This topic contains 6 replies, has 2 voices, and was last updated by  mwpeters75 1 month, 3 weeks ago.

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  • #3213

    LectronFan
    Participant

    Hi all,

    This circuit resembles the circuit 41 but we use a DC coupling between the 2 amplifier stages. This reduces harmonic distortion. Also included is a tone control which reduces the high frequencies of the audio signal.

    But first a remark : nowadays it’s hard to find audio equipment with a DIN plug. You can better purchase a DIN to 3,5 mm mini jack cable. Or you can order at Lectron.de a Cinch block 2127. 

    Just plug the mini jack into your MP3 player, your smartphone or your tablet and the DIN or CINCH cable into your Lectron circuit. Now you can experiment with the Lectron system and use your own music. There also exist a nice and free function generator app for Android called Keuwlsoft Dual channel Function Generator. I use it a lot here at home.

    Now, back to our circuit.

    Here’s the diagram :

    How does it work ?

    After the volume control, we find our first amplifier stage. It’s a common emitter amplifier stage. The bias for this transistor comes from the 100K resistor. The second stage is a bit special. The bias for the 2nd transistor is provided by the first transistor via the 1,5K resistor. This is why it’s important to carefully design the first amplifier stage.

    At the emitter of the 2nd transistor we find 3 connections. The first goes to a resistor of 220 Ohm which sets the amplification factor. The 2nd connection goes to a 10uF capacitor which let pass the audio frequencies otherwise blocked by the 220 Ohm resistor. The 3th connection goes to the 100K resistor which biases the first transistor.

    The cool part of this circuit lies in the fact that the amplifier stabilizes itself. This is done with the feedback provided by the 100K resistor. The 10uF capacitor provides a smooth control.

    The tone control is quite simple, by using the variable resistor, more or less high frequencies will be grounded.

    A challenge for the more advanced experimenters, redesign this circuit for the PNP transistors 😉

    Greetings

     

    • This topic was modified 1 month, 3 weeks ago by  mwpeters75.
    • This topic was modified 1 month, 3 weeks ago by  mwpeters75. Reason: Changed font and size styling for easier reading
  • #3216

    mwpeters75
    Keymaster

    Hi Frank,

    Why is the first 10uF cap needed if we already have a low level audio signal to begin with at the input?

    • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
    • #3221

      LectronFan
      Participant

      Hi Michael,

      The first capacitor of 10uF is needed to separate the DC voltage of about 0,6V at the base of the first transistor and the input circuitry.

      Without this capacitor, a voltage of 0,6V will flow into your smartphone or tablet 😯

      You could also use a 0,1uF capacitor, but the amplified sound will be thinner, due to it’s value. It will then only pass the higher frequencies.

      This is a good circuit to test your new oscilloscope 🙂 . I also have a well functioning Rigol scope which I used here to make measurements.

      Greetings

      • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
      • This reply was modified 1 month, 3 weeks ago by  mwpeters75. Reason: Changed font and size styling for easier reading
      • #3225

        mwpeters75
        Keymaster

        Hi Frank,

        Thank you for the rationale of the 10uF cap.  That pesky DC voltage always hanging around to cause trouble! 

        Looking forward to getting started with your excellent practical lessons.  You are breathing new life into the Lectron System by showing what a great learning platform it is!  Thank you.

        • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
        • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
        • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
        • #3230

          LectronFan
          Participant

          Hi Michael,

          Great scope you have !

          Those scopes are really neat and easy to use. I like them for very short pulses where an analogue scope can’t be used.

          Also the possibility of storing wave forms on USB is fantastic !

          Which is a good idea, I will make some measurements with this amplifier circuit and publish the waveforms here 😆

          Lectron is indeed a magnificent system with lots of learning possibilities. And so easy to swap components. It gives so much inspiration for creating your own circuits.

          Greetings

           

          • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
          • This reply was modified 1 month, 3 weeks ago by  mwpeters75. Reason: Changed font and size styling for easier reading
        • #3233

          mwpeters75
          Keymaster

          I have no where near your abilities with engineering I have to say!  This Rigol scope is fully featured with Freq. Counter, DVM, Serial Data Protocol Analysis, AWG and other goodies.  More capability then I currently have!  So great learning opportunities arm with this scope and your lessons!  MWP.

  • #3217

    LectronFan
    Participant

    And here’s a setup picture : note that I used the double cinch connection block 2127 for ease of use.

    • This reply was modified 1 month, 3 weeks ago by  mwpeters75.
    • This reply was modified 1 month, 3 weeks ago by  mwpeters75. Reason: Changed font and size styling for easier reading

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