I am using FreeBSD 12.1 on a number of computers. Recently I started to
use a mini audio amplifier which is powered by a USB port and receives
the audio signal through the 3.5mm socket on the motherboard.
I can hear a noise through the loudspekers (possibly 50 Hz) as soon as I
plug in the USB connector for power. It appears that the noise is of a different magnitude, depending on the motherboard. The noise also seems
to be connected with the activity of the hard disk.

Is there an explanation for this?

Thank you / Alistair

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On Sat, 08 Aug 2020 21:11:42 +0000, alistair wrote:

I am using FreeBSD 12.1 on a number of computers. Recently I started to
use a mini audio amplifier which is powered by a USB port and receives
the audio signal through the 3.5mm socket on the motherboard.
I can hear a noise through the loudspekers (possibly 50 Hz) as soon as I
plug in the USB connector for power. It appears that the noise is of a different magnitude, depending on the motherboard. The noise also seems
to be connected with the activity of the hard disk.

Is there an explanation for this?

Thank you / Alistair

The explanation is the the DC out from the USB doesn't need to be
100% "clean" to power devices. Except in your case, your mini USB
amplifier clearly doesn't take this into account and has no noise
filters on the DC power input circuitry and the noise on the power
line is hitting the amplifier and then the speakers. The fact it
happens on multiple different motherboards indicates that 99% of
the time it's not an issue and so the motherboard manufacturers
don't spend money cleaning the noise from the DC output line on
the USB ports. If a device needs clean DC power, the device is
responsible for making sure that's what it gets. I'm guessing your
amplifier is a cheap no-name device?

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On Sun, 09 Aug 2020 17:10:48 +0000, NoOne wrote:

On Sat, 08 Aug 2020 21:11:42 +0000, alistair wrote:

I am using FreeBSD 12.1 on a number of computers. Recently I started to
use a mini audio amplifier which is powered by a USB port and receives
the audio signal through the 3.5mm socket on the motherboard.
I can hear a noise through the loudspekers (possibly 50 Hz) as soon as
I plug in the USB connector for power. It appears that the noise is of
a different magnitude, depending on the motherboard. The noise also
seems to be connected with the activity of the hard disk.

Is there an explanation for this?

Thank you / Alistair

The explanation is the the DC out from the USB doesn't need to be 100% "clean" to power devices. Except in your case, your mini USB amplifier clearly doesn't take this into account and has no noise filters on the
DC power input circuitry and the noise on the power line is hitting the amplifier and then the speakers. The fact it happens on multiple
different motherboards indicates that 99% of the time it's not an issue
and so the motherboard manufacturers don't spend money cleaning the
noise from the DC output line on the USB ports. If a device needs clean
DC power, the device is responsible for making sure that's what it gets.
I'm guessing your amplifier is a cheap no-name device?

Thank you for your remarks. Indeed, the amplifier is a very cheap no-name device, not much filtering going on there. I tried a capacitor across the
power connections and it helped. Maybe my mentioning of 50 Hz is not
correct: it is rather more, possibly it could be the polling frequency of
the USB socket (1000Hz)? Be it as it may, I'll connect the amplifier
directly to a 5V outlet on the motherboard which should help. The PSU is
a branded one. I had a very basic PSU on the board before which caused
some noise extra through the amplifier as well. I think I can treat this
issue as solved.

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