Neophyte questions re: selecting an audio interface - Linux-audio-user

7 Jan 2012

On 01/07/2012 01:05 AM, linux-audio-user-request(a)lists.linuxaudio.org wrote:
...
  On Fri, Jan 06, 2012 at 10:50:12AM -0500, Chris
Metzler wrote:
  >  1.  Some PCI interfaces (like the Delta
1010LT) have RCA jacks rather
 >  than 1/4".  What about issues relating to noise/degradation introduced
 >  by a 1/4"-to-RCA adapter?  I've read elsewhere (on the M-Audio forums)
 >  of people complaining about unpleasant noise with guitars when using an
 >  adapter that's then plugged into an RCA plug on the Delta 1010LT. 
Adapters don't normally introduce noise or degradation. But they
 are often used to connect things that are not compatible anyway,
 and they tend to fail (bad contacts) after some time. Try to avoid
 them, they are in general a nuisance.

 RCA inputs are 'consumer line level' and unbalanced, which is
 a disadvantage.

 Line inputs are not meant for guitars, so those people who
 are complaining get what they ask for.

  >  2.  Some interfaces seem to have a
combination jack that allows XLR and
 >  1/4" inputs.  Are these good?  Or is there a tradeoff between
 >  convenience and sound quality?  Sound quality is not determined by the
connector. But the use of
 such connectors can indicate 'budget quality'. Very often they are
 used as a combined mic/line input (mic = XLR, line = TRS). Then
 it depends: if the line input is just an attenuator feeding the
 mic input that's not really very good. A real line input does not
 pass via the mic preamp.

  >  3. ... Put another way, if you have the
chance to pick up at a
 >  really good price Z-Audio's UberSuperDuperInterface, with a gazillion
 >  ins/outs and a lot of features, and right now you only need a small
 >  fraction of what it can do, are there non-cost reasons why you
 >  *shouldn't*  get it?  No.

  >  a)  Guitar cables typically use a TS
connector while line-level devices
 >  (analog outs on drum machines, keyboards, etc.) typically use TRS,
 >  right?  I'm presuming there's a difference in voltage range as well
 >  (hence why guitars aren't referred to as line-level)?  Guitars are not
line level, they produce a much weaker signal and
 need a preamplifier with a high input impedance. Some sound cards
 have 1 or 2 inputs that can be switched for use with guitars.
 Without those you need a DI box.

  >  b)  Am I correct in thinking that a cable
with TS connectors is
 >  unbalanced and a cable with TRS connectors is balanced?  A cable with TRS
connectors*can*  be balanced, one with TS
 definitely is not balanced.

  >  c)  My understanding is that in order to
plug a guitar into a balanced
 >  input, there needs to be a DI box in-between to change the impedance.
 >  But in advertising blurbs, some 1/4" inputs on interfaces are billed as
 >  accepting balanced or unbalanced input. Does that mean that both
 >  guitars and line-level devices can be plugged into them?  Any balanced input
will accept balanced and unbalanced signals.

  >  If an interface only has mic/line jacks,
does that mean you*must*
 >  get a DI box to plug in a guitar?  Yes, unless the line input can be switched
to high impedance and has
 enough sensitivity. As already mentioned, some interfaces have such
 inputs. But you can't use a standard mic or line input for a guitar.

  >  d)  Since guitars aren't at line level,
do they need preamplification
 >  like microphones do?  Yes. Normally the DI box is connected to a microphone
input.

  >  Or if an interface has unbalanced inputs,
can you just plug a
 >  guitar in and go?  In most cases not, see above.

 Some other things to consider:

 Usually you get what you pay for. Quality is not only sound
 quality, but also construction and reliability, and it has
 a price.

 If you are a musician, consider the combination of a simple
 but good quality soundcard having only balanced line inputs
 and outputs (even fixed level) with one the many small
 musician's mixers. These usually have mic and guitar inputs,
 a headphone output etc. Such a combination is much more
 flexible than any soundcard.

 Ciao,

 -- FA 
Fons, interesting view. Could you give specific examples of such 
soundcards and small musician mixers?

Regards,
Dirk