help with this very simple Op-Amp FM receiver

Discussion in 'The Projects Forum' started by tsukaranz, Sep 28, 2011.

  1. tsukaranz

    Thread Starter New Member

    Aug 23, 2011
    24
    0
    i found this on the net while googling...
    i was amazed of it cause it has a few parts in it...
    [​IMG]

    my question is does it really work? because i made my own, but it didn't work...
     
  2. tsukaranz

    Thread Starter New Member

    Aug 23, 2011
    24
    0
  3. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    Many of the replies to that post cite good results, so yes it can work. It's basically the old crystal radio set updated to use an op-amp. Many others report bad results like your's. The devil is in the details.

    I suggest you post more details about your build and maybe someone here will spot a problem.
     
  4. tsukaranz

    Thread Starter New Member

    Aug 23, 2011
    24
    0
    well i guess my problem is with the coil and with the trimcap,... i don't know the specific values for them...
    i am hoping for at max range of 108Mhz...
    hope you can help me
     
  5. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Um, well this is only going to receive AM stations. FM receivers require a more sophisticated detection method.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Even an AM detector normally requires a DC load and filter capacitor! Try adding a resistor of about 10kΩ to ground from the junction of D1 and C2. Also add a capacitor of 10nF in parallel with the resistor. Finally, change the coil to about 300μH with a variable capacitor of a few hundred pF to tune the AM broadcast band and it might work, in very a limited way.

    Basically this is an elementary AM detector plus AF amplifier circuit which has lost a few bits along the way. Confusingly, you might even hear something with the circuit as shown if some RF got coupled via the diode's self capacitance into the input of the op-amp, where some non-linearity might crudely demodulate the signal.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The circuit is supposed to be an AM radio, not an FM radio.

    The opamp has one input (pin 3) at the same voltage as its negative supply pin (pin 4) so it will not work. Pin 3 must be at least 3V more positive than pin 4.

    If the opamp is biased properly then the radio still might not work because parts are missing.

    I fixed it 4 years ago:
     
    KJ6EAD likes this.
  8. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
    363
    Very nice AG, two different op amp versions no less. What would it take to make it work with ordinary headphones or earbuds, another amplifier stage?
     
  9. tsukaranz

    Thread Starter New Member

    Aug 23, 2011
    24
    0
    thanks, audioguro....
    this much help me....
     
  10. tsukaranz

    Thread Starter New Member

    Aug 23, 2011
    24
    0
    how about using lm741 instead of lm386 in your circuit??
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Ordinary modern headphones are 32 ohms for each ear.
    An opamp is overloaded when its load is 1k ohms or less.
    The LM386 is not an opamp, it is a power amp that can drive a speaker or headphones. But an LM386 has hiss that can be heard through headphones so you should make your own little power amp with a low noise opamp driving output transistors.

    Don't use a 741 opamp (it is 43 years old) because it is extremely noisy and limits its high frequencies to 9kHz.
     
  12. #12

    Expert

    Nov 30, 2010
    16,257
    6,759
    Well done AG. When I saw that suggestion last night, I broke out in laughter from expecting you to blow a fuse. Only a beginner or a troll would suggest a 741 in a reply to you.

    Troll: an internet poster trying to cause a reaction by posting incredible or outrageous comments.
     
Loading...