Hum on hi gain. Cause?

Discussion in 'The Projects Forum' started by Bigbizkit, Dec 26, 2015.

  1. Bigbizkit

    Thread Starter Member

    Oct 29, 2015
    44
    1
    Hi made a bass preamp by Elliot Sound.
    I am having problem eliminating the hum.
    Everytime I turn the volume control up, the hum gets louder.
    And when i increase the gain level, it produces a hissing sound.
    But @ lower volume/gain, the preamp is working fine.
    I am planning to put a voltage regulator using 7815,7915.
    Wondering if it will work
    Note: my wiring arrangement is messy,
    Im thinking its one of the reasons why my project is noisy!humming.
    http://sound.westhost.com/project152-1.htm
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    You could use this circuit from the same site:
    http://sound.westhost.com/project05a.htm
    You can connect the AC inputs to the AC15/1 and AC15/2 connections in the given powersupply circuit of the project.
    You will likely not need the auxillary supply given in the preamp supply project.

    Bertus
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    You need to use a pcb really for amplifiers, veroboard is notoriously noise prone with stray capacitance. If you can post the circuit, and have a separate audio amp to test the preamp output.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    That is an impressive looking circuit board!

    Yes, it is a very good idea to use a regulated power supply for your preamp because the opamp has a a finite Power Supply Rejection Ratio.

    You might want to go back and re-read the author's advice about the power supply "The power supply will not be inexpensive..."

    Hum can also be picked up by capacitive coupling from AC mains wires near the circuit. Some shileding might be required. I've noticed that the fast transitions on the output of triac lamp dimmers, in this case a soldering iron power control, is one of the worst offenders.

    Hum can also result from getting the power supply grounds "mixed up" with the signal grounds, resulting in a ground loop.

    About the hiss: There will always be some hiss, the challenge is to make it so low that you don't notice it. Since the design is by Elliot Sound a safe working assumption is that the design is good. If, when you set the gain to its normal working range and the hiss is still objectionable then you might have a damged opamp (assuming you are using the opamp version of the preamp). One of the potentiometers might be noisy.
     
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