Re: [LAU] Neophyte questions re: selecting an audio interface

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To: <linux-audio-user@...>
Date: Sunday, January 8, 2012 - 11:43 am

On Sat, Jan 07, 2012 at 08:11:09PM -0500, Chris Metzler wrote:

> How do you find out? This doesn't seem like something they'd go out of

Indeed they don't. OTOH, some very good equipment does
share the mic preamp for the line input. But doing this
well is not as simple as just sticking a few resistors
in front of the mic preamp.

> >> d) Since guitars aren't at line level, do they need

A magnetic pickup requires an input impedance of a few hundred
kilo-ohms to work well. A piezo pickup as used for acoustic
guitars needs at least a mega-ohm. There are in fact 3 types
of DI box:

1. Passive. This is just a transformer. To provide the required
high input impedance it needs quite a high ratio, meaning that
the output signal voltage will be 20 to 30 dB lower than the
input. So you need quite a sensitive mic input for those, and
you may get some noise added as a result.

2. Active, but no gain. The amplifier just ensures that the
input impedance is high and the output impedance is low
enough for a mic input. Most DI boxes are of this type.

3. As (2), but with voltage gain. These could be used to
drive a line input.

Since an active DI box needs power, and mic inputs can
provide phantom power while line inputs don't, most DI
boxes are designed to be used with mic inputs, i.e. the
second type above.

> > Usually you get what you pay for. Quality is not only sound

Most reviews on the web are quite useless. To get an idea of
how much you can trust a manufacturer have a look at the specs
of his product. Incomplete or no specs are a sure sign of
trouble. For example, if you read something like 'Signal to
noise ratio XX dB', that can mean all sorts of things. To be
valid the specs need to mention how and in what conditions this
was measured. Without such information the spec is essentially

To get an idea of how welll something is constucted there is
no substitute for actually taking it in your hands.

> > If you are a musician, consider the combination of a simple

If your sound card is just stereo you are limited to two tracks
of simultaneous recording anyway. In the other case (e.g. an 8
channel soundcard) you need a mixer that supports this. That
doesn't mean it needs 8 groups. Many mixers have per channel
direct outputs or insert points which you can use for multitrack
recording. It all depends on what you actually want to do...

One other point: 'digital gain' is no substitute for correct
analog levels. If your signal is -40 dB on the AD converter
you can boost it digitally to normal level. But you also
amplify any noise and interference. In some cases it may work
but don't count on it.



Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.

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Messages in current thread:
Re: [LAU] Neophyte questions re: selecting an audio interface, Fons Adriaensen, (Sun Jan 8, 11:43 am)
Re: [LAU] Neophyte questions re: selecting an audio interface, Ricardus Vincente, (Sun Jan 8, 1:01 am)