# Guitar signal

Discussion in 'Electronic Basics' started by Music Man, Mar 17, 2005.

1. ### Music ManGuest

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

2. ### John PopelishGuest

Yes. And AC voltage, also.
No. It increases the amplitude of the AC voltage.
The impedances are fixed. The output voltage is proportional to the
input voltage, just larger. The power amplifier makes the AC voltage
even bigger and that AC drives the speakers.
The guitar gets whatever ground from the input jack on the amplifier.
This is a zero volt reference that the input signal is measured
against, but it doesn't have to actually be a connection to Earth.
The main value of having the amplifier actually connected to Earth is
that this means there is no difference in voltage between the cases of
the pickups (that act as shields around the coils) and Earth. This
helps reduce hum pickup because there is less capacitive current
traveling up and down the guitar cable shield. But having a good
ground connection also means that you will feel a shock if you are
touching the metal on the guitar and also touch something else that
has voltage with respect to Earth on it. So many tube amplifiers have
a floating guitar common (outside barrel of the cable plug).

3. ### Roger JohanssonGuest

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:

http://www.muzique.com/lab/lowvolt.htm

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

4. ### Active8Guest

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

5. ### Roger JohanssonGuest

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)

6. ### Active8Guest

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?

7. ### Kevin AylwardGuest

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

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

8. ### Roger JohanssonGuest

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
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.

9. ### Rich The PhilosophizerGuest

The Ultimate Cause of Everything There Is is Desire:
http://www.godchannel.com/spirit.html

Cheers!
Rich

for further information, please visit http://www.godchannel.com

10. ### Active8Guest

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?

11. ### Kevin AylwardGuest

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

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

12. ### Jon DannikenGuest

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

13. ### Roger JohanssonGuest

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.

doh!<blush>

15. ### Active8Guest

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.

16. ### Fred AbseGuest

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.

17. ### Active8Guest

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.

18. ### Kevin AylwardGuest

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

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

19. ### Fred AbseGuest

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,

20. ### Fred AbseGuest

Does it matter?

If it matters, does it matter that it matters?