Power Supply questions - isolated vs hum filtered

Discussion in 'Effects, Pedals, Strings & Things' started by x-ray specs, Feb 20, 2012.

  1. x-ray specs

    x-ray specs Member

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    What is really the difference between a regulated/hum filtered PSU and an isolated PSU, other than one has isolated outs and the other doesn't? Is there a great advantage to having an isolated PSU when the other is hum filtered and regulated? Wouldn't a regulated/hum filtered PSU be about as quiet as an isolated PSU?
     
  2. chervokas

    chervokas Member

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    You're talking about pedal power supplies? You're talking about two different aspect of the power supply and two different potential sources of noise. Regulation is just about smoothing the AC ripple from the DC power (remember you're talking 120 VAC from the wall and sending out 9V DC to the pedals) so you don't have hum or buzz from ripple --just like in the power supply of your amp. Isolation just is about avoiding ground loops. Ground loops can cause hum because the loop acts like an antenna so you want to avoid that by avoiding multiple paths to ground.

    So, two separate aspects of power supplies that address two different potential sources of noise.
     
  3. jnepo1

    jnepo1 Silver Supporting Member

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    I can't think of any PSU that has isolated taps w/o a toroidal transformer. The toroidal transformer is designed for audio use to help defeat/minimize noise. Primarily, a combination of the isolated taps and the toroidal transformer work in conjunction w/ each other in combatting potential noise in audio power units. Any power unit w/ a wallwart will not have isolated taps nor a toroidal transformer, as a toroidal transformer has a limited range in maintaining low noise from the source and needs to be in close proximity to the taps.
     
  4. chervokas

    chervokas Member

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    Well the toroidal transformer -- which is doughnut shaped -- has a smaller EMF radiation field than a typical laminated transformer, so you can put sensitive equipment closer to the toroidal transformer than to a laminated transformer without the likelihood of inducing hum (another potential source of hum), but it still has some field...I mean, stick a vintage wah on top of a PP2+ and there will be induced hum, even with the toroid.

    So good regulation reduces AC ripple, one potential source of hum. Isolated taps reduce group loops -- which can be antennas for induced hum. And toroidal transformers reduce the size of the radition field from the power supply itself in an attempt to limit the potential for hum to be induced into nearby devices from the power supply's xformer itself.
     
  5. drbob1

    drbob1 Silver Supporting Member

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    Actually, I don't think folks are grasping the electronics of isolated taps. The issue there isn't ground loops, because we're talking DC pedals with ground being at the same level in all of them. Any ground loop would come thru the amplifier and would be eliminated by proper isolation of the input power.

    Isolated taps means that the individual pedal power outputs are electrically separate from each other. They don't just share a ground, they don't just share a 9v buss. If I understand correctly, each will have its own 9v voltage regulator and a ground tied directly back to the power supply. So, any power irregulaties (like one pedal loading down the dc to 8v for example, or a pedal needing a different ground) shouldn't cause problems to the other pedals...
     
  6. chervokas

    chervokas Member

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    Well Voodoo lab specifically sells it's ground isolation scheme as designed to avoid the ground loops that can result from daisy chaining from a single power supply used to power multiple pedals as well as a scheme for, as you say, being able to run a neg tip output on one tap and pos tip output on another. I haven't cracked one open to look at it or seen a schematic but now I'm curious to do so.
     
  7. amp_surgeon

    amp_surgeon Member

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    I think the kind of ground loop problems they're talking about are the kind that arise when you try to use a daisy chain power supply with pedals that would otherwise have a different ground level. For example, trying to use daisy chain power with a combination of pedals in front and pedals in the effects loop of an amp. There shouldn't be any ground loop problems if you daisy chain pedals that have the same signal ground potential, and have the same ground polarity. For example, a string of negative ground pedals in front of the amp. Ground loops are only one sort of problem that isolated power outputs are meant to address, though.

    I'm not sure "isolated taps" is the correct term for the type of transformers they're using. The term "tap" usually refers to a connection at some mid-point on a transformer winding. If a transformer had one secondary winding with multiple taps then those taps would not be isolated. It also wouldn't be possible to establish a common ground level if you had more than one tap at any mid-point on the same secondary winding, unless your objective was to generate different voltages relative to that ground point.

    It's complicated to explain without pictures. If you wanted multiple voltages, then this secondary with two taps would work:

    Code:
    +-- 27VAC
    (
    (
    (
    +-- 18VAC
    (
    (
    (
    +-- 9VAC
    (
    (
    (
    +-- GND
    
    Likewise, if you had a secondary with a single center tap, and wanted two separate power supplies, then this would work, though it would NOT be isolated (assuming rectifiers, filters, regulators):

    Code:
    +-- pedal 1 +
    (
    (
    (
    +-- GND
    (
    (
    (
    +-- pedal 2 +
    
    However, if you tried to use that two-tap secondary to make three different 9V supplies then you'd have to wire it like this (adding rectifiers, filters, regulators):

    Code:
    +-- pedal 1 +
    (
    (
    (  +-- pedal 1 -
    +--+
    (  +-- pedal 2 +
    (
    (  +-- pedal 2 -
    +--+
    (  +-- pedal 3 +
    (
    (
    +-- pedal 3 -
    
    The above would NOT work. When you tied the grounds together through the patch cables you'd end up shorting both of the taps to the bottom end of the secondary winding. The only way to get completely independent and isolated outputs is to use multiple secondaries:

    Code:
    +-- pedal 1 +
    (
    (
    +-- pedal 1 -
    
    +-- pedal 2 +
    (
    (
    +-- pedal 2 -
    
    +-- pedal 3 +
    (
    (
    +-- pedal 3 -
    
    "Multiple taps" isn't what you want, and "isolated taps" isn't really even possible. What you need are multiple secondary windings, each isolated from the others.

    Anyway, this allows each output's ground to float relative to the other outputs, which allows you to use one output for a pedal in front of the amp and another output for a pedal in the effects loop. It also allows you to connect a negative ground pedal to one output and a positive ground pedal to another output, and then connect those two pedals together with a patch cable without causing any shorts.

    It also makes pedals on one output relatively immune to power line noise caused by pedals on another output. For example, a digital delay on one output would be less likely to couple clock noise over to an analog dirt pedal. This is a problem with daisy chain power supplies as there's not much that can be done inside the power supply for noise which is injected into the DC power line AFTER the power supply.
     
  8. sstweed

    sstweed Supporting Member

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    The biggest deal to guitar players concerning multiple secondaries is that you can power a positive ground pedal like a fuzz face and not short the rest of your pedals and power supply out. Also you can stack the independent outputs to get different voltages. If you don't have those requirements, usually the common ground types will work fine.
     
  9. chervokas

    chervokas Member

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    Anyone dissected at PP2+? I think a single toroidal xformer with 8 separate secondaries might be the topology, at least that's how it was once described to me. But as I said I haven't had a look under the hood of one.
     
    Last edited: Feb 21, 2012
  10. donnyjaguar

    donnyjaguar Member

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    This is misinformation. All CSA and UL listed power supplies have outputs completely isolated from what goes into the wall. Whether they are regulated or not.

    I found the output voltage from so-called 9Vdc wall-wart adapters was anywhere from 10 to 18Vdc when unloaded. That's crazy. I build my own 9Vdc fully regulated supply and things sure are a lot quieter now!
     

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