| I'm not a pro, but I hate seeing questions go unanswered.
Have you listened to any mariachi records? That would seem like the first thing to do.
I'm certainly no marichi expert, but being that they are all acoustic instruments, I think you could treat it similar to the way you would treat acoustic jazz or bluegrass. Probably more like bluegrass.
I would think the musicians would prefer to all be in one room without headphones. If it were me, I'd start with a stereo pair and arrange the musicians (maybe marking spots on the floor with tape) around the mics until the balance is what everyone wants. Add spot mic(s) for any vocalists, and maybe one for the guitarrón (bass), though I would avoid it if you can.
Maybe the KM184's in ORTF or XY in front of the group, the RE20 on guitarrón and the SM7 for vocal? If your room is decent, a stereo pair should work well, I would think.
My $.02.
Edit: Since your only stereo pairs are cardiods, you probably would need a spot mic on the guitarrón to get enough low frequencies from the bass. Too bad you don't have omnis. |