The NL844 is a small, four channel, low noise, high impedance, differential pre-amplifier designed for electromyography (EMG) or evoked potential (EP) use close to the preparation or human subject. The unit is intended for use with the NL820A Isolator, from the NeuroLog™ system. It has controls for LF-cut, Gain and Impedance checking of each amplifier electrode. The front panel inputs use 1.5mm DIN 42802 safety connectors. This unit is particularly suited for use where there are very large stimulus artefacts, for example when used close to a magnetic stimulator coil. In these cases the stimulus artefact would normally swamp the pre-amplifier inputs resulting in a ‘tail’ with a long time constant that obliterates the response. The special circuitry of the NL844 eliminates this as long as good recording practice is followed. The input circuit automatically adjusts to the DC input conditions and a non-linear filter subsequently modifies its time constant if the differential input signal exceeds its normal ±20mV working range. Therefore fast, short artefacts do not block the subsequent stages. This results in an amplifier with a very fast recovery time from stimulus artefact pulses.