I read a lot of posts on other forums to the effect that the musician wants to simply capture the sound of his or her instrument and amp to be able to share it with others. Here's how to do it.
There are a few things to remember before we get into mics:
1. A microphone doesn't selectively process sound the way your brain does.
Your brain is designed to filter out sounds that your ears hear to concentrate on a particular sound. This evolved as a natural safety tool. For a million years in nature, our ancestors depended on their brains to filter out non-threatening sounds to be able to concentrate on threatening ones. Or to listen for prey to hunt.
Think of yourself talking to someone at a noisy party. Your brain filters out all the other conversations so you can listen to the person talking to you.
In a similar way, you filter out a lot of information that's in a room when you play, say, a guitar amp. You filter out most room reflections, hums and buzzes, structure-borne vibrations such as light bulbs vibrating and making ringing sounds, etc. A mic can't do this.
2. You don't listen to your guitar amp with your ear an inch from the speaker.
Though you can filter out a lot of the sounds around you to concentrate on the tone of your amp, you do hear them, and they do affect your perception.
3. Don't expect to truly capture the sound of "live."
Your ears are omnidirectional. Even the best multichannel recording techniques do not convincingly capture real sound. You know you're not listening to a live band, for example, even when listening to 5.1 mixes. Or 8.1 mixes. It's not the same.
With that, let's explore a few types of mics and their strengths and limitations:
Your ordinary Shure SM57-style dynamic mic is actually a very fine recording mic, and an excellent transducer. Its strengths are that it's extremely robust and is able to capture the sound of a guitar speaker up close, and it can also do a nice job at a reasonable distance. The weakness of dynamic mics are that the method of transducing sound waves requires that the physical movement of a relatively heavy diaphragm. So it's more difficult to reproduce transients, and the attack portion of the waveform is a little more rolled off. It's also more accurate at the midrange than the low end or high end. People say mics like the 57 are midrange-heavy. Actually, they're high end and low end deficient with a relatively slow transient response. They are not horribly sensitive to proximity effect.
They're absolutely great for capturing certain elements of a guitar amp tone, and depending what you want to do with them, dynamic mics are well worth using. The key to getting a more natural sound with them is to use them close-in in combination with other mics.
Ribbons are little corrugated pieces of aluminum suspended between two magnets. They are inherently fragile; a puff of wind can blow out the ribbon. But their lightness makes their transient response very fast. They have a very natural sound. The drawback is that even the most robust and best ones are very, very sensitive to proximity effect; close-in they exaggerate bass, and they therefore sound pretty dark. They also don't exhibit a lot of high end detail. Most roll off around 10KHz. EQ is needed to balance out their sound. The good news is that they have extremely low distortion and self-noise, and you can goose the high end with EQ to balance their sound out without increasing harshness. They're also figure 8 pattern mics, so even close in, they will pick up some room sound.
Blended with, say, a dynamic to capture the sound of the room, they can bring a nice naturalness to your recording. Alone, with EQ, they can sound very natural and real as well. The drawbacks are their fragility, their need for EQ, and the good ones are priced like a good condenser mic. The cheap ones aren't very good, but that's also the case with cheap condensers.
Condenser mics have a capsule that consists of a polarized backplate made of metal, with a charged diaphragm made of a very light material, such as mylar. This allows for fast transient response, and great sensitivity to detail. Because of their need for electrical polarization, they have to be phantom powered, or powered by a power supply (in the case of tube mics), and they also have to have a built in preamp to amplify the tiny signal they generate to even microphone level.
They go into distortion if too close to a guitar amp, and sound horrible when that happens. Or they blow up. So they need to be placed a bit farther back than a dynamic or even some ribbons, such as the Royer models.
While condensers' frequency response is theoretically more accurate than dynamics or ribbons, many are voiced to sound good with vocals, so they have a "presence bump" that brings a gentle rise in frequency response starting around 1-2 KHz and sometimes extending the rest of the way until they roll off around 15-18KHz.
The active electronics and nature of a condenser makes them a little more like a microscope, and there are distortion artifacts that can make recordings made with them a little harsh if the EQ is pushed beyond a certain point.
There are probably thousands of methods that can be used in combination to capture the sound of a guitar amp, but keep in mind that none is perfect. I like to use a dynamic up close, blended with a ribbon about 3-6 feet back, and higher up to approximate what my ears are hearing. I don't worry about stereo so much, simply because I still have to fit the amp sound into a mix with other instruments.
On a clean amp, I might add a condenser like the Blue Dragonfly, to give the blend a little more presence. But as you add mics, you have to be very careful not to cause phase cancellation issues, and a phase switch can help determine if you are screwing this up or not.
Some folks advocate miking the back of an open cab in addition to the front. I think this isn't a good idea; just use a room mic, and you'll pick up plenty of wall reflection from the back of the cab. I don't know anyone who listens to their guitar amp standing both in front and back of it at the same time.
Where do you put the mics? You MUST experiment. Every amp, every room, every mic is different. Have a friend play and move the mics around with headphones on until you're satisfied. An inch can make a difference! If you're alone, it's just going to take longer, but if you are after that elusive natural sound, you can record different placements, and listen back, until you're happy.
There is no substitute for experimentation, and no book can give you more than a starting point! Finally, take notes when you've found a good spot. Take a picture of the setup. Use a tape measure to keep track of distances and mic heights. Do what you have to do to document everything so that you can do it again.
But also remember, it's going to change when you use a different amp, or move anything so much as an inch. And that includes the amp being in a different room, or a different part of the room. It's going to sound a little different. Get used to not having a "formula." That's the art of it!
So that's it for today's pointers.