Small and portable unit, Can be built on a veroboard
The amplifier's gain is nominally 20 dB. Its frequency response is determined primarily by the value of just a few components-primarily C1 and R1. The values of the schematic diagram provide a response of ±3.0 dB from about 120 Hz to better than 20,000 Hz.Actually, the frequency response is ruler flat from about 170 Hz to well over 20,000 Hz; it's the low end that deviates from a flat frequency response.
The low end's roll-off is primarily a function of capacitor C1(since RI's resistive value is fixed). If C1's value is changed to 0.1 pF, the low end's comer frequency-the frequency at which the low-end roll-off starts-is reduced to about 70 Hz. If you need an even deeper low-end roll-off, change C1 to a 1.0 pF capacitor; if it's an electrolytic type, make certain that it's installed into the circuit with the correct polarity, with the positive terminal connected to Q1's base terminal.
Circuit Diagram:
Audio Booster Circuit Diagram| Parts | Description |
| P1 | 100K |
| R1 | 47K |
| R2 | 470K |
| R3 | 10K |
| R4 | 560R |
| R5 | 270R |
| C1 | 0.1uF-25v |
| C2 | 3.3uF-25v |
| C3 | 470uF-25V |
| D1 | 5mm. Red Led |
| B1 | 9v Battery |
| J1 | RCA Audio Input Socket |
| J2 | RCA Audio Output Socket |
| S1 | On-Off Switch |
Source :http://www.ecircuitslab.com
