Computer Science Project



Preamplifier diagram

Building the preamplifier


Why a preamplifier?

 The microphone have a small voltage at the terminals (2,4mV), we need to boost this small signal to increase the voltage. I have built a preamplifier (Vellman-Kit) that has a gain of 40dB.

The dB (Decibel) is a logarithmic unit that express a ratio between two values, in this case two voltages, the input voltage and the output voltage.

GdB=20 log (Vin / Vout)

With a gain of 40 dB and voltage of 2.4 mV, I hope to have 41.68mV at output. The converter (ADC) need a voltage between -5 V to +5 V, with a small voltage I donít know how is working , but when I  make the first tests, I have the opportunity to see if is necessary more gain.



Searching & Reading

 I started  studing the characteristics of the converter ADC and make some recording tests. The tests were performed with a recording microphone electret and the program "Sound Forge Audio Studio" from Sony.

 It is important to do previous tests to see if there are distortions or very high level of signal. I discover some noise that come from the structure of the window that is in aluminium. But this noise it is a bad noise for me, I was interested in the noise of the street. I change the microphone and the noise disappears. I have read about this noise and the problem is the RF (electromagnetic waves), the aluminium work like a antenna and the microphone receive this signal and send it to the system.     



JM Freixo Nunes © 2009