How to record very faint sound from the analogue input?

[Joep L. Blom]

Hakan Koseoglu wrote:
> Dotan,
> On Fri, Mar 5, 2010 at 6:59 PM, Dotan Cohen <dotancohen at gmail.com> wrote:
>> I need to record a phone conversation, but the sound from the analogue
>> input is so weak that recording and playing back in Audacity I hear
>> almost nothing. How can I either boost the playback or (even better)
>> boost the input so that the file is recorded in a hearable way?
> 
> Your best chance is to boost the signal before it arrives to the
> microphone input. You should be able to get one from your local Radio
> Shack-clone or if you want to do it yourself
> (http://www.epanorama.net/circuits/micamp.html)...  Also make sure
> that the impedances are not way off. That would increase the loss even
> further.
> 
> Software solutions will invariably end up with a LOT of noise since
> you will be amplifying all of the internal noise from your PC/laptop.
> I always found that making sure that the incoming voltage level is
> good before recording at my home studio. Since your signal to noise
> ration is so horrible, whatever modification you do after it gets
> converted to digital (i.e., pulse audio mixer settings or Audacity
> levelling), you will just amplify the noise along with the signal. Do
> it before it arrives to the PC.
> 
I agree with Hakan. A bad signal is never improved by digital 
amplification: it only amplifies the noise.
However, it could be a good idea only to match the inpput impedance of 
the signal output to the audio input. I would never use the microphone 
input for a line signal as:
1.	The impedance matching is normally way out
2.	The analog signalpath is especially adapted to the microphone signal 
which is much differeing from a line signal.
with 3 resistors and a transistor a simple impedance matching circuit 
can be build. You don't need a cicuitboard. A soldering iron and some 
wires is sufficient.
Joep