When I'm performing a song (well, any) on stage, with an acoustic-electric guitar, the soundman tells me that I should use direct output into the mixer, but when I ask for it to be plugged into an amp first (which is a Roland JC-120), he says that it can't lift up the signal.
Why is it? Why does he tell me to go direct output instead of plugging in to an amp?
Amp miking is basically a ridiculous thing to do: a speaker is, in principle, a device to make an electric signal audible. A microphone is a device to do the opposite. Neither does its job perfectly, in particular it's very hard to design a speaker with truely even frequency response. A purely electrical transmission is obviously much better, cleaner, from a technical point of view: the signal you want comes through virtually unaltered, and interference (AC hum etc.) is much less of an issue as well compared to mic bleed.
There are basically two reasons why it is nevertheless often a good idea to mike an electric-guitar amp:
An electric guitar's signal is not itself “meant to sound well”. Back in the day when these were invented, there wasn't even really a notion of how an electric signal “sounded”; sound was always an acoustic thing and if some particular system used electric transmission then the meaning of this signal was defined by what you would plug the guitar into. And because electronics were clunky in the day, it was kept to a minimum in the guitar (from a modern point of view, an absolutely archaic and rather badly-designed minimum) and the sound was adjusted mostly by altering the amp's characteristics. You could say, an electric guitarist is playing two instruments simultaneously: the guitar that is largely a “control device”, like an organ manual. The real instrument being – the amp, or organ pipes. So, taking away the “imperfections” of the amp's response is problematic; if you do that you probably need to compensate in some way (e.g. amp modelling).
Electric guitarists tend to make a lot of extra sound adjustments as part of their performance, almost a third instrument. To have proper control about pedal etc. FX, they need to be sure what the audience hears is comparable to what they hear themselves from the amp, including stuff like controlled feedback. The easiest way to ensure this is to have the audience hear the very output of the amp cabinet, via a microphone.
Neither of these points really apply to acoustic guitars:
An acoustic guitar is, well, an acoustic instrument! Under ideal circumstances (studio), it's largely agreed that “correct” sound is always the direct sound from the guitar, as you can only capture with your ears or a microphone. Pickup systems generally strive to approximate this as closely as possible (not in some vague “so-it-sounds-well-through-some-particular-amp” sense, but in the modern sense of: the electric signal is similar to what a microphone might emit, and when you plug into a good (almost-linear) PA / monitors / headphones, it'll sound similar to the original guitar itself).
Most acoustic guitarists don't use a great lot of effects, only gentle “thickeners” like reverb and chorus, set up more or less always the same way. You thus don't really need fine-control over these effects during the performance; indeed, IMO the best thing is if the engineer sets them up from the FOH.
So really, the main purpose of an acoustic guitar amp is as a monitor for your playing on the guitar. Now, that's in fact not a trivial task, mostly because acoustic guitar has to deal with uncontrolled-feedback problems much more than in the electric. Acoustic-guitar amps tend to focus on surpressing feedback quite a lot, and that means
In summary: the reasons why you'd mic an electric amp don't apply (or indeed apply negatively!) for acoustic guitars. Therefore, I too would very much hesitate to mike an acoustic-guitar amp, and instead opt for either a good DI box, or the amp's built-in direct out.
Feedback, bleed, clarity in the mix. All these apply to both miking an amp and miking the guitar directly without using an amp. These apply to using microphones in general, in any instrument, with or without amps involved.
It depends on the venue (dimensions, materials, shape), the equipment (mics, amps, PA, monitors), and the performance (# of instruments, type of instruments, the mix). In many scenarios, there is a very big potential of feedback. It doesn't need to be a complex scenario. One guitar alone can have problems in a small and reflective venue (both standing waves and reflections being an issue).
Mixing engineers know this, and they often recommend to use direct out in all or some instruments. It also makes mixing simpler, since you don't have to worry about bleeding (which again, depending on the scenario, can be a big issue).
When he said that "it can't lift up the signal", I'm sure he meant that he can't bring the guitar to the amplitude levels he wants because it would induce feedback. This is why feedback can impact your performance even if it's not manifested as feedback, it will be manifested as a limitation of volume (which will vary among mics and instruments), this will be reflected in the mix. Some instruments won't be able to sound loud enough to clearly be heard (this is because feedback will be triggered at the same volume as the instrument needs to be heard through the mix).
What might have happened there is that the soundman noticed that he wouldn't be able to capture the guitar correctly with a mic (volume, tone, dynamic, whatever), and he considered that by using line out he could achieve a better individual sound and/or mix as a whole. It's a pros and cons game. You lose the guitar amp, but you gain in other areas. How good is miking your amp if the audience won't be able to hear it? At that point it really doesn't matter how beautiful it sounds, all the details will be lost since it won't be able to sound loud enough, or the details are being masked by other things going on around the mic (as good isolation is not always possible).
If you find yourself in this situation a lot, and you don't like the sound of raw line out or whatever process the soundman chooses, do yourself a favor and have amp or speaker-mic modeling as an option, as a plan B, in your performance. There are a lot of options out there, in both hardware and software form. This way if amp miking can't be done, you will still be able to have the amp sound.
The most popular sepeaker-mic simulator (that will simulate the speakers and mic combo, but not the amp), is Hughes & Kettner's Redbox. You feed it with your amp's output, and it simulates being the speakers and mic. I see this one the most often since it's inexpensive, easy to use, and it's hardware (doesn't require you to carry a laptop around).
For amp-speaker-mic simulator there are a lot of options. The most populars are Native Instrument's Guitar Rig, Waves' GTR, and IK Multimedia's Amplitube.
Get an LR Baggs Para-Acoustic DI and never haul your amp again. Ever! We usually do our own sound system in our acoustic duo. I DI both my keyboard and my acoustic Breedlove Atlas Black Magic. It has a built in LR Baggs Piezo pickup and the Para-Acoustic DI box uses Phantom power from the mixer. It's an active DI and that is what you want. I can color the sound as there are EQ's on the DI as well. I also run a Godin A6 ultra through it as my second guitar. I pump both my keyboard and guitar back through our monitor along with vocals. Works slick! Dump the amp man! You don't need it!
