The 1588B is a solid-state, discreet, class A mic preamp module found in a variety of vintage Altec mixers. In it’s stock form, the 1588B is capable of up to only 33.5dB of gain, with the rest being provided by the master section of the mixers they are usually taken from. The modules can be modified to provide extra gain by lowering the amount of negative feedback. This makes these nice sounding modules more usuable as standalone mic preamps.
The shematics below were originally posted on the TapeOp message board and were drawn up by Ned Clayton in reference to his mods on the 1592b mixer.