A low-noise and fast preamplifier is needed for the new boron-coated multiwire proportional chamber (MWPC) neutron detector with a delay line readout system to improve position resolution. A transimpedance preamplifier with a rise time of 30 ns, a signal-to-noise ratio higher than 40 dB, and an automatic gain control function are designed to meet the aforementioned requirements. On the other hand, we also compare it to commercial preamplifiers. Then, the advantages and disadvantages of commercial and in-house preamplifiers are analyzed. The preamplifier is used and tested on a MWPC neutron detector, and results are presented.