The SR550 Voltage Preamplifier is designed to work with any SRS lock-in amplifier. Preamplifiers provide gain close to the experimental detector, before the signal-to-noise ratio is permanently degraded by cable capacitance and pickup. The preamplifier minimizes noise and pickup in the connecting lines and reduces measurement time in noise-limited experiments. When used with the SR510 or SR530, the preamplifier gain is automatically chosen to optimize noise performance without compromising dynamic reserve. When used with the SR810, SR830 or SR850, the preamplifier gain is automatically set to its maximum value. Power and control signals are brought from the lock-in by a 9-pin cable (included). The SR550 may also be operated independently from the lock-ins by applying appropriate biasing (Tone20 VDC, +5 VDC).