzjyzy4m2e/build-...-indicator
Button setup - https://howchoo.com/g/mwnlytk3zmm/how-to...spberry-pi

My problem is that the amp turns the pi on/ off. I turn the amp on the pi turns off. I turn the amp off the pi turns on. I think the pi is receiving a power spike from the amp.
I cant figure this out. I think the pi and/ or the DAC is 'floating' but i don't know how to weigh them down.

Any help with this would be much appreciated.
Thank you

Paul

Hi Paul, it might be worth trying to break the common earth by using a galvanic isolator, now this isn't a true device as it doesn't use transformers but its worth a try:
Audioquest Jtterbug £36.00

A lot cheaper! £15.99
Nobsound ha!
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#4
Hello

Thank you both for your replies.
I noticed that if i moved the PI and the button away from the amp the problem didnt occur. 
I had the button on quite a long bit of wire (30cm+) so i cut it right down to <5cm, refitted it and that solved the problem.
I think the button wire was acting as some sort of ariel and was picking up interference when the amp was turned on/ off.
Lesson learnt - keep wires short.

Cheers
Paul
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