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Milliamps, please explain in great detail.

Discussion in 'Effects [BG]' started by Josh Ryan, Jul 19, 2002.


  1. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    Hi all, I've recently been told that everything I know about mA, (which could fit in a fortune cookie, no problem) is wrong!!! Can someone give me the technical rundown? the definitive answer? the final word? the TRUTH with a capital TRUTH? I'm sort of confused about it again.



    edit: pedal power is the reason, of course.
     
  2. pudgefactor

    pudgefactor

    Mar 28, 2002
    Boston
    mA in terms of what? A milli-amp is one-thoasandth of an amp. An Amp is a measure of the
    flow of electrons. The good ole engineering formula is [ Voltage = Current * Resistance , V=IR ].

    Impedance is basically Resistance in this formula. A 4 ohm cab sucking up 20V will be using 5 Amps, or 5000 mA.
    (Impedance is resistance with a phase altering relation to sinusoidal signals and involve more complex formulas than V=IR)

    An example might be: a signal from your bass is a +/- 1.0V 10Khz signal. This feeds into a filter with
    a bad bypass that offers 50 ohms of resistance. Your amp is only getting a +/- 20mA 10Khz input signal (+/- 0.02A).

    I'm a little rusty on this...........
     
  3. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    In a nutshell, if I have a wall wart with a mA rating of say, 350, and a pedal that says 9v 100mA, can I use that wall wart for the pedal? And what makes that OK? My friend had his theremin repaired recently, and I went with him to pick it up. The technician who repaired it said that the high mA of the plug he was using cooked a resistor inside the theremin.
     
  4. That's 1/1000th of an amp, very low resistance and voltage... like pudgefactor said.
     
  5. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    OK, I'm not getting my point across for some reason. Maybe if I break it down. I have a wall wart that's for a 9 volt 500mA pedal. Can I use this for any pedal that is 9 volt, and LESS than 500mA?
     
  6. pretty much... yeah
     
  7. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    OK, the guy who fixed the theremin said that would burn out the pedal, that's where they confusion comes in. I'm wondering what the juice box or the Voodoo labs thing do to not destroy pedals.
     
  8. If you're affraid it will burn it out why not just but a new power suply? They're pretty cheap already, and besides I would rather trust the guy who fixed the theremin than us. :p :D
     
  9. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    Well, I wanted to use an all in one to minimize plugs, and you guys haven't steered me wrong yet. In general you all seem to know what your talking about here, so I thought I'd ask in the hopes of getting a response like, "the power brick things send only the mA needed" or something like that, if that is the case. I can't imagine Dunlop or Voodoo labs make a products that actually destroys pedals, but I was hoping for some confirmation.
     
  10. Jeff Moote

    Jeff Moote Supporting Member

    Oct 11, 2001
    Beamsville, ON, Canada
    I'm not sure I would, here's why: max current rating of the supply (500 mA in this case) is how much current any loads may draw from this. If a pedal draws 13mA, then that's what it draws. The power supply doesn't force electrons through the circuit. Where the opposite is true, is if you use, say a 12VDC adapter for your 9VDC pedal. This puts the full 12V through it, and can cook it, but current is drawn, not pushed.


    Hope that helps :D
     
  11. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    Yes ,that is EXACTLY what I was asking! Thanks MontyP!!!!
     
  12. Jeff Moote

    Jeff Moote Supporting Member

    Oct 11, 2001
    Beamsville, ON, Canada
    No problem ;)
     
  13. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    The theremin repairemin is full of beans, and Monty is correct. I remember at least that much from when I used to teach electronics (back in the 60's, LOL).
     
  14. Phat Ham

    Phat Ham

    Feb 13, 2000
    DC
    We just studied power supplies last semester and MontyP is right on the money. The 500mA rating is just the maximum the wall wart can supply. Like Munji said the theremin repair dude doesn't know what he's talking about. just goes to show just because someone is a "professional" doesn't mean that person knows anything.
     
  15. Paul A

    Paul A

    Dec 13, 1999
    Hertfordshire U.K!
    I think they are 'bout 2 inches long with loadsa legs...:confused:
     
  16. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    Thanks all, thats what I was looking for!
     
  17. In the 60's, eh? :p
     
  18. This is absolutely........correct. Time to get a new repair guy! If what he was saying was true, you wouldn't be able to plug, say, a TV that only draws 4A into a wall socket that can provide 15A. How inconvenient would that be?:p
     
  19. Josh Ryan

    Josh Ryan - that dog won't hunt, Monsignor. Staff Member Supporting Member

    Mar 24, 2001
    True! It rung false to me, I'm glad you guys could clear it up.
     
  20. Jeff Moote

    Jeff Moote Supporting Member

    Oct 11, 2001
    Beamsville, ON, Canada
    Wow, I didn't know I was that correct :D

    That's three more experienced TBers in this area confirming me, wow. (Munji, Phat Ham, Spacegoat)

    These guys could surely elaborate (as Spacegoat did [extra example]) where that was the extent of my knowledge here. I wish I knew as much as them, but all in time I'm sure :p