How to record very faint sound from the analogue input?

[Joep L. Blom]

Dotan Cohen wrote:
> On 7 March 2010 17:20, Joep L. Blom <jlblom at neuroweave.nl> wrote:
>> 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
>>
>>
> 
> I can solder! If you can recommend or link to a wiring diagram, I can build it!
> 
> Thanks!
> 
Dotan,
You ask me to dig up some things I did > 25 years ago. I don't know it 
by heart any more (did when I was younger!) but will look for it 
tomorrow although any entry-level book on amplifiers should do. Or is it 
such old stuff and can people only work with IC's nowadays?
Joep