If you purchase a audio interface there is a mic-pre amp build in.For some the quality ain’t fine, so the user likes to buy an external separate box unit.
Read about mic-preamps.
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Budget mic-pre amps
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1.Golden Age Project Pre 73
A low-cost, one-channel preamp with classic sound that is warm, sweet, and punchy.
It’s based on the circuitry used in Neve’s legendary 1073.Price $350.
Review
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2.Studio Projects VTB1
$150
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3.Radial JDV-Pre & PowerPre
$400,500$
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4.Grace Design M101 and M501
560$,540$
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5.True Systems P-solo
1-channel Ribbon Mic Preamp with Higher Input Impedance and Gain
$600
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6.Audient Mico 2
The Audient Mico 2-channel microphone preamp packs major sound quality and great analog-to-digital converters into a very compact package that’s perfect for desktops. Audent’s renowned Class A design is used in the Mico, giving it great transparency.
$660
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7.True Systems PT2-500
$650-$700
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8.Summit Audio 2BA-221
~$700
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9.Vintech 573
$700
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10.Millennia’s HV-35
$720
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$800
Tonelux MP5
A Designs P-1
A-Designs Audio 500
Universal Audio 710 Twin-Finity
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HIGH END
Avalon
Neve
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Suggest other good ones below.