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
> their way to tell you in the product documentation!
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
> >> preamplification like microphones do?
> >
> > Yes. Normally the DI box is connected to a microphone input.
>
> Sorry, does that mean that the levels of the guitar signal upon output
> from the DI box are comparable to that of a mic, and thus a mic preamp
> associated with a microphone input will do the job? I had it in my
> head that you used either (an active DI) or (a passive DI + specialized
> guitar preamp) to bring the guitar up to line level, and then plugged
> into a line input. If I'm understanding you correctly, another option
> is to use a passive DI and plug into a microphone input (assuming the
> interface has one). Is that correct?
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
> > quality, but also construction and reliability, and it has
> > a price.
>
> Are there particular ways you recommend to find out about that, other
> than trolling the list archives for positive/negative comments? I've
> had mixed results with trusting customer reviews on vendor sites,
> because even if honest, those reviews tend to be posted very soon after
> purchase, when what I want is to see how people feel after more
> extended use.
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
useless.
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
> > 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.
>
> Can you elaborate on this a bit more? The mixers I'm familiar with
> have a limited number of outputs (since the point of the mixer is to
> mix down the signals it's receiving); so if I intended to record
> multiple channels simultaneously, I'd lose some post-recording
> flexibility (like the ability to manipulate tracks independently in a
> DAW) by mixing down before passing the signal to the audio interface.
> Or maybe you're not suggesting using the mixer *as a mixer*, but rather
> as a mic preamp or guitar preamp for just one of my analog signals
> before passing that one signal to the interface? Or do I still not
> follow you?
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.
Ciao,
-- FA Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl. _______________________________________________ Linux-audio-user mailing list Linux-audio-user@email-addr-hidden http://lists.linuxaudio.org/listinfo/linux-audio-userReceived on Sun Jan 8 16:15:01 2012
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