Volume control with 10k pot

Discussion in 'The Projects Forum' started by peter_morley, Mar 22, 2011.

  1. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    I have included my circuit diagram of what I have set in place. My problem is I can only hear sound when the resistance on the pot gets around 1kΩ and then when I turn the dial all the way at about less than one Ω i get a squeal sound. What I'm thinking is that the sound is only audible when a certain power is sent to the speaker. So when the current is small at about 10kΩ I can't hear anything and when the current is almost short circuiting at less than one Ω I get a high pitch squeal. I know that humans interpret loudness at a logarithmic scale so I started with a log pot. It seems that something is wrong, any guidance is appreciated. thanks

    Peter
     
  2. Wendy

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    Mar 24, 2008
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  3. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    the other replies are not specific to what i'm asking. I can't seem to get an even steady increase in sound when I turn the pot ie get less and less resistance so more current. I only get an audible sound around 1kΩ so what i'm asking is how do i get a linear sound increase. Well a linear increase in sound that humans interpret ie logarithmic scale in decibals.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Your 10k pot does not have its slider connected so turning the pot does nothing.
    The circuit is missing an output coupling capacitor to block DC from destroying the speaker and to reduce the current a little.
    the circuit is also missing a resistor to limit the output current to 200ma.

    Look at your circuit if the slider of the pot is connected to the top connection of the pot:
    The output of the 555 will try to go to a high of +6.5V. The current will be (6.5V/8 ohms=) 813mA which will burn out the 555. If the pot is turned down a little then it will also burn out.

    Add a coupling capacitor between the output of the 555 and the pot. Then the output will try to swing to +2V and to -2V. The peak current is (2V/8 ohms=) 250mA which might burn out the 555 and might burn out the pot if it is turned down a little.

    Then add a current-limiting resistor in series with the capacitor and the pot with a value of about (2V/200mA=) 10 ohms minus 8 ohms= 2 ohms.
    The pot still might burn out if it is turned down a little.

    To get a smooth adjustment of volume then the pot should have a resistance much less than 10k ohms. Try a 100 ohm pot and connect it as a volume control, not as a rheostat.
     
    Last edited: Mar 23, 2011
  5. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    This is the explanation I was looking for. I'm kind of bad with multisim so I forgot to connect the slider but on my circuit at home I have that all configured correctly. Thank you for telling me that the speaker is AC i had no idea. And I didn't realize a speaker needed a capacitor and a current limiting resistor. Are most speakers 200mA max or did you figure that out from a calculation. V^2/R = 3.5Watts so max Voltage drop across speaker should be about (8Ω*3.5W)^0.5 = 5.3V. From there you could find the max current which is ((8Ω*3.5)^0.5)/8Ω? but that gives me 661mA. So I'm lost on how you got 200mA, I know to limit that current i could put a resistor in series and that would drop it. So as you said the voltage becomes about 2 volts from capacitor so in order to limit the current to 200mA I would do this...2V/(8Ω +R1Ω) then R1Ω = 2Ω? Can you show me how you did this so I can learn? Im rambling so hard i dont know what i just wrote.

    Peter
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    We don't know where you connected the slider.

    The speaker doesn't need a current-limiting resistor. The 555 is not an audio power amplifier so it needs a current-limiting resistor.

    The max allowed output current from a 555 is only 200mA. A speaker draws as much current as it needs. But an 8 ohm speaker is like a dead short to the output of a 555 so the current mst be limited to 200mA with a resistor in series with the speaker.

    From there you could find the max current which is ((8Ω*3.5)^0.5)/8Ω? but that gives me 661mA. So I'm lost on how you got 200mA[/quote]
    The max allowed current from the output of a 555 is 200mA so you must add a resistor in series with the speaker so the max current is limited to 200mA.

    2V/200mA= 10 ohms. My calculation was wrong. the 8 ohm speaker must have a 2 ohm resistor in series to limit the current to 200mA.

    Please show how you connected the slider on the pot.
     
  7. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    I just put in a 8Ω resistor to resemble the speaker but i dont know if that works. I think the pot is setup correctly now, well here it is. The reading I have on the schematic isn't correct the current starts at about 600μA and slowly gets smaller and smaller. This must be the transient effect from initial to steady state. Whatever is going on in multisim is not giving me a clear picture of what is going on.

    Peter
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The value of the pot is much too high.
    1) With the control turned up to maximum, the volume is maximum.
    3) With the control turned up fairly high the volume is fairly low.
    4) At halfway, the attenuation is 5k/8= 625 times so you will barely hear the speaker.
    5) At minimum, the speaker is not turned off completely.

    You should use a 100 ohms pot connected like a volume control.
    But when the pot is turned down only a little from maximum then the pot might burn out because only a small portion of its thin resistance track is dissipating a fairly high amount of power.

    Try this as a volume control:
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Which was imbedded in the link I gave. Here was my version suggested.

    [​IMG]

    That is funny, it is the drawing I made with details added. Cool!

    I didn't realize it until I saw them side by side. :D
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Thanks for your drawing, Bill.
    I copied and pasted it into Paint, added some text then made it big enough to be seen easily.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    All my drawings are open source. If you need anything for future reference I will draw on request.
     
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