For the recording of minute bioelectric signal voltages in the fields of medical research and diagnostic applications, a high-performance amplifier is needed to provide the signal levels necessary for a significant analysis. Irrespective of the concrete application, the major parameters required for a comparative evaluation of such systems are sensitivity, bandwidth and noise. Some applications also make special demands on electrical safety measures. With the aim of using an amplifier with a particularly powerful magnetic stimulator, and in view of the special requirements for brain voltage recording in a fully conscious patient, a universally applicable amplifier system has been developed that incorporates safety measures that ensure virtually risk-free application. The stimulation artefact can be suppressed completely by means of automatic offset regulation of the preamplifier. The circuit elements in electrical contact with the organism are separated from the rest of the measuring system by a high-voltage insulation barrier. The main innovation and central subject of this article is an active protective system with an extremely short response time, which reliably cuts off the relevant amplifier circuits as soon as the ceiling signal level is exceeded. In accordance with relevant regulations, the user is warned of any first failure by a program-controlled self-test run by the protective unit and done automatically when the amplifier is activated. Exhaustive tests with a prototype have confirmed, among other things, that the integration of the new protective device does not reduce the amplifier output to any measurable extent.