The Sansui AU-6900 is a pre-main amplifier introduced in the mid-1970s, blending advanced design concepts targeting audio fidelity and user flexibility. It features a pure complimentary service circuit for its power amplifier section with a dual transistor configuration at the first stage and a Darlington three-stage setup for improved drive stages. The output stage employs a parallel push-pull design using four matched power transistors per channel, which significantly enhances linearity and results in low distortion rates. With a total output of 60W per channel at 8 ohms across a frequency range of 20 Hz to 20 kHz, the AU-6900 aims to deliver robust sound performance. The unit also incorporates a sophisticated NF-type equalizer circuit with four low-noise transistors, ensuring high-quality sound processing. Noteworthy features include a three-step impedance switch for the Phono1 input, a well-regulated dual power supply, and a tone control system allowing adjustment of bass and treble frequencies. The amplifier includes a loudness control, muting function, and stereo/mono switching capabilities, all housed within a chassis-less structure that minimizes interference and sound degradation. Its power supply boasts a large transformer paired with capacitors providing ample reserve power, while various protective circuits help safeguard against DC detection and abnormal temperature scenarios. Overall, the AU-6900 represents a comprehensive approach to audio amplification, catering to both audiophiles and casual listeners alike.
