Magnets & Copper Wire: A Pickup Building Thread

[41hzthunder]

Haha, that gave me some laughs, the alcohol and stuff

Agree totally on point 2 with you. I guess we mean the same thing, we just worded it differently.

Yeah, about the string magnetisation, ideally if you could only magnetise part of the string above the pickup in a way that the field is perpendicular to the pickup core. If those kind of strings could be made, we wouldn't need magnets for sure.

In reality the string is magnetised by the magnet in a much more complex way - we're talking 3D here, so the field is a vector with x, y, z components. And when it moves the sum of components that go through the pickup core are the ones that matter. But that's complex math analysis. Doesn't help much here. In practice we can just assume if you point a magnet north up below the string, the string gets magnetized in that direction only. And when vibrating, moves in that direction only. I'm pretty sure that's accurate enough for pickup makers and that's how they've been designing them form the start

I might have a paper on that somewhere, might be able to find it one day....

Its a good discussion. I'm not sure that the magnatizing of the string is really what is at work in a pickup. The string is not in contact with the magnet, so its note being magetized by stroking. The question is whether the magnetic field of, say, an alnico 5 rod magnet is enough to align the free electrons in the string. If so, is that going to create a strong enough magnetic field to induce a current in the coil? I have some doubt on that.

I did get curious about Lenz's law and found a video from MIT that shows the difference of an iron ball passing through copper vs a magnet. The magnet is slowed because it is literally moving the loose electrons in the copper. Kinda' cool to watch. Love the lab coat! Also makes me wonder is a single wind pickup is possible.

 

[RichterScale]

Note: When you dig deep into the physics, using liquid flow as an analogy for magnetic fields and electricity isn't completely accurate. I know that. But the analogy is fairly good at describing how they work together.

I've used water and plumbing as the most basic of analogies to explain the basics of electricity a few times to people I've worked with. Yes, it's not entirely technically accurate, but they usually understand the basics of plumbing and how water behaves. When they find electricity intimidating or think it's voodoo, comparing it to water is the easiest way to help them make the connection to the very basics.

 

[41hzthunder]

I've used water and plumbing as the most basic of analogies to explain the basics of electricity a few times to people I've worked with. Yes, it's not entirely technically accurate, but they usually understand the basics of plumbing and how water behaves. When they find electricity intimidating or think it's voodoo, comparing it to water is the easiest way to help them make the connection to the very basics.

It is useful, as long as you understand the limitations. I think eddy currents aren't that difficult to understand, but are often poorly explained. They are what's at work here. While strings may not be magnetized, they do disturb the magnetic flux, which ultimately creates the induction. I'm attaching another (last one) video that goes into eddy currents. Current is movement of electrons, so that's why loose electrons count. You just have to imagine that a string moves up and down (actually in multiple directions) compared to the single direction in the video. If the current from this action is passed to the grid in, say, a 12AX*** tube, it opens the "valve" between the cathode and plate for current matching the pattern, but with greater amplitude. The current from a pickup actually dies at the grid and a sort of "exagerated photocopy" complete with distortion goes to the next step. Okay, went way over it there. I've bored everyone enough.

 

[41hzthunder]

I like Bruce's coffee cup analogy too! Gets the concept across.

 

[rudy4444]

Just 4 fun...
I have worked with magnetic pickups a little and can verify that a string is not magnetized enough by simply being above common rod magnet material to create a signal in a plain coil. The disturbance in the magnetic field generated by the magnets (as a function of the movement of the string within that field) is what creates the induced voltage in the coil.

ANY movement of the coils, no matter how minimal, will induce a voltage, and that's precisely what happens when you tap the pickup body with a fingertip. No string motion necessary, and it's what causes magnetic pickups to be microphonic.

Any time these sort of discussions come up there's always plenty of pseudoscience that gets thrown in at no extra cost.

Now for the fun part...

