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noise from usb dac after powering down
#1
i recently installed moode on a raspberry pi zero w to build a internet radio device for my stereo system.

installation and setup went quite well despite me being a novice.

for sound output i use a usb powered dac.

i have one small problem with the system though.
after powering down the unit via the shutdown command, the dac still makes some noise.
(i sound like a loud hum)
it seems the dac is still supplied with some power.

is there a way to get rid of this ?

thanks in advance and best regards
Chris
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#2
(05-03-2020, 05:40 PM)shadowlord Wrote: i recently installed moode on a raspberry pi zero w to build a internet radio device for my stereo system.

installation and setup went quite well despite me being a novice.

for sound output i use a usb powered dac.

i have one small problem with the system though.
after powering down the unit via the shutdown command, the dac still makes some noise.
(i sound like a loud hum)
it seems the dac is still supplied with some power.

is there a way to get rid of this ?

thanks in advance and best regards
Chris

Hi, Chris.

The shutdown command doesn't shut off power, it puts the Raspberry Pi CPU into a "shutdown/halt" state but power to the USB ports is still live.

Hearing a loud hum, however, may also be indicative of a ground loop problem. 

Regards,
Kent
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#3
Well it isn’t really a hum (the noise is hard to describe)
And it’s not there when the unit is normally operating.
Just after shutdown the noise starts. It‘s about normal listening volume ( not loud but not faint either)
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#4
(05-04-2020, 10:57 AM)shadowlord Wrote: Well it isn’t really a hum (the noise is hard to describe)
And it’s not there when the unit is normally operating.
Just after shutdown the noise starts. It‘s about normal listening volume ( not loud but not faint either)

+1 on the probability of a ground-loop

I'm starting off under the presumption that you are listening to RPI/USB DAC combo through an amplifier and speakers. So:

- Make sure that the 'shutdown' is followed by 'powerdown' (turn the RPI power supply OFF). Is the 'noise' still there?
- If the noise is still there unplug the DAC from the RPi... as the DAC is powered by the RPi the noise should disappear
- No noise... it is very likely that the noise comes from the switch-mode power supply that you are probably using to power up the RPi
- Borrow/buy a 5V regulated power supply (something like this JAMECO  or AUDIOPHONICS ) or a 'ground loop' eliminator ( AMAZON  or CHOICE )  

A silly question but I have to ask it... If the USB DAC is removed (unplugged from the amp) has the noise disappeared (provided that the amp's volume setting is at the same level  as before)?
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#5
i tried 3 different power supplies with no success so far.
the noise starts during the shutdown process and remains as long as the psu is connected.
( the noise is not there during normal operation)

i assume it has something to do with that particular usb dac, since i don't have the problem with another usb dac i tried.
the dac in question is an older PCM2704 based design from xd218.com

the noise itself is not really a 60hz ground hum. it sound more like digital noise.

the rest of the chain has no influence. (tried it on the stereo system and with battery powered headphone amp)

to me it seems while the raspberry pi is shut down, power is still supplied to the dac causing it to generate that noise.
don't know why though and how to get rid of it.
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#6
Have you tried a in-line power switch between power supply and Pi?
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#7
(05-05-2020, 05:40 PM)shadowlord Wrote: i tried 3 different power supplies with no success so far.
the noise starts during the shutdown process and remains as long as the psu is connected.
( the noise is not there during normal operation)
................snip......................

What is stopping you to switch the power supply OFF...?
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#8
small update to my problem:

since i wanted to be able to make use of the automatic shutdown at specific times, i unsoldered the xd218 dac and used a hifime one instead.
as i wrote earlier this dac does not output any noise when the pi is powered down.
but i found another strange behaviour with this other dac during shutdown state of the pi.
this dac does have an optical digital out in addition to its analogue output.
now when the pi powers down, this optical output often stays active (which is clearly visibly by it'S red light)
the funny thing is that this does not occur every shutdown. ( id say at 6 of 10 shutdowns the digital out remains active)

to me it seems it has something to do at how the pi shuts down, that sometimes part of the usb dacs remain active.

anyone has any idea what might be wrong here.

(i'm not worried by the active digital out, but clearly something is not right with the shutdown process )
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#9
This hum problem is is an issue your dac, my suggestion is replace the DAC or don't power your pi off.
The Raspberry Pi is an extremely low power device, you won't be making any noticeable cost savings on your electricity bill by powering off the Pi when not in use so just leave it on.
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