Distortion in Transistor Switching

Discussion in 'General Electronics Chat' started by DamageDealingMeatShield, Jul 18, 2012.

  1. DamageDealingMeatShield

    Thread Starter New Member

    Jul 18, 2012
    5
    0
    Hey all,

    Lately I've been experimenting with control voltages (CV) that will turn on or allow audio signals to pass into either a filter, graphic equalizer, distortion, etc. The attached circuit is a simple glide. It has a buffer on the input and output, and between the two buffers is a potentiometer which sets the the time it takes for the capacitor to charge, sustain, and discharge. From the last buffer, the CV is feed into a resistor that is connected to a transistor. As the voltage ramps up the transistor slowly turns on and off based on the time factor of the potentiometer and capacitor. When the transistor turns on it allows the audio signal to flow through, which is represented in this circuit as 100 mV at 600 Hz.

    So the problem that I've run into is that at the beginning of the turn on and at the end of the turn off cycle, I get distortion in the audio signal. I do not get distortion when the voltage surpasses a certain voltage. I've tried lowering the audio voltage signal and high pass filters.

    How do I remove this distortion?

    Please note that the transistor that is represented in this circuit diagram is not the one I am using. I am using the 2N3904.

    Thanks so much.

    The Damage Dealing Meat Shield
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your bipolar transistor causes distortion.
    Years ago I faded background music in a huge intercom system using a series resistor and a Jfet to ground. It is the circuit also used for amplitude stability on an audio oscillator. Here is the circuit:
     
    DamageDealingMeatShield and #12 like this.
  3. #12

    Expert

    Nov 30, 2010
    16,283
    6,795
    AG, can you describe the jfet as a part number or an idle current range or a cut-off voltage range? Control voltage range? +Ve value?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Jfets have a wide range of cut-off voltage. I use 2N5459 (2V to 8V) or 2N5486 (2V to 6V).
     
    #12 likes this.
  5. DamageDealingMeatShield

    Thread Starter New Member

    Jul 18, 2012
    5
    0
    Awesome. I just tried something similar to the circuit you posted. I placed that same transistor that is in the circuit and ran it to ground. And then sent the CV to the transistor base. And now it's working as I want it to. There is no distortion. It slowly fades on, sustains, and then fades out. Thanks for the help. :)
     
Loading...