Microphone to Speaker Circuit

Discussion in 'Analog & Mixed-Signal Design' started by sdcruz, Apr 25, 2019.

  1. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    Hi All
    Does anyone know of a circuit which I can build up using an IC Chip with a electret microphone as input and 4 Ohm or 8 Ohm speakers as an Output. I know the LM386 will not be sufficient enough to take the mirco volts coming in from an electret microphone and amplify it high enough to a speaker/s.

    Regards
    Shelton
     
  2. MrChips

    Moderator

    Oct 2, 2009
    19,114
    6,145
    Use a preamp stage with a gain of about 100 and then feed this into the LM386 amplifier.
     
  3. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    Ok - so you mean use a BC547 transistor and feed the output to the LM386? Like this guy here:



    Regards
     
  4. MrChips

    Moderator

    Oct 2, 2009
    19,114
    6,145
    Or you could use an op-amp IC.
     
  5. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    The output from a electret condenser mic is typically around 10mv, not micro volts.
    If you set the gain of the LM386 at max (200) you might not need a preamp.
    SG
     
  6. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    I tried that - volume was too faint. I will look at adding a pre-amp in front of the LM386.
     
  7. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    Did you bias the mic element?
    SG
     
  8. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    I added a resistor to pull it up to the positive rail - if that's what you mean? Anyway I will try a pre-amp - more fun.

    Regards
     
  9. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    The value of the resistor is important.
    Generally the resistor value is selected to provide half the supply voltage across the mic element.
    Is that what you did?
    SG
     
  10. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    Well with a 12v source what resistor value do you recommend?
     
  11. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    It depends on the mic element.
    Read the voltage across the mic element starting with a 22K resistor. If the voltage is higher then 6 volts increase the value.
    You are looking for about 6 volts or even a little lower.
    SG
     
  12. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    A better approach uses a decoupling circuit between the amp and the mic circuit composed of C6 and R2.
    SG
    EEE Lm386 with mic.PNG
     
  13. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    I haven't built this circuit yet as I don't have 47 to 100 uf caps. Ionly have 33 uf caps.
     
  14. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    Close enough to test circuit.
    SG
     
  15. Audioguru

    Expert

    Dec 20, 2007
    11,249
    1,349
    You said the sound was faint:
    1) Did you use an electret mic and did you speak at a conversation level at a distance of 10cm between the mic and your mouth?
    2) Is the LM386 producing its maximum output power of only 0.5W (like a cheap clock radio) into an 8 ohm speaker when its supply is 9V or producing a maximum output power of only 0.6W but lots of heat when its supply is 12V?
    3) Is the sound howling due to acoustical feedback when the mic can hear the speaker causing sounds to go around and around?
    4) Did you use a tiny speaker with bad sound? Did the speaker have an enclosure designed for its spec's?
    5) Did you use a low value 33uF output capacitor that blocks 606Hz and all lower frequencies?
     
  16. MrChips

    Moderator

    Oct 2, 2009
    19,114
    6,145
    AG makes a good point. A lot of people think that a loudspeaker is that metal cage thingy 2"-3" diameter.
    That is just the transducer. A loudspeaker consists of the transducer and the speaker cabinet. The cabinet plays as much an important role as the transducer itself. If you want decent sound you need a speaker cone of 4"-6" diameter or greater. Then you need to mount the speaker in a proper cabinet enclosure.

    33μF capacitor on the output means you have lost all your low frequency sound. You need a series capacitor of 470-1000μF or greater.
     
  17. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    Hi Guys
    Have been busy with work and other things so haven't had to chance to try this out or answer any questions - will come back to this when I get a chance.

    Regards
    Shelton
     
  18. sdcruz

    Thread Starter New Member

    Apr 23, 2019
    13
    0
    Ok I built the circuit with 11v DC and I am using 22K Ohm resistor for Rm and I get 6.6V - I speak really close to the mic and nothing on the output.
    Also, on PIN 3 of the IC I am getting 0v but on Pin 5 (Output) I am getting 4.8v
     
  19. sghioto

    Well-Known Member

    Dec 31, 2017
    1,039
    157
    Those voltages sound correct. Suspect wiring error.
    Give me the voltage readings on all the pins of the LM386.
    SG
     
  20. Audioguru

    Expert

    Dec 20, 2007
    11,249
    1,349
    You have not shown us a schematic showing how you built the circuit.
    You must have the 10uF capacitor between pin 1 and pin 8 of the LM386 for the gain to be high enough. The output capacitor value must be at least 250uF. The capacitor between the mic and input pin of the LM386 can be any value from 0.1uF to 1uF.

    Maybe the electret mic pins are backwards. Its metal case pin must be at the circuit's 0V ground.
     
Loading...