I've been experimenting with making magnetic pickups for BANJO use where the string might be steel OR synthetic. I embed tiny neo magnets in opposing magnetic polarity in the feet of the bridge and induce the voltage in a pair of open coils oriented in opposing directions to cancel induced EMI and "hum".

It works fine, but requires more fabrication than I want to put into the process. The super nice thing about this process is using dirt cheap sewing machine bobbins and the ease of winding 7000 turns of #42 wire on them. It's easy enough to glue in 1/4" magnets once the winding is completed if you wind them like I do.

You folks that are doing multi-coils might want to think about doing that and incorporating a preamp to boost the ultimate output level. Remember, a lower number of turns has other ramifications other than a slightly lower output.

Just a bit of food for thought.

 

[Jon Clegg]

Seems like a similar principal to @Bruce Johnson 's 'mystery pickup'

 

[41hzthunder]

Now for the fun part...

I've been experimenting with making magnetic pickups for BANJO use where the string might be steel OR synthetic. I embed tiny neo magnets in opposing magnetic polarity in the feet of the bridge and induce the voltage in a pair of open coils oriented in opposing directions to cancel induced EMI and "hum".

It works fine, but requires more fabrication than I want to put into the process. The super nice thing about this process is using dirt cheap sewing machine bobbins and the ease of winding 7000 turns of #42 wire on them. It's easy enough to glue in 1/4" magnets once the winding is completed if you wind them like I do.

You folks that are doing multi-coils might want to think about doing that and incorporating a preamp to boost the ultimate output level. Remember, a lower number of turns has other ramifications other than a slightly lower output.

Just a bit of food for thought.

View attachment 4601028

So, its acting microphonic? How low/small is the output without the preamp?

 

[Bruce Johnson]

Seems like a similar principal to @Bruce Johnson 's 'mystery pickup'

Yep, my Mechanical Percussive Pickup is similar: The pickup senses the movement of the bridge, rather than the strings themselves. The bridge is mounted on a frame of spring rods which allows it to bounce and roll in 3 axis when the strings are plucked. Two large round coils are mounted stationary in the body. Two strong rod magnets are attached to the underside of the bridge, and hang down through the hollow centers of the coils. Pluck the strings, the bridge wiggles, and the magnets wiggle in the middle of the coils. Signal is created!

Rudy's arrangement has the magnets mounted in the bridge, on the front side of the head, directly over the coils, which are on the back side of the head. That'll work, but not as efficiently as dangling the magnets down through the hollow centers of the coils. He may end up adding large stationary magnets in the centers of his coils, opposing the moving magnets in the bridge, to boost the signal level.

Here's a bunch of pages all about my pickup, from 2008:

Building Scroll Basses- Pickups 1

 

[Bruce Johnson]

Just 4 fun...
I have worked with magnetic pickups a little and can verify that a string is not magnetized enough by simply being above common rod magnet material to create a signal in a plain coil. The disturbance in the magnetic field generated by the magnets (as a function of the movement of the string within that field) is what creates the induced voltage in the coil.

ANY movement of the coils, no matter how minimal, will induce a voltage, and that's precisely what happens when you tap the pickup body with a fingertip. No string motion necessary, and it's what causes magnetic pickups to be microphonic.

Any time these sort of discussions come up there's always plenty of pseudoscience that gets thrown in at no extra cost.

Now for the fun part...

I've been experimenting with making magnetic pickups for BANJO use where the string might be steel OR synthetic. I embed tiny neo magnets in opposing magnetic polarity in the feet of the bridge and induce the voltage in a pair of open coils oriented in opposing directions to cancel induced EMI and "hum".

It works fine, but requires more fabrication than I want to put into the process. The super nice thing about this process is using dirt cheap sewing machine bobbins and the ease of winding 7000 turns of #42 wire on them. It's easy enough to glue in 1/4" magnets once the winding is completed if you wind them like I do.

You folks that are doing multi-coils might want to think about doing that and incorporating a preamp to boost the ultimate output level. Remember, a lower number of turns has other ramifications other than a slightly lower output.

