Guitar signal

At the risk of sounding pedantic, it's a *voltage* that is induced by a time varying mag field.

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Best Regards,
Mike

I was referring to Faradays law of induction but EMF is the correct term, not voltage. It is a simplification. Good thing I reread the above, because I read "electromagnetic force" as "electromotive force" and thought, "wait!"

Because it's the electric field that causes the conduction electrons to move.

You wouldn't say that if you knew how much auto crap I've dealt with lately ;)

Yeah, the current can't change from on to off instantly.

The coil does what it has to do resist the sudden change in current. More like the charges keep moving to one end where they're so dense in comparison to the other end it effects a great potential. Poisson.

Despite the wording of Faraday's law, that EMF could not occur if charge weren't moved (current) to one end of the coil. It's when I hear "induced" that the old boy comes to mind.

Hey. Why is it that I recall hearing of you or some other Johansson outside of this group? It's would have been many years ago, but it rings a bell. Are you "famous" or is it just that I haven't seen you in SED for a while?

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Best Regards,
Mike

Seeing the guitar pickup is a magnet,it produces an A/C current.Yes?

Does the Preamp input turn it into a DC current?

How does the input deal with the voltage supplied,and does the impedance change when the signal declines.

Also in a guitar preamp why would you earth if the amp had earth problem,wouldn't it go to the earth plate in the guitar,or is that a new thing.That is ,did old electric guitars have no earth?

Thanks

Yes. The vibrating string in a magnetic field changes the induced current in a coil.

No, that would make it impossible to hear. The guitar preamp has two important functions. To amplify the signal, and to convert the high impedance to a lower impedance. That simply means making it stronger, both voltage-wise and current-wise.

The impedance of the guitar pickup is always high, no matter how strong the signal gets.

Not sure if I lost you there, but the guitar pickup has two connections, two ends of the coil which is wound around one or six permanent magnets. One end is usually connected to the metal parts of the pickup and should be used as the ground side of the pickup. Connect it to other grounded metal parts in the guitar. This reduces noise from outside.

Sometimes guitarists want to connect one pickup backwards and mix it with another pickup, to create more harmonics. Then they ignore this precaution and use the ground side of one pickup as the signal side and vice versa. So it is not necessary to connect a guitar pickup correctly, it is just the preferred way to do it.

Here is a very popular guitar effects box, the Ibanez Tube Screamer.

Look at the transistor Q1, it is the preamp which converts the guitar signal from high to low impedance. In this case the next stage is an op amp so no amplification is needed in the first stage.

Here is another input stage which both fixes the impedance and gives amplification, in the Q1 transistor.

Here is a web page with a lot of information about guitar preamp circuits built with transistors of different types:

In all these links you can go up one or more directories and find a lot more guitar electronics information and schematics.

--
Roger J.

At the risk of sounding even more pedantic.. :-)

The electromagnetic influence causes an electromagnetic force which is both a voltage and a current in a conductor. And magnetism around it.

It is scientifically incorrect to see voltage as a cause of current or vise versa, but we often do both to simplify understanding.

Think of the ignition in a car. A voltage from the battery "causes" a current in the ignition coil. The power supply is suddenly cut off, the current continues to flow through the coil, that is the nature of coils. This current "causes" a very high voltage to appear at the ignition plug and results in a spark.

...and things of that nature.. (Schwarzenegger)

--
Roger J.

Not really, but the point is a bit subtle. The ultimate cause of *all* of E&M is "charge", sort of. "Charge" directly produces the electric field, to which voltage is associated with. Current is the flow of charge. It is the electric field by (static) charge that actually causes the flow of this charge. That is, the *accelerating* electric field produces a force that accelerates charges, i.e. Electric field (voltage) is causing current. The *motion* of charges is really secondary.

"Charge" is in quotes because, charge itself is nothing more than a number describing the exchange of photon momentum between "charges". Ultimately, *any* motion can only be instigated by other motion, i.e. exchange of motion from one entity to another, so in this sense it is always motion causing other motion, i.e. Newton's conservation of momentum. However, the motion that is being exchanged in E&M to do this, is not the motion associated with the "current" itself, so in this sense current itself doesn't cause anything.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

I have been around here and in SED off and on for a few years, since

2001. And elsewhere in usenet since 1996/97, in alt.philosophy, for example. Before that I was active in the global amateur computer network Fido from 1990-91.

