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Building a low noise isolated PSU

  1. Sep 13, 2008 #1
    Hi there,

    For a personal project of mine, I have been designing a low-noise linear power supply to provide power to guitar effects pedals. A couple guitarists I know use a hodge-podge of AC adapters and switched PSU's to run their various pedals (sometimes 8 at a time). This mixture of devices is messy and I am working on a single unit to power all the pedals. They require fairly low voltage ~9v or so at maybe 300mA maximum.

    I have started with a 120v transformer rectified and filtered into eight 78L09 voltage regs. The only problem is that each reg is not completely isolated because they share common grounds. I would like them 100% isolated so that noice on one pedal's supply doesn't effect the supply and therfore the "sound" on another.

    Is there any way to do this in short of putting a transformer on each reg? Do you think I would get clean enough power without isolating each regulator?



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  3. Sep 13, 2008 #2


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    i can't rember what I was makeing but I used diodes between the componant and ground. This ment even if the ground rail was distorted it wouldn't effect anything else (or atleast that was the idea).
  4. Sep 14, 2008 #3
    ok, I will look into that. Havn't heard about it before, but I am an electronics newb.
  5. Sep 16, 2008 #4


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    Yea, I'll ask my teacher if he can rember, not sure how well it will work for something like this though.
  6. Sep 16, 2008 #5


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    To truly isolate them, yes, you would need one transformer per isolated output.

    The next best technique would be to use a "star ground" configuration, so that none of the output channels share any ground impedance. That is, the ground point for the input storage cap (after the rectifier), and the grounds of all of the regulators must be grouped together, and connected to a very low impedance ground island. If you do that, noise currents on the ground of one channel coming in will not be able to generate any ground voltage noise in any of the other channels. For more info, you can google "star ground" "shared impedance".
  7. Sep 16, 2008 #6
    Good suggestions already, but I wanted to add that pursuing low noise power supply for guitar pedals is only going to pay off so much: the circuits you're powering are not generally of great fidelity in the first place. You'd probably get more mileage out of replacing their guts with higher precision components, although this would of course run into asthetic considerations when it comes to stuff like fuzz pedals.

    A very easy way to get isolated, clean power to your pedals is to simply use 9V batteries instead of external power supply. Isn't this what most guitar players do? If you're using too many pedals for that to be practical, I'd personally recommend ditching all but a couple of fuzz/distortion/wah pedals and doing everything else in a dedicated effects processor. In addition to simplifying power supply issues, this would improve sound quality by removing all of the (probably cheap) input/output stages in the effects pedals.
  8. Sep 17, 2008 #7


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    I suspect that the reason you're looking into floating grounds is that you are having ground loop issues with your equipment setup.
    Berkeman's star ground approach needs to be applied to the entire system, not just the one power supply.
    Given the attention to detail required to make this work system wide you are probably better off with quadraphonics suggestion of batteries.
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