How far can you get from your 25' wall? You will need to do some calculation to find a lens that will cover it. If the distance is too small, it's going to be an odd perspective. The versions of you that are toward the edge will be angled away unless you angle "everyone" in a bit to compensate...
Get closer to the mic. Put the pop screen within an inch of the grille and your lips practically touching the screen. Adjust the gain so the signal stays well clear of clipping.
This will tend to accentuate the lows, but eq can compensate. If the mic has a high pass filter, engage it.
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Since the voice coil is behaving like antenna, you give it a twin nearby (which is not capturing any sound) and wire it in with reverse polarity. It's virtually the same idea as balanced connections.
A humbucking coil generally needs to be close to the coil that it's protecting from interference. I have an Electro-Voice 635a that has a coil around the outside of the capsule (which is unfortunately coming unwound).
There are isolation boxes that I expect would work. My desktop at home is in a big case with a lot of ventilation and a couple of large fans. The fans are so big that they barely have to move to keep the case cool, so they're pretty quiet. It's so overbuilt that it barely breaks a sweat, even on...
Condenser microphones generally need to be powered by the device they're connected to, typically a mixer. It's called phantom power, and a 1/4" mic input won't supply it.
Many digital mixers are capable of recording multitrack onboard and/or to a DAW.
I have a UI24R, and it's pretty effective for live mixing and recording. I haven't tried it in a studio setting, but I think it would do reasonably well. It records to a USB stick, which has to be fast enough and...
You should probably engage the pad on the mic itself. The active circuitry in the mic can be overloaded by loud sources. Decreasing gain downstream won't undo that clipping.
I use two mics on a snare and I never worry about polarity because the sound of the bottom has almost no resonant elements. It's pretty much just the white-ish noise of the wires. And I high-pass it so what little it has in common with the top is further reduced.
On a guitar cab, I definitely...
I record the whole band live, maybe without the vocal if they know the song well enough, and replace as needed. My goal is to get a complete drum take to build on. I'll generally have the bass, guitars etc. go direct at this stage, then mic them for real takes.
But this all requires having...
Bit depth (word length) mostly sets the digital noise floor. I would bet the guitar has enough noise that 16 bit audio won't be the weak link, as long as your gain structure is decent (don't go into the reverb with very low levels).
What unit?
If it's a tape machine, you're going to be spending enough on tapes to make a computer look pretty attractive in not very long. Getting it serviced will seal the deal.
You don't need a super special computer or an expensive DAW to make great recordings. I record on an old Win7...