Adjustable regulated power supply

Discussion in 'The Projects Forum' started by kriksis, Feb 26, 2016.

  1. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    Hi everybody,

    I had a post about transformers (link bellow), but as questions drift away from the topic, here is another one.
    http://forum.allaboutcircuits.com/t...rom-center-tapped-vs-dual-transformer.121203/

    As mother google has told me, better power supply means better performance for audio amplifier. After finally getting my hands on massive enough transformer, I have designed general layout of the supply.
    What I would like is, for someone to comment if I have not made any silly mistakes. Or maybe done something too complicated (unnecessary). Please find the schematic attached. Any productive criticism / advise is welcome.

    Amp is based on two lm3886 ICs.


    For whoever finds time to look through this, - thank you in advance!
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No obvious problems. Comments:

    13,700 uF is a lot for a 1 A output current. What is your calculated peak-to-peak ripple voltage?
    Where do you get a 3700 uF capacitor?
    Add a 0.1 uF capacitor to each regulator output to reduce high frequency noise.
    Connect the floating ends of each pot to its wiper. This reduces noise during adjustment, and prevents the regulator output from jumping to the maximum possible voltage if the wiper contact fails.

    ak
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Lm317/337 will give 1 Amp each, thats 25W per regulator, i would use the 317/337K regulators, big heatsinks needed, use 4700uF caps on input and output.
     
  4. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    So, what you are suggesting, is that due to fact, that LM317 can handle a wide range of voltage, the caps after bridge can be smaller? Just I have them laying around with no particular use.
     
  5. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    If I understand it correct, then heat that I would have to dissipate is calculated based on difference between the input and output of regulator, is it not?
    That is actually the only reason, why I included pots instead of fixed value resistors. So I can fine tune the difference.
    Lets say, input is 22.5, output is 20.
    (22.5 - 20) x 1A = 2.5w of heat to dissipate. Heatsink required, but reasonable one.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    25 W *output* per regulator. It might be only 2 W of internal dissipation, depending on the regulator's input voltage.

    ak
     
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
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    The two small capacitors on theLM337 side appear to be upside down, assuming they are polarised types.

    Regulated PSUs are not always the best choice for audio power amplifiers as their necessary current limiting can prevent them from delivering the large current peaks required for music reproduction at higher levels; a simple unregulated supply with plenty of capacitance can be superior in this respect.
     
    eetech00 and #12 like this.
  8. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    As I wrote, I'm aiming high - to get the input only 2V higher. Just enough for regulator to output the smooth voltage, so it would not have struggle with turning the excess voltage into heat.
    About the transformer, - yes. In fact, I found a nice (I hope), large transformer, that according to my tests, seems to be CT and provides all the volts I need. By the looks of it (size), it should be with enough umpfff as well to not overheat. I took a picture and posted it on the other discussion about ct vs dual rail.
     
  9. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    Thanks for the advice. So what you're saying, is that I could just skip the regulators and drive the amp directly from rectifier?
    This is my first amp build, and although datasheet for this op-amp claim, that input voltage can vary 20v - 84v, I assumed, that steady voltage goes a long way. Sort of, don't yank it and it will last longer.
     
  10. #12

    Expert

    Nov 30, 2010
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    No. It's more like you have a dog barking at your speakers and you put a leash on it with a regulator. Take the leash off and let those transistors bark!
     
  11. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Yes, take the power connection from the capacitors and forget the regulators. The LM3886 has a high PSRR (Power Supply Rejection Ratio) so is largely immune to mains hum and fluctuations in rail voltage. The majority of linear audio power-amplifiers ever built use simple unregulated supplies, and that includes most high-end hi-fi amplifiers.
     
  12. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    Ohh, dogs. Sure, I like dags.
     
  13. kriksis

    Thread Starter New Member

    Feb 23, 2016
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    Thanks, makes sense. This actually eases up some things. Thanks for advice!
     
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