[LAU] OT(ish): Connecting balanced out to line in

Ken Restivo ken at restivo.org
Wed Dec 8 20:18:31 UTC 2010


On Wed, Dec 08, 2010 at 05:00:34PM +0000, linuxdsp wrote:
> James Stone wrote:
>> On Wed, Dec 8, 2010 at 3:49 PM, Gabriel M. Beddingfield
>> <gabrbedd at gmail.com> wrote:
>>>
>>> On Wed, 8 Dec 2010, James Stone wrote:
>>>
>>>> I guess in hardware?
>>>>
>>>> Thinking more about it, do I need to plug the DI out into a preamp
>>>> before it goes into the line in for the soundcard? (the instrument is
>>>> a bass guitar unamplified).
>>> If you can, just skip the DI and plug the guitar straight into the sound
>>> card's line-in.
>>
>> I've tried that before, but (at least with an electric (non-bass)),
>> the sound was rather woolly and lacking in treble - I though due to
>> the mix of Hi-Z and line level input.
>>
>> James
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>> Linux-audio-user at lists.linuxaudio.org
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>>
>
> If you are connecting a guitar to a soundcard input you will almost  
> certainly need some kind of DI box to provide the necessary high  
> impedance input for the guitar otherwise you will get a dull "woolly"  
> sound - as you describe.
>
> A balanced line has three conductors instead of the usual two.  These  
> are the screen, and the two signal wires.  The signal wires carry the  
> audio signal, but in opposite polarity.
>
> In a conventional balanced input stage, the two signal inputs are  
> subtracted from each other - and being of opposite polarity, the result  
> is the original audio.  However, the important advantage of this method  
> is that any noise or interference induced on the wire(s) will be of the  
> same polarity on both signal conductors (in theory) and so will be  
> cancelled out (this is what common-mode rejection is all about).
>
> You can get a signal for your single-ended input to the soundcard  
> between either signal phase and the screen.  To avoid any phase  
> inversions, connect the positive signal out from the DI box (XLR pin 2)  
> to the signal in on your sound card input (Tip of the jack connector),  
> and the screen (XLR pin 1 ) to the ground on the soundcard input (Body  
> of the jack connector).
>
> This is not the ideal way to convert between balanced and unbalanced  
> signals, but it will provide a working solution.  There's some more info  
> here:
>
> http://www.rane.com/note110.html
>
> As for whether the signal level will need more amplification, that  
> depends on the DI box.  If you can feed a line level mixer input, then  
> probably not, but try it and see.
>


I have a slightly different question: I need to convert a non-balanced consumer-grade headphone input (from a laptop or iPod), to a non-balanced consumer-grade amplifier (Cambridge SoundWorks).... but with complete ground isolation. I'm guessing a simple transformer should do the trick, but the question is: WHICH transformer? What should the specs be in order to get the impedance right for both sides?

-ken


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