help with a circuit for boosting a line input signal

[danrogers]

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?

thanks

 

[MikeMl]

Google Boosteroo

 

[danrogers]

Thanks Mike but like I said I want to make one.

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?

 

[audioguru]

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.

 

[danrogers]

thanks guru

what else would need to be changed to get the circuit running from 5v? I see 9v is on pin 7 of the TL71?

 

[audioguru]

The minimum supply voltage for a TL071 is 7V.
An OPA134 audio opamp has the same pins and works with a supply as low as 5V.

 

[danrogers]

thanks Guru, I will give it a whirl

 

[danrogers]

one thing Guru, is it possible to measure the input impedance of the amplifier, via the audio input?

Thanks!

 

[audioguru]

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.

 

[danrogers]

Sorry I got confused and originally thought that you meant the input impedance of the main amp this pre-amp circuit will feed.

Many thanks