Best type of Preamp for guitar/mic FX

Discussion in 'The Projects Forum' started by lofiboy, Dec 31, 2010.

  1. lofiboy

    Thread Starter New Member

    Dec 26, 2010
    15
    0
    I need to boost a guitar signal to a minimum 4.5v for a project involving a 555 timer. What would be an effective, single stage, discrete transistor amp to utilize for this?
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Let's see, a guitar pickup jack output is usually around 10K - 47K and the output is between 100 mV to upwards of a volt - all specific to the type of pickup(s), player, type of strings used etc. No real standard on this.

    An op amp would be the ideal solution but if you can put up with a bit of distortion I'm sure our resident expert on audio gear can suggest something using a single transistor with a gain of 400. He'll be along soon enough sometime in the morning.
     
    lofiboy likes this.
  3. lofiboy

    Thread Starter New Member

    Dec 26, 2010
    15
    0
    A bit of distortion could possibly be a good thing in this project. If an op amp would offer better fidelity, then I think I could use one at the output stage instead.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Can you show a schematic or block diagram of what you are trying to do?

    On this page of geofex there are a lot of circuits for guitar effects:
    http://www.geofex.com/

    Bertus
     
  5. lofiboy

    Thread Starter New Member

    Dec 26, 2010
    15
    0
    Well, I don't really have one but I can try my best to describe it; it's basically an astable oscillator using a 555 timer. The amplified guitar/mic would be fed into pin 5 to modulate the oscillation, and a variable capacitor would control the effective sample rate, hopefully attaining a decimator kind of effect.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    What modulation scheme are you using?
    Pulse width or pulse position?
    In the chapters 4 and 5 of the attached datasheet you will see the difference.

    Bertus
     
    lofiboy likes this.
  7. lofiboy

    Thread Starter New Member

    Dec 26, 2010
    15
    0
    Pulse width modulation, as of right now. I didn't know about pulse position modulation. Thanks!

    As the purpose of this circuit is to imitate low resolution digital sampling, I think I best keep the PWM scheme, though PPM seems like it could be used to make a cleaner amp out of the 555.

    What is, if any, the original purpose of the 555's PWM and PPM abilities? I'm sure it wasn't intended to be used in audio fx processing!
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The pin 5 was intended to create a for example an FSK (frequency shift keying), a modulation technique used in the old days to transmit a telex signal.

    The pin 5 can also be usd for a low frequency FM modulation.

    Bertus
     
  9. lofiboy

    Thread Starter New Member

    Dec 26, 2010
    15
    0
    That makes sense, thanks!
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A guitar magnetic pickup needs a preamp with an input impedance of at least 1M ohms which is much too high for an ordinary transistor.
    Here is a recommended preamp for a guitar magnetic pickup that has low gain that can be increased by reducing the value of R2.
     
Loading...