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PCI audio mixer for hum-free line-in recording?

Discussion in 'Electronic Design' started by Jax, Feb 27, 2007.

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  1. Jax

    Jax Guest

    I run XP on a old VIA266A mobo + Duron 1800 MHz processor. I am in the


    I want to plug a line-input from a tap on my home telephone line or from
    my audio player which runs off the mains.

    I get a lot of hum plugging the line input into my mobo (Syntax SV266A
    with a VT8235 chip for AC'97). Probably is a ground loop.

    Earthing one end or another of the line input lead doesn't help much.
    I'm told that a good way to minimise hum is to use an isolating

    If I get a modest audio mixer card which fits into a PCI slot, then is
    it likely to have an isolating transformer (or some other hum isolating
    device) already built in?

    I am thinking of a card costing about 15 to 20 UK POUNDS (20 to 30
    DOLLARS). This is not trying to be audiophile but is for high quality
    speech files.
  2. It's very unlikely that a PC audio card would be transformer isolated.
    The general method of interfacing is:

    I suggest that you could use a 1:1 600 transformer, almost anything
    would do, I like OEP tranformers, more than adequate for the job

  3. Pen

    Pen Guest

    A modest card will not have a transformer on it. What you might be able
    to find is one with an electronically balanced input. This is done by
    usually by employing a differentially connected opamp.
    If you're getting hum from the telephone line then an isolation
    trasformer is not going to help much if at all as the noise is likely on
    the line.

  4. Definately Not recommended, you NEED transformer isolation.
    1) to protect your equipment from HV crap on the line
    2) to protect the Telco from you putting crap on the line

  5. You are unlikely to find an audio card OF ANY PRICE that includes
    transformer isolation of input or output. Easiest to use an external
    iso transformer device.
  6. bet Neve would make one.......

  7. bet they wouldn't....

    1) Not in the business of making computer plug-in cards.
    2) Wouldn't want the hassle of maintaining drivers, etc.
    3) Not enough height for any decent transformer.
    4) Hostile environment for high-quality audio circuitry.

    Likely other reasons, besides :)
  8. JAD

    JAD Guest

    What is the input level into the 'line' in?
    can you put a pot in between?
  9. Paul

    Paul Guest

    There is a nice long web page here, on interfacing. characteristics.htm

    And one sample device here:

    Search terms: "telephone audio to computer".

  10. Arny Krueger

    Arny Krueger Guest

    Use this with your existing card. Some plug adaptors may be needed.
  11. Geoff

    Geoff Guest

    There is 50VDCon telephone lines.

  12. DaveM

    DaveM Guest

    What Martin has suggested is absolutely the right (and probably the cheapest)
    way to eliminate the hum. The reason you're getting the hum is that your phone
    line is a balanced line, whereas the inputs on your sound card is unbalanced.
    When one side of your phone line is grounded (the shielded side of the sound
    card input), the line is unbalanced, and tremendous hum results.
    The best way to solve the problem is to keep the phone line balanced by use of a
    transformer as shown in the link, and let the transformer's secondary handle the
    unbalanced input on the sound card.
    You can use almost any 1:1 600-ohm transformer for the job... phone line
    fidelity isn't good enough to cause any worries about transformer quality.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Some days you're the dog, some days the hydrant.
  13. Mr.T

    Mr.T Guest

    3) required by law in many countries.

  14. Mr.T

    Mr.T Guest

    That old furphy!
    Funny that companies like Lynx can obtain PCI audio performance as good as
    Neve equipment then.

  15. Mr.T

    Mr.T Guest

    Whilst it may be perfectly adequate, I had to laugh at the listed "Tech

    Model 270-054
    Product Type Isolator
    Enclosure Color BLACK
    Body Material Multi

    Tells you all you need to know I guess.... it's BLACK :)

  16. Lynx doesn't have any transformers on their boards.

    Perhaps you have never seen a system with the slots right next
    to a whomping 400W switchmode power supply transformer.
  17. Mr.T

    Mr.T Guest

    No, they don't need them.
    In fact I've never seen one that didn't. The results still speak for
    themselves though.

  18. jakdedert

    jakdedert Guest

    Also need some kind of voltage limiting (back to back zeners?). If the
    phone rings while it's hooked up to the card....

  19. John A

    John A Guest

    You can see that the overwhelming, and correct, advice is to get a
    one-to-one isolating transformer and connect it between the telephone line
    and a PC audio input.

    However, when you connect it all up, you must connect a capacitor - say 0.1
    microfarad (aka 100nF) - in series with the transformer's connection to
    the telephone line.
  20. No, normal audio PCI cards have no isolation transformer.
    If you can find an antique telephone you can get the transformer from that,
    I also once designed some circuits with an optocoupler as isolation,
    worked very well actually.
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