Hum on audio out.

Discussion in 'AV Pre-Amp/Processors & Power Amps' started by RichD1, Nov 7, 2017.

Tags:
  1. RichD1

    RichD1
    Novice Member

    Joined:
    Nov 7, 2017
    Messages:
    5
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    1
    Location:
    Weymouth
    Ratings:
    +0
    Not sure which category to put this question in, so here goes.

    I want to take sound from the Avtex TV in my motorhome to the media player in the dash to utilise the better speakers.

    The Avtex has a coax digital output and I have bought a cheapy DAC from Amazon to convert the output so that I can plug in the AV 3 x RCA phono cables fitted to the motorhome which go up to the dash. When I got it all connected there was a discernible hum coming thro' the speakers. I tried another AV phono cable but still the same. The plugs mounted in the cheapy DAC were quite wobbly so I was thinking that these might be the problem so I returned the unit to Amazon and bought the more expensive FiiO DAC. This unit was of a much better build quality. Connected this up and still the hum was there, reading the manual the DAC wants to see PCM but the Avtex only outputs in a compressed Dobly digital. The sound seemed to be OK regardless but for the hum. I then decided to get my home hi-fi Cambridge DACmagic and try that. I thought I had solved the problem but when the volume was turned up very loud with no TV channel selected the hum was still there but much reduced. Guess the DACmagic has better input filtering.

    Having sat and thought about the problem I suddenly realised that I had to convert the 3 phono AV cable running from the TV to the dashboard into a 3.5mm jack to plug into the Kenwood media player. But I had used a 3 pin stereo rather than a 4 pin 3.5mm adapter and only plugged the red and white cables in!

    Would this cause the hum?

    Richard
     
  2. Mark.Yudkin

    Mark.Yudkin
    Well-known Member

    Joined:
    Sep 19, 2007
    Messages:
    10,059
    Products Owned:
    0
    Products Wanted:
    1
    Trophy Points:
    136
    Location:
    Zurich, CH
    Ratings:
    +1,548
    The hum is likely a ground look with the TV, or more likely a source (SKY installations in UK are a common cause of ground loop hums as the installer fails to ground properly).

    Remove all leads carrying signals going into the TV (this means your TV cannot be used to watch anything); does this fix the hum? If so, work out which source(s) cause the issue and post back. However if it's SKY, contact the installer to get it fixed.
     
  3. RichD1

    RichD1
    Novice Member

    Joined:
    Nov 7, 2017
    Messages:
    5
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    1
    Location:
    Weymouth
    Ratings:
    +0
    Hi Mark,

    This a 12v portable system mounted in my motorhome. Source is a roof mounted DVB-T aerial.

    I'm convinced it is a ground loop problem but I've now connected a wire between the chassis of the Kenwood media player mounted in the dashboard and the 0V DC supply to the TV. The hum does decrease slightly but not enough to make it listenable. Should I try linking the shielding on the phono connectors too?

    I also think that the output level of the headphone socket in the Avtex TV is not sufficient for the Kenwood AV input. The Kenwood has to be virtually on full volume to get a reasonable listening level from the speakers.

    The hum is level controllable on the Kenwood which suggests that the noise is coming from the TV end.

    Richard
     
  4. Mark.Yudkin

    Mark.Yudkin
    Well-known Member

    Joined:
    Sep 19, 2007
    Messages:
    10,059
    Products Owned:
    0
    Products Wanted:
    1
    Trophy Points:
    136
    Location:
    Zurich, CH
    Ratings:
    +1,548
    Automotive systems often have such issues, the resolution is to correctly ground the amplifier.

    Getting Rid of Ground Loops

    There is no shortage of isolators, but you need to know what you need for your kit.
    https://www.amazon.co.uk/Ground-Isolator-Stereo-Systems-Included-Black/dp/B01LYYRT7M
    https://www.amazon.co.uk/Behringer-MICROHD-Hum-Destroyer-HD400/dp/B000KUD2G4/

    When using the headphone output of a TV to drive the amplifier, you will always need to set it to (near) maximum volume, so that's normal.
     
  5. RichD1

    RichD1
    Novice Member

    Joined:
    Nov 7, 2017
    Messages:
    5
    Products Owned:
    0
    Products Wanted:
    0
    Trophy Points:
    1
    Location:
    Weymouth
    Ratings:
    +0
    I've now taken a wire from the chassis of the Kenwood to the 0V wire feeding the TV. I've also taken a wire from the 0V of the TV and held it around the shield side of the AV connectors that go to the dashboard. The hum reduces slightly and a bit more when touched on the AV connectors but is still audible as the volume of the Kenwood needs to be so high.

    Waiting for our local car audio shop to get some stocks of ground loop isolators. He said he is getting one that has some amplification in it which might be just the answer.

    Richard
     

Share This Page

Loading...