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  • noisy USB

    From alistair@21:1/5 to All on Sat Aug 8 21:11:42 2020
    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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  • From NoOne@21:1/5 to alistair on Sun Aug 9 17:10:48 2020
    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?

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From alistair@21:1/5 to NoOne on Mon Aug 10 22:05:05 2020
    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.

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)