Just a bit of food for thought.

View attachment 4601028

That's all looking good, Rudy, but there's a really big inherent problem with adding pickups to banjos, that you are probably about to discover.

If your pickup system is directly coupled to the movement of the head (via the movement of the bridge), then you will have created a large sensitive 12" diameter microphone! That large flexible head is very efficient at collecting sound waves out of the air, and any vibrations in the frame, and pumping them through the pickup to the amp. That head is more sensitive than your ear. As soon as you can hear sound coming from your amp, it will already be going into feedback.

This was an incredible problem with Banjozilla, which was a 24" diameter microphone. It could detect sounds that I couldn't hear, and start feeding them back.

I've got to get out into the shop now, but I'll write more about this later. Think about. This is a really big problem.

 

[rudy4444]

That's all looking good, Rudy, but there's a really big inherent problem with adding pickups to banjos, that you are probably about to discover.

If your pickup system is directly coupled to the movement of the head (via the movement of the bridge), then you will have created a large sensitive 12" diameter microphone! That large flexible head is very efficient at collecting sound waves out of the air, and any vibrations in the frame, and pumping them through the pickup to the amp. That head is more sensitive than your ear. As soon as you can hear sound coming from your amp, it will already be going into feedback.

This was an incredible problem with Banjozilla, which was a 24" diameter microphone. It could detect sounds that I couldn't hear, and start feeding them back.

I've got to get out into the shop now, but I'll write more about this later. Think about. This is a really big problem.

Hi Bruce,
I've been building banjos for several years with my main focus on shifting focus away from conventional designs which have their own inherent problems. An example of my design work can be seen HERE.

I'm fully aware of the "amplification dilemma". It's probably the single biggest factor that has held the banjo back from wider acceptance in more modern forms of music such as alt country, progressive old time, etc.

There are various ways of using magnetics for banjos but they all have their drawbacks. Peter Tork (from the Monkeys) had a P-90 anchored directly under the strings in front of the bridge when performing live, and there have been various attempts to use mags, up to and including the Kavanjo pickup which extends the pole pieces through holes in the mylar head.

I've even wound some 5 pole buckers that were positioned under the head below the strings. Those work well on banjos with smaller distances from head to strings at the heel, but not as well for standard fare resonator instruments.

Problem is, they all sound like guitars and they only work with steel strings.

My focus has been more on developing non-intrusive methods that work for steel or synthetic strings and incorporate SOME of the head resonance that somewhat defines the characteristic sound of the banjo. My interest in the banjo world has always been open backs, so the good news there is that it's somewhat easier to control the feedback situation by using carefully applied head dampening techniques. In really loud settings you can simply concentrate amplification of string movement and reduce the amount of head energy coupled back to whatever transducer you opt for.

I'm all ears on your thoughts.

Please pardon my slight sidetrack of the topic here. I really only threw this out there to serve as a way of provoking "outside the box" thought on this whole "individual coil" discussion.

 

[Honkey tonk]

It's pretty easy to make A Low Z Transducer out of non ordinary materials? could posibly cut out alot of what you don't want with different materials.
You would be very surprised as to what material combinations will Transduce at LOW Z.

 

[rudy4444]

So, its acting microphonic? How low/small is the output without the preamp?

No, it isn't microphonic.

The coils are held at a fixed distance from the bottom of the head by attaching the coil block to the internal dowel stick within the banjo pot. The neo magnet's lines of force travel through the mylar head and the actual bridge movement is what is being transduced into an output signal by the bucking coils.

The output of the pickup is probably around half of what you would get from a conventional bucker, but you have to consider that the way this works is quite different from how a normal guitar pickup functions.

 

[Kinkh]

Just 4 fun...
I have worked with magnetic pickups a little and can verify that a string is not magnetized enough by simply being above common rod magnet material to create a signal in a plain coil. The disturbance in the magnetic field generated by the magnets (as a function of the movement of the string within that field) is what creates the induced voltage in the coil.

