Is this circuit diagram correct?

[That one guy]

Hi guys, i need to follow this circuit diagram to make a speaker-mic-amplifier circuit but i have my suspicions that the the diagram is incorrect. Could you please let me know what you think

 

[davenn]

Hi guys, i need to follow this circuit diagram to make a speaker-mic-amplifier circuit but i have my suspicions that the the diagram is incorrect.

at first glance I don't see anything obviously wrong
what did you think was wrong with it ?

You can google the datasheet for the LM386 and confirm that part of the circuit
the transistor is just a pre-amp for the electret condenser mic



Dave

 

[OBW0549]

Looks OK to me, too. Depending on which electret mic you are using, it's possible you may need to use a different resistance value for R1; but other than that it should work.

 

[Audioguru]

1) The Jfet in an electret mic draws 0.5mA so the value of R1should be reduced to about 10k to prevent the Jfet from saturating.
2) The transistor has a high input impedance then the high 10μF value of C1 passes low earthquake frequencies so its value should be reduced to about 0.22μF and it should be a non-polarized film type.
3) The 2N3904 has an hFE of about 200 so the value of R2 should be increased to about 1M or 1.5M ohms.
4) The low value of the volume control is killing the signal level from the preamp so it should be increased to about 100k ohms.
5) The high 0.1uF value of C3 is killing all medium and high audio frequencies. Remove C3.

Hey, the transistor has a voltage gain of only 10 so it is not needed and a 10μF capacitor can be connected between pin 1 and pin 8 of the LM386 to give it 10 times more gain.

 

[That one guy]

There were comments on the website that i got this from that it wasn't working so i wanted to ask you guys.
Thanks for all the suggestions davenn, OBW0549 and Audioguru. They helped alot.