AGC preamp for audio of a TV set

[mhtplsh]

I want to make agc preamp for audio.
Input will be from satelliete receiver & outputwill go to audio in of TV.
The different channels in satelliete receiver hv different audio levels.
so everytime u hv to set the audio level after changing the channel.
I am thinking that using such preamp will solve the problem.

 

[A.Anand Srinivasan]

Re: agc preamp

such a circuit would be needing variable feedback or controlling which would need a variable resistor or some other component.... since it is in the preamp you have to take care that the noise contribution by this extra component must be very low...

 

[mhtplsh]

Re: agc preamp

I found this circuit on the net. I assembled it but it is noisy. I used 2N5109 as fet.

 

[IanP]

Re: agc preamp

To switch from channel-to-channel you don't need 50dB gain ..
Most likely a gain of 6db (x 2, max 4 ) will be enough ..
Try to reduce the gain of the opamp stage by decreasing 33kΩ to, say, 1kΩ and, if you can, replace LM358 with a "low noise" amplifier ..
After all, the LM358 is just a general purpose opamp and as a such is rarely used in audio applications ..

Regards,
IanP

 

[mhtplsh]

Re: agc preamp

Yes. I will reduce the gain & change the opamp.
Any other suggestion welcome.

 

[A.Anand Srinivasan]

Re: agc preamp

i think J176 is a low noise FET... so try finding an equivalent one and use.... i think reducing gain will have a lesser curative effect on the noise [that is what i remem learning]