简单实用的
指标:
frequency response: 20 hz to 20 khz +-1db 
noise level (a-weighted): -85 dbm 
amplification: 35 db 
元件清单:
r1      4.7 kohm
r2      220 kohm
r3      2.2 kohm
r4      120 ohm
c1..c4  10 uf 16v electrolytic 
c5      100 uf 16v electrolytic
d1      red led
q1      bc547b (9013)
sw1     on/off switch