choking off magnetic permanent magnetic 'pull' with electromagnet 1

[peskywinnets]

Trying to wrap my head around the optimum design for a sustaining device for guitar.

What would be the best arrangement (material/magnet wise) for the following...

Think of (loosely) a guitar pickup...permanent magnet on the bottom, a coil with a steel core. This results in a 'throw' (for want of a better word) of magnetic field from the top of the pickup. What I want to do is augment & retard that magnetic 'throw' by way of running an AC current through the coil. Another way of looking at this is to say I wish to know the best way to modulate the permanent magnet 'pull' on the guitar strings. (i.e. I'm trying to 'gate' the magnetic pull on the guitar strings)

Some observations of tests that I did ( a good while back)...

using a standard guitar pickup, but with a small thin coil I built placed on top of the pickup (circumferencing the guitar pickup pole pieces)...had good results modulating the magnetic pull (the guitar strings sustained well) ...this suggests that it was possible for my coil to successfully impede & augment the 'static' pull of the magnetism permeating out the top of the pickup pole pieces).

using a custom built 'sustaining coil' which was physically much more squat (about 5mm tall vs a standard pickups 20mm height), with strong neo magnets on the base ....had somewhat more erratic/worse results.

Now I don't know a whole heap about magnetism, but my thoughts are that because the device I built was so squat & the magnets on the bottom so strong, that it was difficult for the ac coil surronding the core to choke off the permanent magnet 'throw'.

therefore to my questions, to modulate the static permanent magnetic 'throw' from the top of such a device, what would the optimum set up be? By this I mean, neo magnets on the bottom, ceramic magnets on the bottom, type of inner core (steel, or even alnico magnets), height, should the coil surround the whole core, or just the top etc etc.

Any input welcome!

 

[IRstuff]

Please do a search; there may already be sufficient information in this site:

http://www.eng-tips.com/gsearch.cfm...=FORID:10&ie=ISO-8859-1&q=guitar+string&sa=Go

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[peskywinnets]

I did search, & yes whilst there are threads discussing such a device, my actual reason for posting is less to do with the makeup of a sustainer (which seems to be what most other threads discuss), but how best to choke off a magnetic pull using an AC coil.

In other words, If I put alnico pole pieces in the core, would it be tricky to choke off their magnetic pull on the strings at the top via an AC current through the coil surrounding the alnico magnets.

Would it be easier to choke off the magnetic pull at the top of the device if using a steel core (vs. using alnico magnets in the core)

Would it be better for the AC coil only to surround the top of the core (nearest the strings) ...again, with the purpose of choking off the magnetic pull being the end goal.

Would it be easier to choke off the [pull if the core is tall (vs. squat)

Should neos be avoided (on account they are *so* strong & therefore more diffiult to 'fight' with the 'controlling' magnetic field generated by the coil)

& so on.

this is a bout one specific aspect of controlling the magnetic pull at the top of the device ...& not how do I build a guitar sustainer.

 

[dgallup]

My first question is why would you use AC coil excitation? You are trying to modulate a permanent magnet field which is essentially a DC device. Your AC coil is going to augment the the permanent magnet field in half it's cycle and decrease the the field in the other half.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[EdStainless]

Ok, a few basics.
You need a balance between a magnet that is difficult to demagnetize (so that it keeps working) and one that you cannot change at all.
The reason for the long steel pole piece on the Alnico magnet (or a long thin pin style magnet) is to raise the operating point on the demag curve to minimize the risk of demagnetization.
In order to make such a device that works easily (low powered coil, low heat) and reliably (does not change the magnet over time) I would suggest a long pole piece with the coil near the top of it.
If you use Neo (or any RE magnet) it would need to as small and thin (yes, real thin) as you could make it. It would be easier to use Alnico. But you will need to test to make sure that you are not over driving your coil and demagnetizing the magnet.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[peskywinnets]

Thanks ed...that's just the type of input I seek, alas because I want to mount this device on the guitar body (as opposed to fit into an existing pickup routue ...which is deep), I'm somewhat constrained by height, but I'll take those points on board.

dgallup - re augment/decrease the pull - that the point, I want to 'bias' the permanent magnetic pull, such that as the AC guitar signal (from the guitar output) flows through the coil, it will both augment & impede the pull at the top...this will in turn pukll the string down further & 'release' with each positive & negative phase of the AC signal ...end result is the guitar vibrates in sympathy with the AC signal.

 

[MagBen]

This is a broad question, and you can not expect a specific answer.
Both Alnico and Neo work, both steel and magnet pole pieces (for making the coil) work. All depend on what you want.
There is no external AC current applied, the AC signal is created by the vibrating of strings (so alternatively increase and decrease the flux through the coil). This AC singal will increase the field the permanent magnet created (or by the coil with steel core) when fulx through the coil decrease, it will decrease the field when flux increase, this is what the so-called demagnetization comes from. Not sure how much is the demag field, but I feel it is pretty small compared with the permanent magnet, demag shoud not be a big concern.

neo does give you a strong field, and so you can decrease your turns, or the length of the pickup coil. if it is too strong, you may put it on hte bottom, use steel core to conduct the field.

 

[IRstuff]

The sustainer generates the external AC to create the sustain, by matching the frequency and phase of the string's initial strum

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[MagBen]

It is possible there is an additional external AC signal created by an external power supply, but the signal from pick up coil must be AC, which is created by the viberation of strings. Maybe these two AC signals interacts, making good sounds. But again, the external AC field could unlikely demag the permament magnet.
i never played electric Guitar, but would think the principle of magnetism involved is not complicated.

 

[peskywinnets]

This thread seems to be going off in a tangent!

The AC signal fed into the coil comes from the guitar's bridge pickup (it's buffered & amplified & fed into the sustainer driver coil)...the resulting current through the coil generates an alternating magnetic field. Because a magnetic force can only pull a ferrous material (as op[posed to repel it), a permanent magnet is need to have a quiescent pull on the string, the AC signal through the coiuld both augments & impedes this magnetic pull (if you lieke I'm modulating a permanent magnet;s pull with an alternating AC current through a coil)

This isn't a question about sustainers per-se, but more so the best way to design the core & coil around it, to strangle off the static magnetic pull of the magnetic flux which is routed through the coil

 

[MagBen]

you are talking about the same thing as my first post on how the AC magnetic filed is created. And this is directly related to how you design the core and coil. without understanding how it works, all desgins are in the air!

The second external AC singal is a bit off the topic. But peopele do use additional coils/AC signal for noise cancellation, harmonics control, etc. which may or may not affect the magnetism on pick up coil.

 

[peskywinnets]

In your first post you said...

"There is no external AC current applied, the AC signal is created by the vibrating of strings"

perhaps we're at crossed lines, but there is very much AC current applied to the driver coil (the bridge pickup AC signal get amplified & then it's fed into an amplifier which drives AC current through the sustainer coil.

 

[MagBen]

What is the conflict? I did not see any.
Back and forth movement of strings induces an AC current in the coil, this AC current/signal gets amplified. This AC signal is created by the strings and the coil, not by an external source (e.g., a power supply). Am I wrong?

 

[peskywinnets]

Ok, now I can see that we're in agreement ...it's the word 'external current' which made me raise an eyebrow.

Clearly the pickup signal has no capacity to drive a sustainer coil, so an 'external' power supply is needed (to buffer the pickup signal, then amplify it ...then driving AC signal current through the sustainer coil).

i think it might have been clearer if you'd said "there is no external signal in play here, the AC signal comes from the string itself" ....anyway, as you say, there is no conflict...