Difference Between Mic Line and Instrument Channels

Posted: Sat Jan 19, 2008 8:42 pm

Hey,
I've got a question about every interface I see. They all seem to have both Mic Inputs (XLR) and instrument inputs (1/4 In.) Whats the difference between them? For example. I've got a Firepod with 8 inputs. 1-2 are instrument/mic, 3-8 are mic line ins. They all have XLR / 1/4in jacks so why can't they all be instrument/mic inputs?

Thanks

Posted: Sun Jan 20, 2008 8:29 am

It's to do with input impedance and sensitivity.

An input labelled as "instrument" is usally designed to handle the output of a guitar pickup, and for that it needs to have an input impedance of at least 100KOhm, preferably 500KOhm to 1MOhm, and a medium to high sensitivity, as the output from a pickup is quite low. Instrument inputs are usually unbalanced, meaing that there is a single signal wire plus a ground connection, so the input is a 2-pole (TS) jack.

A "mic" input is conventionally via an XLR connector (male on the mic cable and female on the pre-amp). It is a low-impedance balanced input, meaning that the signal is developed between two signal wires. Its input impedance is usually in the range 2KOhm to 10KOhm. The sensitivity is high, because the output from a microphone is often very low amplitude. Because of the balanced nature, the two signal wires can be raised up to a relatively large voltage (e.g. 48V) with respect to the ground, and this is the way that d.c. power can be supplied to a condenser or other sort of microphone that needs a polarising voltage or power for internal electronics. This method of powering is called "phantom power".

A "line" input is designed to handle high-level signals from other pieces of equipment. They are usually balanced and about 10KOhm differential impedance, so can be sent through an XLR connector like for a microphone (but with phantom power turned off) or through a 3-pole (TRS) jack. Because line-level signals are already at a high amplitude, the gain of a line input is low, sometimes needing an attenuator switched in to reduce the level to what the receiving equipment can handle.

To relate all this to your FirePod, all the channels can receive microphone signals on XLRs. The jack sockets on channels 1 and 2 are unbalanced (TS), high-impedance, medium sensitivity. The jack sockets on the other channels are balanced (TRS), medium impedance and low sensitivity.

Hope that helps.

Posted: Sun Jan 20, 2008 8:48 am

Thanks Boswell. This should probably be stickyed somewhere.

Posted: Sun Jan 20, 2008 11:38 pm

Well I don't plan on have more than 2 guitars go DI anytime soon.

Thanks

Posted: Mon Jan 21, 2008 4:57 am

Drewslum wrote:

Well I don't plan on have more than 2 guitars go DI anytime soon.

That's OK then, if by "go DI" you mean plugging the guitars into the instrument inputs of the FirePod. Usually, going in via DI means using a separate DI (="direct injection") box which takes an instrument-level input and converts it to a balanced microphone level, so you can use standard microphone inputs on your pre-amps. If you find later that you need more than two instrument inputs, just get a DI box and go in via one of the other microphone inputs.

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