DI box vs. Preamp

[rkmase]

Are these the same things?

From what I've read, all active DI boxes contain a preamp, and it seems that most units labeled "preamps" have DI box functionality.

Is there any real difference in basic features of a DI box vs. preamps? Now, I know depending on price, bells and whistles will change the variables a bit, so i'm just wondering about the basics.

Also, on the subject of DI boxes, what does that little ground-lift button actually do? I understand it reduces hum in some way - but could anyone explain how it does so in laymans? Wiki and google had nothing.


Thanks!

 

[Farview]

A DI box takes a line level high impedance signal and turns it into a low impedance lower level signal that a mic pre can deal with. A DI box will have parallel inputs so you can plug the guitar into it and have the signal go to your amp as well as going to the preamp. DI boxes are useless for mics.

Mic Preamps are designed to take a mic level signal and bring it up to line level. Preamps that have DI inputs generally do not have the parallel feed for your amp. The DI input on a preamp fits into the 'bells and whistles' catagory.

There is a difference between a preamp with a 'line input' and a preamp with a 'DI' input. The difference is the impedance that the inputs expect.

The ground lift switch does just what it says. It lifts the ground, it disconnects the ground between the input and the output. The gets rid of ground loops.

 

[rkmase]

Why are DI boxes useless for mic's? According to the all knowing wikipedia "An active DI unit contains a preamplifier". Wouldnt this bring the mic level up to line, or is there something else a mic preamp does that the DI box does not?

Thanks for the reply - lots of good info.

 

[Farview]

No. It isn't a pre-amplifier, it's an opamp used to match levels and impedances. A marshall amp has a preamplifier, you wouldn't plug a mic into it.

Anyway, passive DI boxes are nothing but a transformer. The only reason to use an active DI is so you don't load down passive pickups.

 

[rkmase]

so, just to clairify:

mic > mic preamp > soundcard

but if you want to go direct

guitar > di box > preamp >soundcard (without a high impedence input)

am I close?

 

[Farview]

Yes.

If you have a preamp with a DI, you won't have separate pieces, but it will still be going through the same type of path.

These all-in-one units tend to confuse people because it's hard to visualize the signal path. It also blurs the difference between things like preamps and DI boxes.

 

[RAK]

No. It isn't a pre-amplifier, it's an opamp used to match levels and impedances. A marshall amp has a preamplifier, you wouldn't plug a mic into it.

Anyway, passive DI boxes are nothing but a transformer. The only reason to use an active DI is so you don't load down passive pickups.

To add on to what Farview has said, (and maybe this is too obvious,) but the active DI, being active, needs phantom power. Passive DIs are also better noise cancellers because of the transformer.