The first time I came to SED I recognized several author names from books and articles in electronics magazines like Wireless World, Popular Electronics, Everyday Electronics, Elektor, etc..which I had read since the middle of the 60-ies when I started studying electronics.

Nobody takes me seriously, but there aren't much people around with no bodies.

--
Roger J.

The Ultimate Cause of Everything There Is is Desire:

Cheers! Rich

for further information, please visit

I like it, but when you say "accelerating E field", what do you mean? IOW, if a stationary or static charge causes another charge (charge 2) to accelerate toward it, I can understand that the field associated with charge 2 as accelerating, but not the field of the original static charge. Maybe you're referring to the net E field?

--
Best Regards,
Mike

I always thought that was due to the collapse of the magnetic field back onto the coil, creating a potential between the two ends of the coil.

Jon

doh!

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Best Regards,
Mike

I used to think of it as inertia since the mag field lines or flux is drawn as circles. Like a wheel that has to decelerate, thus giving rise to reactance or resistance to change. But the flaw is that you'd then have to think of the current also having to decelerate as if the charge carriers (electrons) really zipped along, but they don't. It's the charge that propagates quickly while the electron drift velocity is snail paced.

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Best Regards,
Mike

All I am doing here is trying to highlight that the conventional terminology is to use the word "accelerating". That is, its is recognised that a electric field generates a *force* and a force will

*accelerate* an *object* as per Newton.

I am referring to the actual *object* that is moving, not its field. The object is accelerating, hence the name accelerating voltage or accelerating electric field. It isn't referring to the field, only that fields accelerate objects.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

Maybe that is a way to see it, but the cause is rather that the magnetic field is _not_ collapsing immediately when the power supply is cut off. Instead the magnetic field continues to exist, generating more current to flow, which results in high voltage in the spark plug.

A coil is like a pump without a motor, it is started by pushing current through it, and it continues to pump current automatically when the power supply has been removed. It slows down because of resistance, like a motor slows down because of friction.

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Roger J.

No, it doesn't, the current stops (more or less) instantly. In this example, there's no capacitor across the contact breaker, to simplify things.

No, it works like this, in inductors, the relationship between voltage and current is:

V = -L di/dt

If current is cut off suddenly, di/dt is very high. in a perfect world, infinite, but we're not in a perfect world, so there is finite decay time.

L is fixed, hence V is also very high.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Sorry, V = L di/dt di/dt is negative, decreasing and sets up a voltage opposite to that which was present before.

You can also look at it as a collapsing mag field (changing flux) which induces an EMF. But is d_phi/dt dependant on di/dt or is there really some inertial thing going on with one or the other or both? Either one is just a buncha photon spin action, eh?

Kevin!!!

And the current very badly wants to *not* turn off instantly. It's one of the premises used to determine initial conditions for a transient analysis. And a cap doesn't want to change voltage instantly.

Hey this is cool! We're OT and still talking electronics.

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Best Regards,
Mike

Our general terminology in E&M is very useful but not how things "really" "are". Of course we all use concepts such as, changing magnetic fields "causing" electric fields an other such heuristic descriptions, but one should always keep in mind that it is just macro descriptions of things we don't even know the "true" operation of.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

Hmmm. Difference between voltage drop and counter EMF. Let's see how many screw ups I can find.

I see Haliday and Resnick managed to foul things up by first writing

lineIntegral [ E.dl ] = -d_phi/dt

and then in the chapter on Maxwell they drop the negative sign and reference the above eq as if they didn't change it. That's Table

37-1 with no mention of why it's dropped. In Table 37-2 the put it back and never mention why but they discuss adding a missing term to Ampere's Law from a prior chapter.

No where else to look for screw ups. Oh well, at least I remember which way the protection diode goes.

--
Best Regards,
Mike

No, the direction of the induced EMF is such as to oppose the change of current that produced it, hence:

V = -L di/dt

As you say, di/dt is negative, hence the induced voltage is positive,

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)