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my amplified ear wont work!

Discussion in 'Beginner Electronics' started by pil, Jul 13, 2004.

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  1. pil

    pil Guest

    I built the amplified ear found at

    I do get some noise out of it that sounds a lot like one of these listening
    devices but thats it. No sounds are picked up by the mic.

    I used a condensor microphone taken from an old panasonic tape recorded. It
    has a blue and a red wire. I also swopped them around. Nothing.

    What could be wrong. All trannies have hfe's of 140-180.

    Also, what does the dashed line between C6 and SW1 in the circuit represent?
    I don't have such a connection in my circuit.
  2. Zorknob

    Zorknob Guest

    You probably have inadequate supply voltage for the microphone you're
    using. You need a microphone that'll work at ~1v. Beyond that, the
    usual stuff: check your wiring, etc. etc.

    The dashed line indicates a mechanical connection, in this case it
    shows that the on/off switch and volume control are are a single unit
    where you turn the volume down until you hit a click stop, then
    turning it further shuts power off.

    Obviously you can do this with the volume control separate from the
    power switch.
  3. Andrew Holme

    Andrew Holme Guest

    1. You should hear mains hum (buzz) if you touch the input with your

    2. Check orientation of semiconductors.

    3. Test the diode.

    4. Test the diode junctions in the transistors (BE and BC).

    5. Check gain using your multimeter by measuring resistance between C
    and E whilst applying a wet finger between C and B.

    6. Measure voltages at all circuit nodes and post to

    The semiconductors must be removed from the circuit for 3, 4 and 5.

    Test 4+5 will also confirm that you have the correct device pinouts.
  4. Sam B.

    Sam B. Guest

    Well, the noise tells you that your headphones are working, and that
    you are getting some amplification of (possibly) transistor noise.

    But if you mean that you used a true "condensor mic", don't those
    require a high impedance input stage, like a FET? They may not work
    at all with the bipolar stage that you are using. If you used a
    regular electret mic, the microphone actually incorporates the FET in
    with the element, if I remember correctly. But whatever you do, don't
    clean them, unless you know that they are resistant to solvents. A
    lot of them will be damaged when cleaned. Also, a lot of them are
    quite sensitive to heat, soldering leads to the mic will destroy it,
    unless you are quick, and use the proper amount of heat. Static
    discharge can will also damage certain types. If everything else
    looks good in the circuit, them just replace the mic. Before I did
    this kind of thing for a living, I used to scavenge old cordless
    telephones for the mic elements. Even regular corded phones will
    have these as long as they are not very old. The local thrift stores
    seem to have an endless supply of this kind of used stuff.

    It's also possible that you may need to adjust your bias to the mic
    (R1). But that's just a guess.

    Best Regards,

  5. pil

    pil Guest

    that really helped alot. Pinouts of my transisors are correct because I
    measured their HFE's on my DMM.

    One question, about the mains hum - I am using a 1,5V cell. Will I still
    hear the hum if I use this cell instead of using a mains PSU? And if so,
  6. Rich Grise

    Rich Grise Guest

    pil top-posted:
    You'll hear it when you touch the input point, because your body is
    picking up the 60Hz power freq that's everywhere. Also, your circuit
    itself could be acting as that antenna.
  7. I built that circuit, and played around with it trying to get the AVC to
    work a bit better, but with little success. The bias on that stage is
    very low, and may be part of the problem. As the input to the mic
    changes, so does that bias, so it's difficult to tell what the votlage
    should be. But with the input at minimum, the circuit should have
    maximum gain.

    Also, I used an earpiece out of a telephone handset instead of a
    headphones. It's louder, and uses less battery current.

  8. Soeren

    Soeren Guest


    Use the specified transistors, they have higher h_fe.


    * If it puzzles you dear... Reverse engineer *
    New forum: <URL:>
  9. rchen


    Dec 30, 2009
    I have built this project twice with the same result - I can't get them to work. Not a peep out of them. I don't know what I have done wrong.

    Can anyone tell me how to trouble-shoot this circuit?

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