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voice processor

Discussion in 'General Electronics Discussion' started by mantyi themba, Nov 30, 2014.

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  1. mantyi themba

    mantyi themba

    Nov 30, 2014
    hi everyone please help me. I am doing a design project using a voice processor chip (ISD1420p) using the attached. the problem is that I am not getting a clear and loud sound at headphones, instead I get a noisy sound with a low sound that I recorded. I then used an audio amplifier to amplify the output, still no changes what can I do?

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Do you have a clean power supply? What value do you use for C11? What do you use for volume control (VR2)?

    It would generally be very helpful if you'd annotate the schematic with component values.

    Amplifying a noisy signal will not improve the signal to noise ratio. You need to get a clean audio signal in the first place, that is at the output of the chip.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    and if its noisy at the chip output, then you need to sort out what problem there may be with the microphone input side
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Here's the suggested application circuit from the data sheet. The design from looks very similar. Coincidence? LOL

    ISD1400 application circuit.png
    The only differences are:
    • the trigger circuit: the design use a relay instead of a pushbutton to trigger playback; this could be done without the relay;
    • the speaker circuit: the "designer" didn't understand bridge-tied load amplifiers and has messed up the connection of the headphone socket and volume control;
    • the electret microphone circuit: I think the "designer" just made a mistake or a deliberate change when copying the circuit.
    To fix the headphone output, make the following changes.
    1. Disconnect everything connected to the "SP+" and "SP-" pins (14 and 15).
    2. Don't use the "SP-" pin (pin 15) for anything.
    3. Add a new capacitor, 100 μF 16V electrolytic, with its positive terminal connected to "SP+" (pin 14).
    4. Connect the negative side of the new capacitor to the clockwise end of a 50Ω potentiometer. This will be the volume control.
    5. Connect the negative end of the volume control potentiometer to the 0V rail (VSSD, VSSA).
    6. Connect the tip and ring contacts of the headphone socket to the wiper of the volume control potentiometer.
    7. Connect the sleeve of the headphone socket to the 0V rail.
    The electret microphone circuit should work as shown but if you want to change the design to match the data sheet's suggested design, disconnect the negative side of C12 from the 0V rail and connect it instead to the -RECLED pin (pin 25). And check that the component values match the diagram from the data sheet.

    Electret microphones are polarised, and for this circuit to work, the negative side (usually also connected to the case of the microphone) needs to be the side that connects to C13 and R26.

    If you still have problems after making those changes, post the details of the electret mic you used and I'll suggest some other tests you can make.
    Harald Kapp likes this.
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