ANY movement of the coils, no matter how minimal, will induce a voltage, and that's precisely what happens when you tap the pickup body with a fingertip. No string motion necessary, and it's what causes magnetic pickups to be microphonic.

Any time these sort of discussions come up there's always plenty of pseudoscience that gets thrown in at no extra cost.

Now for the fun part...

View attachment 4601028

I think we're all referring to the same thing here but just using different wording. If by magnetization you mean becoming permanently magnetised like when you rub iron off a magnet, then definitely no, the strings don't get magnetized.

The magnetization I'm referring to is the orientation of magnetic dipoles in the string material, induced by the proximity of the field of the permanent magnet. If this effect didn't occur, the vibrating string wouldn't do anything to produce the signal. The movement of these magnetised dipoles is what causes the disruption of the field of the permanent magnet, a change in magnetic field.
That's just a property of ferromagnetic materials, and that's why steel and nickel is used for electric guitar strings and not other metals.
No pseudo-science in that.

Here's a bunch of pages all about my pickup, from 2008:

Building Scroll Basses- Pickups 1

Omg, that's your page. I've seen this ages ago and was amazed how awesome all this work was, the details and attention. No wonder your pics here seemed familiar

 

[Bruce Johnson]

Hey Rudy;

Yeah, I assumed that you were at least aware of the problem. I wrote that more as a background warning to anyone else who is thinking of amplifying a banjo.

It really is a serious problem, and I saw it to the largest extent on Banjozilla. I tried a lot of different types of pickups on Banjozilla, and only one of them sort of worked with tolerable feedback levels.

Even with a magnetic pickup mounted on the end of the fingerboard, and steel strings, it still had a horrible feedback problem. The reason was because the head is connected to the strings (through the bridge). Pluck the string, the pickup gets the signal and sends it to the amp. The amp's speaker pumps the air in the room. That air pushes on the banjo head, which then adds more vibration to the string....which sends a bigger signal to the amp....

And you have the same problem with any kind of pickup. I tried magnetics, piezos under the bridge feet, piezos on the rim, even a microphone inside the rim. Banjozilla was/is nearly impossible to amplify in a club stage environment. Even with a directional mic set three feet in front. If you could hear Banjozilla on the stage, through the monitors or the front PA, it was already feeding back. Even the electronic feedback gates didn't work, because it would feedback at any frequency. The feedback gate would run up and down the frequency spectrum, trying to shut down everything. Like I said, a big problem.

The one thing that somewhat worked was putting a pair of Fishman piezo discs on the back side of the bridge, standing up vertically. I think the reason that somewhat worked is that the piezos were flexing with the for/back rocking of the bridge, rather than the up/down motion of the head. I think that put the amp signal 90 degrees out of phase with the head signal. That reduced the feedback quite a bit.

With that setup, I could plug Banjozilla into my little Carvin cube amp, setting it 8' away, facing away from the head. I could play that at low session-on-a-porch volume, and keep it from feeding back.

That's the best I've ever been able to with it. I eventually took the Fishmans off and have kept it as a purely acoustic instrument. I've had Banjozilla in a pro studio a couple of times. We put a good mic in front of it, and didn't put any signal into the monitors. I listened to myself play it acoustically.

Unfortunately, the only way to truly stop this feedback problem is to disable the head. Mount the bridge solidly to the frame. Use the pickup of your choice. Put soft padding behind the head to make sure it doesn't buzz. Then you've got an electric bass that sounds like...an electric bass. The rim and head are just decorative props. Don't do that. Don't build a fake banjo. It ain't right.

 

[Bruce Johnson]

Omg, that's your page. I've seen this ages ago and was amazed how awesome all this work was, the details and attention. No wonder your pics here seemed familiar

Yep, that's me. I'm about the only person who has been messing with Mechanical Percussive style pickups. It's cool to see Rudy giving them a try. I'm still developing the idea. I have a whole new version in the works for my (someday) upcoming Series V AUB-2 Scroll Bass. Some new tricks in the design.

