Electret microphone amplifier doesn't work unless I touch it with my finger

Discussion in 'The Projects Forum' started by Leav, Nov 5, 2012.

  1. Leav

    Thread Starter New Member

    Nov 5, 2012
    I'm just getting into electronics now, so I have little experience with designing my own circuits, let alone debugging them,

    My goal: DIY hardware only (no microcontroller) Clapper.

    right now i'm trying to amplify the signal from an electret microphone, and used this schematic as a base LINK.

    I'm using an LM324N in an open-loop configuration to achieve maximum gain, and have chained the 4 op-amps to again achieve maximum gain (since the single stage didn't seem to cut it). The last stage has a voltage divider (pot) connected to the negative input of the op-amp in order to control the cut-off voltage (put it there in case I needed it).

    Here is a link to the circuit
    Link to Falsted circuit simulator (of the circuit)

    Notice the white alligator clip? the circuit doesn't work unless I touch that (also my bare feet have to touch the ground).

    Why is this happening and how do I fix my circuit?

    Further information: I'm using ArduinoScope as a very-poor-man's oscilloscope. I would also appreciate any critique you have on my work.

  2. Audioguru

    New Member

    Dec 20, 2007
    The circuit uses a very old and noisy opamp. The LM324 quad opamp (and its sister the LM358 dual opamp) have too much hiss to be a mic preamp.

    The circuit does not produce amplified sound, instead it produces an positive output voltage each time there is sound that is loud enough.

    The first opamp has its (+) input biased at 0V so its output is also 0V. Its output cannot swing positive and negative like audio does because its output cannot go negative. Its output can only go positive which rectifies sounds.
    The voltage gain of the first opamp is 1 + (100k/1k)= 101 times when the pot is at maximum resistance. If you want more gain then simply reduce the value of the 1k resistor.

    The second opamp is a comparator with a reference voltage of +1.5V at its (-) input and 0V at its (+) input so its output is 0V with no sounds. If its input exceeds +1.5V then its output goes as high as it can which is about +3.8V when it has no load.

    Each opamp has an open-loop voltage gain of 100,000 at low frequencies so it amplifies its own hiss as your 'scope shows. It also amplifies its input offset voltage.
    Connecting 4 opamps in series like you did results in an open-loop gain of trillions which is never needed.