Avoiding ground loops.

Discussion in 'The Projects Forum' started by MattP, Apr 2, 2014.

  1. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    I built a microphone preamp for my computer (a kit preamp), and I've been powering it by USB. Because its output goes into the computer's mic socket, which shares the same ground as USB, a ground loop is created.

    To rectify it I put a ground loop isolator between the output of the pre and the input of the computer. Alternatively I guess I could have used one of these instead if I wanted the signal to remain unaltered:

    http://www.ebay.co.uk/itm/360860072660


    Right, so let's say I wanted to power an amplifier with the PC's PSU, as well as receive its signal from the soundcard. There'd be a ground loop of course, so I'd either have to use a ground loop isolator (slightly degrading the signal), or find a way of isolating the power for the amp (like in the above link). The latter isn't really practical because of the power requirements.


    SO this leads me to my main question! How do you design an amplifier, pre or normal, that won't be affected by ground loops? I know it's possible because I've salvaged a few small amps designed for Ericsson mobile phones that are both powered by and receive the signal from a common ground (the phone). Or maybe these little amplifiers have DC isolation ICs on them?

    Thanks in advance - this is something I've wondered for a long time!
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,387
    497
    For some reason I thought that computer sound cards have amplifier on the mic inputs.

    I mean the normal sound card inputs are:
    * Mic
    * Line In
    I think one of them has amplifier, but I don't remember off hand which.
     
  3. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    It's the mic input that SHOULD have a preamp, but they often don't and just rely on boosting the gain with the DAC. This means that if you just plug a mic in you'll get noticeable hiss in the background (even on stupidly expensive cards like the Sound Blaster ZXR which I have), meaning that it's beneficial to amplify your mic signal before it goes into the mic-input.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    The line input is for signal input around 1 volt peak peak (or higher). The mic input is for lower voltage levels that would typical come out from a microphone.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,517
    2,369
    What is Your definition of a ground loop?
    In my area of expertise, it is when a potential exists between two 'Common' points which otherwise are considered essentially the same potential.
    The PC P.S. (unless a laptop) is connected to earth ground inside the PC, so hence the USB common is also presumably at this point.
    So any equipment that attempts to eradicate ground loops will be designed to so that all common points are electrically the same, to me this would involve making sure that the USB common and the common point of the mic socket are electrically the same. i.e. bonded to one another by either an external connection or a large ground plane of a PCCT.
    Max.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,528
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    First, while you said a ground loop is created, you didn't say if this loop affected the sound quality. Is there a real problem to be solved here?

    Second, just because two things are grounded at different places does not mean automatically that the two grounds ae at a different enough potential to be something worth worrying about.

    If there is a loop to be broken, do it by not connecting the USB ground to the preamp. The preamp now is a 3-wire connection to the PC, and only one of them is GND:

    +5 from USB connector
    Audio out to input connector
    GND to input connector

    Yes, power supply current now is running through a signal ground. Auntie Em! Auntie Em!!! But how much of that is there really, a few mA?

    Filter the gorp from the +5 before it powers the preamp to remove any noise that might be audible or cause aliasing problems for the ADC. For extra power supply rejection, use an opamp for the preamp.

    ak
     
  7. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Well, when I said a ground loop was created I mean that a significant amount of noise is introduced to the signal unless either the power is isolated or the signal is isolated. I wouldn't care if there weren't any affects, but the noise is significant, so I'm wondering how to design a circuit that won't be affected by this issue without having to resort to isolation.

    Quite interesting what you say about only connecting one ground. I'll give that a go and see if it gets rid of the noise, but I'm fairly sure I've tried it before and it didn't fix it.
     
  8. GRNDPNDR

    Member

    Mar 1, 2012
    435
    7
    use one of these, They isolate the power source.
    There are a bunch of different types of them but they might just work if you can work it into your circuit.
    http://www.datasheetcatalog.com/datasheets_pdf/N/D/Y/1/NDY1212.shtml

    I had to use one of these to power a small voltmeter display that couldn't receive power from the same source it was measuring, so these little devices solved the problem.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,528
    1,248
    Actually, I have used one of those, and the single-inline version mentioned before, both isolated and non-isolated. For a DVM, sure, but not for audio. Those little critters have relatively noisy outputs, and usually require an external L-C output filter for sensitive work - as indicated on page 3 of the data sheet. I recommend doing just about anything to avoide adding another switching power supply to your noise cloud.

    Another problem with the smaller DC/DCs is that they are terrible at isolating common mode noise on the input, and relatively poor at reducing input ripple, so they might not solve the problem, or replace it with another one. I think a good place to start is with an L-C filter - and nothing else. Filter the noise with a differential or common mode choke and a few caps. The filter might be bigger than your preamp, but thems the breaks. I've done a few of these to keep 12V fan noise off of a 12V powered signal backplane.

    And about that preamp. I forgot to ask questions. Such as

    What kind of ground loop isolator have you been using?
    What is the preamp kit? Gain, circuit type, make and model, etc?

    ak
     
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