microphone and oscilloscope!

Thread Starter

Eric007

Joined Aug 5, 2011
1,156
Well I think Audioguru has provided a 'ready to use' mic circuit...so I'll just get all all the reistor and capacitor values and breadboard it as drawn by Audioguru!

Just a little question tho, it about the RC filter for the mic! What is the cutoff frequency? I know how to calculate it but a bit confused with the power supply filter you added...
Coz I want to cut off the 60Hz surrounding noise and have a clean speech signal...

Pardon my ignorance and Thanks again!!
 

Audioguru

Joined Dec 20, 2007
11,249
If you build the circuit on a breadboard then its wires and connector strips are antennas that will pick up TONNES of mains hum.

The 470 ohm resistor and 47uF capacitor reduces 120Hz full-wave rectified hum from a wall-wart power supply and keep the circuit from motorboating at a low frequency. Its -3dB is simply calculated by 1 /2 pi RC which is 7.2Hz. Then 14.4Hz is reduced 6dB (to 0.5 times), 28.8Hz is reduced 12dB (to 0.25 times), 57.6Hz is reduced 18dB (to 0.125 times) and 115.2Hz is reduced 24dB (to 0.0625 times).

Your highpass filter was a 2.2k mic in parallel with a 1k biasing resistor, in series with the 1k input resistor to the opamp and a 1uF coupling capacitor so its -3db frequency was 95Hz. Music and men's voices will sound bad. My highpass filter is the 2.2k mic in parallel with the 2.2k biasing resistor, in series with the 10k resistor to the opamp and a 0.22uF coupling capacitor so its -3db frequency is 66Hz.
 
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