DI vs line input

[pianoman]

I'd like to learn about DI vs line input.

I can plug an instrument (e.g. electric guitar or bass or keyboard) into a "DI" input (I have only one) of my preamp, or I can take a 1/4" to XLR connector and connect to the line input of my preamp. My limited understanding is that there will be differences in the sound due to the impedance differences. I wonder if there's increased noise as well due to impedance mismatch? Which could potentially affect a mix?

Will any decent passive DI box solve this problem? I.e. instrument into passive DI into preamp?

As an aside, for you audio electronics experts, when impedances are not matched, I get the concept of reflections from a power coupling/loss perspective but don't understand the frequency dependence of an impedance which I thought has a solely real component.

I hope this isn't a "stupid" question.

[Remoteness]

Ah, the old DI vs. line input concept…

Well, a DI takes an instrument’s high impedance (HiZ) output and turns it into a microphone low level signal so you can plug it into the preamps microphone input.

Some mic pres provide instrument level inputs so you can just plug your rig right into the pre without the need of a DI. Some pre amps have a variable switched input with a few impedance options.

Depending on the impedance you could take an adaptor cable and plug your instrument right into the preamp.

I trust our more technical gurus will chime in with more comprehensive details.

Remember, there are no "stupid" questions on this forum, just “stupid” answers.

[Student]

An other thing to consider is that on large format desks the mic. preamps may be of a better or more pleasing quality than the line amp.

On a smaller digital recorder like a Boss or KORG portable digital recorder, the quality of the pres in comparison to one another are irrelevant and either may be pretty clean.

Impedance mismatching is kind of up to the user, the English used to wear white lab coats and looked down upon impedance mismatching. Here in the U.S. people were more relaxed about the matter and did tend to experiment.

As far as DI vs. Line...they may be equally qualified for the task at hand, some old punk music was recorded direct into the mixing board, Kurt Cobain did it on a song. Impedance mismatch may cause an amout of harmonic distortion, that may be inaudble, or unbearable.

But I digress

It's a general rule that the most efficient exchange of signal between to objects happens when their impedances are closely matched.

[pkautzsch]

Originally Posted by Student

It's a general rule that the most efficient exchange of signal between to objects happens when their impedances are closely matched.

If you mean "input impedance = source impedance", then you're wrong. That would mean precisely 6dB of signal loss (or: 6dB more noise). The only reason to use that would be *very* long cable runs (talking a few kilometres or more here) when reflections become an issue. Phone lines, for example.
In recording, we need voltage (U), not power (P). This is achieved by having the input impedance at least 10 times the source impedance, preferrably more. A typical SDC mic has somewhat like 35 ohms, a typical mic pre has about 5k.

To the OP: Both versions have been done. A keyboard usually would have line level outputs, so it would rather go to the line input. In the end, it's the old story: If it sounds good, it is good.