Post by axekicker on Sept 29, 2010 20:25:19 GMT -5
I'm familiar with SD and Dimarzio wiring schemes, but wanted to check in on the Chinese 4-lead pups I'm getting. There is a black wire and a green wire separated. Then there is a bare silver wire that looks like shielding and a red and white wire that are wound together. I'm guessing black is hot and green is ground. The red and white are to split the coil or phase? Not sure what the bare silver shielding is for? Any ideas? Please let me know if it sound right or not. Thanks!
Set your meter to the 20K range if it's not an auto-ranging type. Disconnect the red and white wires from each other.
When you have the 2 wires that comprise one coil, you'll read the DC resistance of that coil, say somewhere between 3KΩ and 12KΩ.
If the wires are for different coils, you'll get an "overlimit" reading ("OL" on my meter), or "out-of-range" on some meters, meaning an infinite resistance (i.e., no connection).
You know the green and black are opposite coils, so test green to white and green to red. Just a process of elimination- once you find the wire that pairs with the green, the other one pairs with the black (but double check 'em just to be sure you've got 2 functioning coils).
Not necessarily, although it might be connected to a grounded point in certain wiring schemes. As Ash points out, it only matters relative to another pickup or pickups- so that the pickups are wired in phase.
Labeling pickup leads as "hot" vs. "ground", or as "+" vs. "-", is a convenient shorthand, but these terms have no real meaning electrically, since we're talking about an AC circuit.
Those terms are all DC terms; in AC circuits, the "hot" and "ground" are continually being swapped back and forth in alternation (that's what makes it "alternating current").
In a DC circuit, "-" and "+" have an absolute meaning; that's the reason you have to install batteries (i.e., DC) the right way for a device to work properly. In an AC circuit, the concepts aren't absolutes, but only relative. A lightbulb socket in your house (AC) can be wired either way and it will still work just fine.
In the old days, AC circuits in a house just had the 2 conductors, so things could be, and often were, wired backwards (i.e., OOP) with respect to other components. Things like light bulbs and toasters don't care which way their wired.
Nowadays, we've added a third conductor for AC circuits, since modern things like computers and other electronics do care which way they're wired- and the third conductor ensures things are all correctly phased in a home AC system. This also helps safety, since 2 AC components not wired in proper phase present a shock/fire hazard.
..... Things like light bulbs and toasters don't care which way their wired.
Nowadays, we've added a third conductor for AC circuits, since modern things like computers and other electronics do care which way they're wired.....
Uh oh, my BS detector just fired off. I'm waiting for an explanation of how any AC-powered device cares which way it's plugged in......
- and the third conductor ensures things are all correctly phased in a home AC system. This also helps safety, since 2 AC components not wired in proper phase present a shock/fire hazard.
That's the exact reason the three-prong plug was dreamt up, safety. For sure, it's not the fact there's a third conductor, it's the fact that the plug can't be inserted incorrectly. This is the saving grace behind the whole idea of wasting yet more copper. You all do know, don't you, that the 'neutral' and the ground wires are tied together at the incoming power box, yes? And this is exactly where "ground loops" come from.
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There is a way to measure the absolute charge of an object. It's as easy as comparing the number of electrons to the number of protons. This really has nothing to do with voltage, though.
Voltage is always relative. One thing has a greater or lesser charge than sone other thing. The AC power in your house is referenced to the earth. It oscillates up and down around the charge of the earth. The reference voltage is the earth, or the ground. We have carried that word along, and now use the words "earth" or "ground" to denote some reference voltage. The actual ground beneath your feet is not really 0V in any absolute sense. Your planet, in fact, has a very significant negative charge. I suspect the spinning iron core, but what do I know.
Think about a car for a minute, though. The electronics in most cars is referenced to the steel frame, connected to the more negative terminal of the battery. This is called ground for all the electronics. The frame of most cars don't have a very good conductive path to ground, though - unless something's gone horribly wrong. Then, as you drive the thing, air molecules will tend to "rub off" electrons, and the frame of the car will start to build up a charge some small amount more positive than the earth. Luckily at these speeds it doesn't build up much, or it might be dangerous to debark. The electronics can't tell the difference at all, since they are firmly referenced to whatever charge the frame holds, even when that changes. Airplanes - which do move fast enough to build up a significant charge - would be in big trouble if this weren't the case.
Back to something more our style. Most stompboxes tie the - terminal of the battery to the chassis, to which the sleeve of the guitar cable is connected, and use that as a reference. We call this 0V. There's a problem with this, though. The voltage on the tip of the cable coming from the guitar is sometime more positive than it's sleeve but also sometimes more negative. The active stages in the circuit are incapable of reproducing any voltage more negative than the bottom of the battery. Unless we're looking for some particularly nasty distortion, we have to raise the reference voltage around which the audio signal oscillates to something greater than 0V by (but less than 9V or we have the same problem) adding some voltage to the signal. This voltage is often labelled Vref (Reference Voltage) or bias voltage, but is also often called a virtual- or audio- ground. This voltage is stripped out before the audio goes on to other devices, but the point is that the box actually has a couple different "grounds" going on at the same time.
It gets weirder, though. Some stompboxes actually tie the + battery terminal to the chassis and jack sleeve, creating what's called a "positive ground" device. All the action takes place at voltages more negative than the reference voltage. Yes, this can cause problems when this pedal shares a board with the far more common "negative ground" variety!
Now back to the guitar and it's pickups. The pickup is a coil which generates voltage as the magnetic field around it oscillates. Depending on the magnetic polarity, one end of the coil will go more positive than the other when the string moves toward the pickup, and they will swap when the string swings away from the pickup. As far as I know, there is no standard for which of these gets labelled + and which is -. As I said above, it doesn't matter when you're talking about one coil. Some folks claim to be able to tell the difference when the speaker moves out (toward them) on the initial attack rather than in (away from them), but I think they're full of hooey. Anyway, the various active stages between the pickup and the speaker are about as likely as not to flip the waveform over (inverting its absolute polarity) so that (unless you research diligently and structure your rig carefully) it's pretty tough to know which way that speaker's gonna go when the string moves a particular direction.
As we've said, though, this "absolute polarity" is important when we combine coils. We generally want two coils to reinforce each other, adding their voltage contributions at least for the fundamental. If we choose wires to where one pickup is "pushing" while the other is "pulling" nothing goes nowhere, and we hear very little. This is that nasty thin sound we call "OoP".