I plan to someday start a special thread just about Mechanical Percussive pickups. But, I haven't gotten to it yet.

 

[1n3]

... I've been experimenting with making magnetic pickups for BANJO use where the string might be steel OR synthetic...

Seems like a similar principal to @Bruce Johnson 's 'mystery pickup'

It's also similar to the electric violin pickup Leo Fender developed in 1958. That had a single coil wrapped around an alnico rod. The bridge sat on a narrow steel plate which could rock slightly. The bass end of that plate had a tab bent down toward that magnet/coil.

While I'm posting, will add that this is one of the most interesting threads I've read on talkbass. There's a lot here for the "pickup curious" to chew on. Some of the concepts are familiar and/or I grasp, and some things - like sidewinder coils - I'm still trying to get my head around. Special mention to Freekmagnet, Bruce Johnson, and slowburnaz. Thanks, guys!

 

[rudy4444]

Hey Rudy;

Yeah, I assumed that you were at least aware of the problem. I wrote that more as a background warning to anyone else who is thinking of amplifying a banjo.

It really is a serious problem, and I saw it to the largest extent on Banjozilla. I tried a lot of different types of pickups on Banjozilla, and only one of them sort of worked with tolerable feedback levels.

Even with a magnetic pickup mounted on the end of the fingerboard, and steel strings, it still had a horrible feedback problem. The reason was because the head is connected to the strings (through the bridge). Pluck the string, the pickup gets the signal and sends it to the amp. The amp's speaker pumps the air in the room. That air pushes on the banjo head, which then adds more vibration to the string....which sends a bigger signal to the amp....

And you have the same problem with any kind of pickup. I tried magnetics, piezos under the bridge feet, piezos on the rim, even a microphone inside the rim. Banjozilla was/is nearly impossible to amplify in a club stage environment. Even with a directional mic set three feet in front. If you could hear Banjozilla on the stage, through the monitors or the front PA, it was already feeding back. Even the electronic feedback gates didn't work, because it would feedback at any frequency. The feedback gate would run up and down the frequency spectrum, trying to shut down everything. Like I said, a big problem.

The one thing that somewhat worked was putting a pair of Fishman piezo discs on the back side of the bridge, standing up vertically. I think the reason that somewhat worked is that the piezos were flexing with the for/back rocking of the bridge, rather than the up/down motion of the head. I think that put the amp signal 90 degrees out of phase with the head signal. That reduced the feedback quite a bit.

With that setup, I could plug Banjozilla into my little Carvin cube amp, setting it 8' away, facing away from the head. I could play that at low session-on-a-porch volume, and keep it from feeding back.

That's the best I've ever been able to with it. I eventually took the Fishmans off and have kept it as a purely acoustic instrument. I've had Banjozilla in a pro studio a couple of times. We put a good mic in front of it, and didn't put any signal into the monitors. I listened to myself play it acoustically.

Unfortunately, the only way to truly stop this feedback problem is to disable the head. Mount the bridge solidly to the frame. Use the pickup of your choice. Put soft padding behind the head to make sure it doesn't buzz. Then you've got an electric bass that sounds like...an electric bass. The rim and head are just decorative props. Don't do that. Don't build a fake banjo. It ain't right.

Hi Bruce.

I hear you loud and clear! The fake banjo sound is one of the reasons I haven't put a lot of effort into the problem, although I think there are better solutions then punching holes in the head and using polepieces under the strings.

I'm firmly in the "If it ain't a banjo then why bother" camp. It's like all of the folks who buy a 6 string banjo tuned like a guitar and then credit themselves with playing "banjo" in their electrified band.

Five strings with a re-entrant fifth string and NO resonator!

My latest experiments have been using two SMALL piezos at the side of each bridge foot and tuned with parallel capacitors across the elements.

