Class B audio amplifier: Can I run it with a wallwort?

Discussion in 'The Projects Forum' started by Romes, Sep 30, 2014.

  1. Romes

    Thread Starter Member

    Sep 30, 2014
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    Howdey guys and gals,
    I built the circuit in the lesson Volume 6 Experiments and since I am new to building circuitry I waned to know what, if any, modifications would be needed to run this badboy off of a wall wort with a regulator and filter circuit. I was thinking a 12v 500ma?

    [​IMG]

    Thanks in advance for anyhelp.

    -Romes
     
  2. #12

    Expert

    Nov 30, 2010
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    No can do unless you create a, "fake" ground at the middle of a 12 volt supply and use that for the ground symbols at the inputs of the two op-amps. Both of them require very little current, so you can just use 2 resistors and a capacitor to make the voltage at the middle hold still. Say...10k ohms each with a 10 uf capacitor connected to the most negative voltage of the wall wart.

    Got it?
     
  3. Romes

    Thread Starter Member

    Sep 30, 2014
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    Thanks for the quick reply, let me draw up what I think you mean and post it for you to review, about ten minuets.
     
  4. #12

    Expert

    Nov 30, 2010
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    I love minuets, especially the one by Bach...Minuet in G.
     
  5. Romes

    Thread Starter Member

    Sep 30, 2014
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    Haha, got to love spellcheck, anywho, I was thinking, what if I used a transformer and a rectifier to make a +12/-12?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You forgot about the speaker ground, which takes a lot of current. For that a common solution is to use a large capacitor in series with the amp output to the speaker. A 1000uF cap will give a -3dB rolloff at 20Hz with an 8 ohm speaker. Connect the plus terminal of the cap to the amp output.

    And you need a 24V supply, not a 12V supply unless you want about a factor of four reduction in the maximum output power.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    For +/- 12V you need a center-tapped 20V transformer (which gives about +/- 14V peak DC output from the rectifier/filters).
     
  8. #12

    Expert

    Nov 30, 2010
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    Oh darn. You caught me again! :(
    That speaker is going to cause quite a lot of current. :eek:
    Crutschow came up with a method, otherwise you're going to need 2 wall warts to get a proper voltage supply with a good, current carrying ground.

    Here's the wrong way:
     
  9. #12

    Expert

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    And, yes, a center tapped transformer with a full wave rectifier and 2 filter capacitors will work, but that's a lot different from a wall wart or two.
     
  10. Romes

    Thread Starter Member

    Sep 30, 2014
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    From your earlier description, this is what I came up with, but where would the -12v go?
    [​IMG]
     
  11. #12

    Expert

    Nov 30, 2010
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    Look at my drawing in post #8 and add the capacitor that crutschow described then change the ground on the speaker to v-.

    Right crutschow?

    I mean, you can't dump all that speaker current into a 10k voltage divider.
     
    Last edited: Oct 1, 2014
  12. Romes

    Thread Starter Member

    Sep 30, 2014
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    Ok disregard that last post, can you explain why you say it is the wrong way?
    What if the speaker was a 4ohm?
     
  13. #12

    Expert

    Nov 30, 2010
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    My drawing did not account for the speaker current.
    If the speaker is 4 ohms, double the capacitor to 2200 uf.
     
  14. Romes

    Thread Starter Member

    Sep 30, 2014
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    Ill give it a shot! Thanks so much for you timely help and patience for my newby self!
     
  15. #12

    Expert

    Nov 30, 2010
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    Por nada. This was a quickie compared to most of the questions we deal with here.
     
  16. Romes

    Thread Starter Member

    Sep 30, 2014
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    crutschow, thank you to you as well!
     
  17. crutschow

    Expert

    Mar 14, 2008
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    That is correct.

    I have an old Dynakit 120 amp (60W/Ch) which uses that output technique that I built many moons ago. Works well and still has a great transparent sound. It was one of the first quality solid-stage audio amps to be sold.
     
    Last edited: Sep 30, 2014
    #12 likes this.
  18. #12

    Expert

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    Here. I added the capacitor and ground changes.
     
  19. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Just get two wall warts, connect them as a standard split supply- done.

    I have been bitten by phantom ground splitters before- hard to make them stiff and stable.
    Once I designed this complex audio device with a single supply, and an opamp driven faux ground circuit, it worked great until the client showed up, then the faux ground went into oscillation, I had to wear a bag over my head for a week.
     
    Last edited: Oct 1, 2014
  20. Romes

    Thread Starter Member

    Sep 30, 2014
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    Thanks for adding the caps and pots. Your a good man.

    Anywho I built the circuit, following the schematic and illustrationhttp://72.135.236.34/shares/USB_Storage/20140930_230731.jpg and all I get is a little hum, and the VR changes volume.
    Ill throw up a few pics of the build, and I would really appreciate it if you see anything obviously wrong.
    I know that the photos arent going to be too detailed, but I can elaborate it need be.

    Again, thanks so much for the effort on your guyes part.

    PS: Since posting I did chane the N on the speaker to -V and still nada
     
    Last edited: Oct 1, 2014
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