MOTU 882 MK2 Mic Preamp Input

Today I disassembled a MOTU 882 MK2 Audio Interface that exhibited significantly increased noise on one of the microphone preamplifier inputs. While I had the unit open, I made this quick sketch of the input circuitry:

From chris' blog

From chris' blog

Interestingly the MOTU does not use a specialized microphone amplifier (such as the SSM2019 for example) but the standard instrumentation amplifier layout known since the dark ages, built from a NJM2068 dual opamp.

The next funny thing to notice: MOTU put a bridge rectifier on top of the SMD opamp (which has been removed in the photograph, it's normally sitting in the four holes on the corner of the small IC). It's protecting the amplifier from voltage swings out of the V- to V+ range. It's also missing from the hand-drawn schematic, it's on the positive inputs to the left opamp. In the end it turned out that this bridge rectifier was the cause of the increased noise. I replaced it with Z-diodes altogether, and now both channels (the unmodified one, and this one) show exactly the same (good) noise-spectra.