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Why wire the wires?

Discussion in 'Hardware, Setup & Repair [BG]' started by Suburban, Apr 30, 2002.


  1. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    or, specifically:
    What is the physics behind grounding the strings (via the bridge)?

    I know that the strings will act like antennas, but I don't get how it can be transmitted to the output, unless the bridge is badly grounded, meaning the bridge ground actually acts as a lead to output.
    The "disturbance" to the electric fields due to this antenna effect would be extremely small - no?

    Yeah, this is out of normal bass bounds, and I do expect a looong and enlightening answer.
     
  2. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    The bridge ground prevents the bridge/strings from being one plate of a capacitor. The other plate of the "accidental" cap would be the input circuitry of the bass.

    It really doesn't have anything to do with antenna theory.

    Too simple?

    Pkr2
     
  3. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    No, but not really complete, either.

    A capacitor needs two connectoers to the same circuit to anything, if I remeber correctly. How is that achieved in this case, and what is the, well, "sonic" effect from that capacitance?

    (Yes, I am the non-regular PIA.)
     
  4. The other connection is you. If you have no ground then you touching the strings becomes the shortest and quickest path to Earth. there's your second connection ;)

    :D:D

    Merls
     
  5. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Electronics 101 in four sentences or less.:) :)

    Good call, Merls!

    Sorry Sub, There are about eight chapters of A.C. theory that one would have to be familiar with in order to understand the answer to your question. Far beyond the scope of the forum. Probably far beyond my abillities as an electronics instructor also.

    That's why Merls is dedicating his studies to electronics.

    I'm sure that merls will agree, it's a tough subject even with college instructors.

    Pkr2
     
  6. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    Oh dear, I just got electricute (N.B spelling is intended to some kind of pun)

    So,I get the circuit thing, sort of. But what does that cap do sound wise? It's supposed to add some noise (when disconneted, meaning no fingers on the strings). What does it induce to the remaning circuit?

    No, I'm not gonna quit until I get some kind of hang of this! (BTW, in my MSc from '85, there were some electronics studies involved, though it does need a refresh...just shoot!)
     
  7. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    That's just it. There is no physical capacitor.

    A cap consists of two conductors seperated by a dielectric.

    The strings/bridge become one plate of a cap. the other plate of the cap is the conductors of the input circuitry. The dielectric is the air between the two conductors.

    You now have a simulated cap with one plate attatched to the input of the amp. The other plate(strings/bridge) are attatched to nothing. An open circuit, being an extremely high impedence, becomes succeptable to A.C. hum which can be called "stray" hum.

    This stray hum couples across the dielectric of the phantom cap (air) into the input circuit of the amp and is heard as AC hum through the speakers.

    When you ground the bridge, you effecively short out the phantom cap.

    disclaimer: I know I was a little fast and loose with theory so, to the techy types, go easy on me.:)
     
  8. pkr2 spot on.

    before the world got all high tech and such, in many radios the tuner was attached to a capacitor (in theory), it involved a series of plates, separated by air, turn the knob and the plates slide in front of each other etc, thus chagning the capacitance.

    Capacitors block DC (direct current) but pass AC, thus they are used in coupling between amp stages so you get no DC component and as such send your speakers to hell. That is why (as Pkr2 explained) we get that AC hum in our amp if you are not grounding. But when you touch it, you ground the other side of the "cap" and bingo, short it out. Take your fingers off and the cap comes alive again.

    Capacitors also "hold" charge so occasionally you hear of people saying "woo i got a slight shock/tingle when i touched my bass!". Thats usually due to poor earthing in the amp, but still has the same effect.

    :D:D

    Merls

    and yes studying it is hard, can you imagine spending all day having topics like this explained?? Ahh i still love it. ;)
     
  9. Figgered that out by playing around the back of the TV set with yer screwdriver didja? :)
     
  10. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    The theory behind that phantom cap can actually be used to some advantage.

    Those lamps that turn off and on by touching are designed around pretty much the same circuit that we try to eliminate in our basses.

    I remember the old variable caps very well, Merle. They were state of the art when I first became interested in electronics. Life was much simpler and so was electronics back then.

    Having been out of electronics for quite some time, I'm sorry to say that I've been passed by when it comes to the current solid state technology.

    Pkr2
     
  11. Suburban

    Suburban

    Jan 15, 2001
    lower mid Sweden
    Thanks, guys, I'm back on the boat now.
    Well done!:cool:
     
  12. my word! and with amps too especially the valve amps...... when you hear someone say "lit up like a christmas tree"... believe them!! pheew learn once and once enough though.... :rolleyes:

    :D:D

    Merls
     
  13. Showdown

    Showdown Supporting Member

    Jan 21, 2002
    Honolulu, Hawaii
    By the way, many, if not most, active basses don't ground the bridge, but leave it completely isolated from the circuit. When I installed a EMG active circuit on a previously passive bass, the instructions said to disconnect the ground from the bridge.
     
  14. Thats probably because the active eq has a ground on board, in form of the ground rail and takes this from the "negative" side of the battery(s), if you earth the whole lot, it might cause hum or a short somewhere. :)

    :D:D

    Merls