Phase cancellation with double Mic

[Vince14]

Hi guys,!
I need a little help! I have bought a new Mic a Sen Md 421 that a I would use with a sm57 in a double mic with my cabinet 1x12.
But the mics are totally out of phase. Why they do that? I could resolve with a Phase invert of my mic preamp ?
Thanks

 

[Steenamaroo]

Here we go.

Phase issues always come from differences in distance. Reflections can cause them but, technically, that's still distance.
Polarity issues come from one microphone being pushed when the other is pulled, or one microphone being wired in reverse resulting in the same effect,
or one of two speakers being wired in reverse.
It's an of/off thing. Right or wrong.

If your microphones have equal distance from the cab and you know the speakers are wired correctly and you're having an issue, flipping polarity (called phase on your interface) should sort it.
If that's not the case and your issue is comb filtering as a result of the mics being different distances from their speakers, then distance adjustments are your friend.

You could run into phase issues simply because technically each mic has a different source (two speakers), and each picks up the other as bleed, although that shouldn't be an issue if you're very close to the grill.
If you suspect this is the problem, try micing close, or try both mics on one speaker just to see.

 

[Farview]

If you are using a daw, zoom in on the two waveforms. If they both start in the same direction, but at a different time, that is a placement problem. The capsule of the 421 is about a half inch or so from the grill, so you may need to back the 57 off the grill by about that amount.

If the waveform move in opposite directions, then your phase switch on the preamp will fix that

 

[rob aylestone]

Two different mics, two different cabs, random distances? This leave you the ears option. You sit there with two faders and the polarity buttons and the panning. You eq, treat and blend till it sounds good (or less bad). Job done. The success is no scientific or set process - it's down to your ears. As all these potential errors have frequency dependency, you can't look and say, ah, 50mm further away and they'll align - because the next note will be higher or lower and the distance will need to change to maintain the same relationship, and it's impractical and things start to cancel and add. Once you start to eq and change the sound, then the blending gets easier because the similarities in the waveform start to change. Don't over think it, just use your ears, and keep in mind that any aligning of waveforms often fails when you add in other processing, because digital processing often delays a signal as a feature.