Bose 901 Series III Active Equalizer

The Bose 901 Series III Active Equalizer, released around 1978, is a critical component for the Bose 901 Series III speakers, designed to balance the output of all frequencies for natural tonal clarity. It features a single-ended PCB with op-amp chips (unlike earlier transistor-based models), RCA connectors, and adjustable knobs for fine-tuning sound. The equalizer connects between a receiver’s preamp output and main amp input, enhancing bass and treble definition. Common maintenance includes replacing electrolytic capacitors (e.g., 470 uF, 10 uF, 1 uF), transistors (BC239, 2N3906), and oxidized RCA connectors to maintain performance. It significantly improves sound quality, making it essential for the 901 speakers.