The Audio-Technica AT8538 is a power module that is only for AT831R, U853R(W), U853R(W)U, U857R(L) and U857R(L)U microphones. It is a phantom power only in-line power module with bass roll-off, input on TA3M, output on integral XLRM 3-pin. It comes included with the U853R, U857R and PRO35 microphones. This power module provides several important functions. First, it converts the 11 - 52V DC phantom power into a smaller DC bias voltage used to power the FET impedance matching circuit inside the microphones listed below. Also, it provides low frequency roll off, supplies battery power if phantom power is not available, converts the signal to the proper impedance and balances the output. The power module is a necessary component when using any of the mics listed below.
Shown above with the PRO35 (not included)