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

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@email-addr-hidden> 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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>
> 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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Received on Thu Dec 9 00:15:03 2010