Part of the reason I'm focused on that is I've been playing acoustic guitar in bands for a metric ton of years and have used several different pickups in the process. I've installed at least a dozen K&K Pure Minis and love the sound, particularly the lack of piezo "quack". Part of the piezo answer lies in the transducer size, part in using multiple transducers, and the particular way in which they are mounted and coupled to the underside of the bridge saddle via attachment to the bridge plate. I'm working toward using these same ideas for banjo, coupled with the correct amount of acoustic attenuation of the head itself.

Piezos are interesting elements, and few people realize that they are capable of generating extremely high voltage if there's no load across them. There's not a lot of difference between a guitar pickup and the spark igniter used to light your barbecue. How an element is coupled to it's real world "next stage" determines a lot.

Probably the best live sound from a banjo I've heard is Richie Stearns who used a close coupled piezo transducer feeding a small Fender tube amp held in a cradle pointed directly up at him. He also had a DI going to the FOH feed, of course. I've seen him play several times with the same rig, so I know it's doable.

 

[Bruce Johnson]

It's also similar to the electric violin pickup Leo Fender developed in 1958. That had a single coil wrapped around an alnico rod. The bridge sat on a narrow steel plate which could rock slightly. The bass end of that plate had a tab bent down toward that magnet/coil.

Yes, Leo did that violin pickup in 1958, and it's a good early example of a basic Mechanical Percussive type pickup. At almost the same time ('57-'58) Zorko came out with their small electric upright bass, which had twin MP pickups; one under each foot of the bridge. Who stole the idea from whom? Whom knows? In 1959 Ampeg bought Zorko, and further evolved the design into the Ampeg Baby Bass. Ampeg made about 5000 Baby Basses from 1960 to 1970. They are the legendary example of a Mechanical Percussive pickup bass.

In 1966, Ampeg developed the AUB-1 Horizontal Scroll Bass, which had the legendary Mystery Pickup. It was a different form of Mechanical Percussive pickup, with dual coils under a single large (5" square) steel diaphragm. Ampeg made about 1250 AUB-1, AEB-1 and Devil Basses with the Mystery Pickup, between 1966 and 1968.

That's about the complete early history of MP pickups. The idea died out through the '70's and '80's. In recent years, several builders have made variants of the Baby Bass, with a similar style of dual MP pickups under the bridge feet: Steve Azola (US), Ray Ramiriz (Puerto Rico), MOG (Columbia), KK (UK). I came out with my Series IV Scroll Basses in 2006, and made 17 of them total.

That's almost the entire history of MP pickups.

 

[Bruce Johnson]

Probably the best live sound from a banjo I've heard is Richie Stearns who used a close coupled piezo transducer feeding a small Fender tube amp held in a cradle pointed directly up at him. He also had a DI going to the FOH feed, of course. I've seen him play several times with the same rig, so I know it's doable.

All the successful amplified banjos have the same basic trick: the head is severely dampened. With foam or pads, they restrict it to making a slight soft thump; no ringing at all. Then they use a magnetic or piezo pickup and dial in a huge lump up high to imitate the head ringing sound. Some guys have made that work pretty well with modern sound equipment. But it's a mostly-fake banjo, to me.

 

[rudy4444]

All the successful amplified banjos have the same basic trick: the head is severely dampened. With foam or pads, they restrict it to making a slight soft thump; no ringing at all. Then they use a magnetic or piezo pickup and dial in a huge lump up high to imitate the head ringing sound. Some guys have made that work pretty well with modern sound equipment. But it's a mostly-fake banjo, to me.

Agreed.

Perhaps the ultimate solution might be something like the Audio Sprockets ToneDexter, the actual desired instrument tone captured by recording a wavemap with a quality microphone, and then dampening the head and using the basic instrument as a trigger to incorporate the desired tone. There are a lot of acoustic guitar players who use it precisely in that way.

I'm personally a bit adverse to applying that much technology to solve the problem, and I'm more likely to accept some of the shortcomings to play a more visceral and basic instrument. Simplicity has a lot to offer.