Mic input vs Instrument input

Question

I just got a Focusrite iTrack Solo. This has two inputs. One has an XLR jack and one has a TRS jack.

It appears that the idea was that you would use the XLR jack for a mic and the TRS jack for an electric guitar.

But I want to hook up two mics, and I have an adapter that goes from XLR to TRS (NOT TS, I know the difference!). According to some posts online, XLR to TRS doesn't lose anything, whereas XLR to TS gives a 6db reduction.

In Ableton, I tried recording with the same mic on each input. The difference I notice is that on the "instrument input" with my adapter, the amplitude of the recorded signal in Ableton appears much smaller.

Questions:

On the iTrack Solo or any similar device, what's the difference between a "mic" input and an "instrument" input? Could it be that the "instrument" input lacks a preamp at all?

If so, what does that mean for my recording quality other than that the input signal available to Ableton will necessarily be more compressed (have less amplitude range)? Is a mic connected to an "instrument" input always going to have less depth and definition than the same mic connected to a "mic" input with a good preamp (such as Focusrite)?

Answer 1

For me a mic input is usually a balanced input and has "extra" electronics inside that give a few dB more gain with a lower noise circuit.

If you put your mic into a normal line-input connector the gain (amplification) will be lower and when you correct this by upping the volume on that channel, you'll hear more hiss and background noise.

When you say "more compressed (have less amplitude range)" this is perhaps a little confusing - the amplitude of the signal will still have the same range but the presence of noise means the useful dynamic range of the input is reduced.

Answer 2

The instrument input in devices like the iTrack solo is very much something different from a TRS-jack microphone input!

• It is unbalanced, the specifications page says so explicitly. Makes sense, because electric guitars / basses etc. virtually never have balanced outputs (don't ask me why this has never caught on). This means, it's actually not even a TRS input, just a TS input.

• Its impedance and gain is optimised for such instruments. You need to know that electric guitars are – from a modern electronics point of view – build very primitively: the signal comes straight from the induced voltage in a coil with a great lot of windings. Because there are so many windings, this results in a quite high signal voltage, so the instrument input doesn't need that much gain¹. However, the fat coil also brings quite a substantial problem: it has a high inductive impedance, so if you a guitar to any noteworthy capacitive or ohmic load (like an ordinary mic input), it will form an LC / RC filter circuit, the pickup will start to cut the treble, the sound becomes "muffled". Similarly but with the oppsite result, passive piëzo PUs have a capacitive innner impedance, so connecting one of those to a low-impedance input will cut the bass range.

  Perhaps more critical, high-impedance connections are very susceptible to all kinds of interference: electrostatic and electromagnetic hum, triboelectric pops, mobile phone stray fields... so you generally need to use pretty high-quality guitar cables for trouble-free operation.

  Microphones (save for some exotic models such as the "bicycle lamp") solve all of these problems by using low-impedance outputs:

  • Dynamic microphones have coils just like guitar pickups, but with much less windings. This lowers the impedance. Unfortunately, it also lowers the output voltage, so you may need quite a lot of gain on a microphone preamp. But due to the lower impedance, it's still rather easier to design a low-noise mic preamp with such a gain capability than it is to design a similarly low-noise guitar input stage. Further, because microphone lines are balanced they have a lot less trouble with hum etc. (particularly magnetic hum) than guitar cables.
  • Condenser microphones have a built-in "pre-preamp". This decouples the (itself very high impedance) condenser capsule from the line connection. It needs a power source for sure, but condensers require 48 V phantom anyway for biasing the capsule. And through the active circuitry, you get a desirable low-impedance output while at the same time having reasonably high signal voltage, so you're basically safe from all interference. This is one of the reasons why condensers excel at picking up quiet sound sources, or for room recordings.

To sum it up – an "instrument" input has, unlike a mic input

1. Higher impedance
2. Lower maximum gain
3. It is unbalanced
4. No 48 V phantom power capability.

All of this makes it not completely unsuitable as a microphone input, but you wouldn't want to use this combination for professional work.

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¹Guitar amps have, of course, lots of gain, but not so much to properly adjust the input level but to grossly overdrive the input stage, by boosting the already high signal even more strongly so it exceeds the supply voltage and becomes thus clipped.