I have a sub with a built in amplifer, but the line in signal that I want to feed it from (home cinema amp) is a bit low. Ive tested it on my pc line out and its alot louder.
Could someone recommend a fairly simple and cheap circuit I could build with adjustable gain to boost the signal?
I have made this circuit before and its working well, but im not sure what changes I would have to make to use it with a line signal instead of a microphone?
Remove R1 and replace VR1 with a 22k pot. Then the voltage gain can be adjusted from 1 to 11 times.
C1 cuts frequencies lower than about 32Hz. Increase ther value of C1 to 0.33uF so that it cuts frequencies below 10Hz.
The output capacitor cuts frequencies below 10Hz if the input impedance of the amplifier is 50k ohms. If the input impedance of the amplifier is less than 50k the increase the value of the output capacitor.
The input impedance of the electret mic preamp you posted is simply R2 and R3 in parallel which makes it 50k ohms.
The TL071 and OPA134 opamps have Jfet inputs with an extremely high input impedance.