Re: How to record very faint sound from the analogue input?

Dotan Cohen wrote:
On 7 March 2010 17:20, Joep L. Blom <jlblom@xxxxxxxxxxxxx> wrote:
Hakan Koseoglu wrote:
Dotan,
On Fri, Mar 5, 2010 at 6:59 PM, Dotan Cohen <dotancohen@xxxxxxxxx> 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


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