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POWERED_SUBWOOFER

2015-03-17 10:56  
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POWERED_SUBWOOFER
The power supply consists of a center-tapped 48-V transformer, a bridge rectifier, and filter capacitors. The rectified and filtered output is about ±35 V. The power supply for op amp IC1 is regulated to ±15 V by zener diodes D1 and D2 and resistors R19 and R20. The input circuit consists of a mixer and voltage divider formed by resistors R1 and R2, potentiometer R3, and unity-gain buffer IC1-a. Potentiometer R3 is provided to adjust the output of the subwoofer to the desired level. Op amp IC1-b provides a 12-dB-peroctave high-pass filter with capacitors C2 and C3 and resistors R5 and R6. The cutoff frequency for this filter is 1/2πRC, or about 34 Hz with the values shown. Resistors R8 and R7 set the gain and Q of the filter. Capacitor C1 and resistor R4 form an additional 6-dB-peroctave high-pass filter at about 20 Hz. A 12-dB-peroctave low-pass filter is formed by IC1-c, C4 and C5, and R9 and R10. The values shown set the low-pass cutoff at 72 Hz. The gain and Q of this stage are set by R11 and R12. These two filters, connected back-to-back, form a bandpass filter. When operated with ±15-V supplies, the output of op amp IC1-d can swing about 10 V peak to drive transistors Q1 and Q2. Resistors R17 and R18 provide negative feedback, and set the gain of the out-put stage at about 3. Hence, the output can swing to about 30 V peak. As long as the transistors are the high-beta types specified, the peak power output into an 8-Ω load is (30×30/8)/2556 W rms. The overall gain of the amplifier is set by resistor R13 and feedback resistor R14.