pickup magnets advice - homebrew

Allen_VA · #1 02-19-2009, 08:37 PM

I'm building a pickup for an electric upright. I'm using 3" x 1/8" x 1/4" Neo bars. I'm trying to figure out the difference between bar (rectangular) and rod (round) magnets - if any. Is there some weird orientation of the North/South polarity? I'm not getting much output! Any thoughts?

Thanks!

l0calh05t · #2 02-20-2009, 04:10 AM

There shouldn't be much of a difference between bar and cylinder shaped magnets IMO

Anesthesia · #3 02-20-2009, 04:26 AM

from the basic understanding i have of how a passive pickup works is...

permanent bar magnet provides element 1 of electricity. magnetism

the copper coil wound around the magnet is part 2.the conductor

the strings are the third part of the equation. movement in a megnetic field. it cuts the lines of force created by the magnets as they pass through the more fields the cut and the closer to the magnet where they are stronger you create higher output voltage.

also as little in the way of shielding between magnet and air is good as well. doesnt make a huge difference but cant hurt to get the magnet closer.

i could suggest trying reversing the polarity of the coil. just swap positive and negative. may have a difference but i highly doubt it.

get the pickup closer to the strings

i know neo's are high power but maybe a bigger magnet.im not really in the know for magnet strength to volume ratrios.

add a battery to the coil make it an electromagnet. that should work. somewhat increase your sound volume

btw i am an electrician so these are based on knowledge not dumb assumptions although i do not build transformers or permanent dc magnet machines.

just a little disclaimer...

Anesthesia · #4 02-20-2009, 04:27 AM

also bar magnetes will have one continues field bar magnets up ended will produce more fields and overlapping fields. more lines of force cut more voltage.

l0calh05t · #5 02-20-2009, 04:42 AM

Well, there are multiple ways of building pickups. Either the pole pieces themselves are magnets (which is what I believe the OP is planning to do, merely with small bar magnets instead of cylindrical magnets) or the pole pieces are metal bars (or screws) in contact with a larger bar magnet underneath. The shape of the bars/cylinders will affect the shape of the magnetic field, but I believe the difference between a 1/4"x1/8" bar from a 1/5" diameter cylinder (same area) shouldn't be significant.

l0calh05t · #6 02-20-2009, 04:50 AM

back to the problem of small output:
1) How are your magnet(s) oriented? All in the same direction? Either north orsouth updward? Sideways won't work correctly.
2) How many windings (approximately)? If you don't know, what is the winding gauge and the DC resistance? Too few windings will result in low output.

Allen_VA · #7 02-20-2009, 08:00 AM

Quote:

Originally Posted by l0calh05t

back to the problem of small output:
1) How are your magnet(s) oriented? All in the same direction? Either north orsouth updward? Sideways won't work correctly.
2) How many windings (approximately)? If you don't know, what is the winding gauge and the DC resistance? Too few windings will result in low output.

1.) They're all oriented end-on-end. I don't know which end is North or South, but they're oriented the same way (I used another magnet and made sure all the ends had the same polarity...but I did this about a year ago!)

2.) # of windings is related to the bobbin turn length and the desired resistance. I wound a little over 8800 turns on the bobbin, giving me a DCR of 8.54k, which is pretty much dead-on what I designed for.

I wound it scattered-wound, not paying much attention to 'evenly distributing' the windings across the width of the bobbin.

I should post a picture - but the bobbin window width is like 1.5" (3.8cm) so I have a wide but thin layer of windings, instead of a typical pickup which has a narrow but thick layer of windings. I figured with a longer magnet I'd want more magnet area covered by copper - but I don't know how much that geometry really matters in a bass pickup.

Last: when I say "low output", what I mean is, I held the pickup near the strings of one of my electric bass guitars and plucked the B and E strings very near the pickup. I measured the AC voltage using a digital voltmeter - The fundamental should be strong, and the AC meters generally respond well between 40 and 400 Hz but I'm getting very little output. I'm not really in a position right now to hook the pickup up to a preamp or an amplifier to see if I hear anything - I did this all in my basement with limited tools and equipment on-hand. I was just expecting somewhere around 100 to 300 mVrms considering the strength of the Neo magnets. I want the pickup to be 'hot' since it will have to amplify the sound of an electric upright bass with a fair amount of distance between the strings (probably nylon core, nickel or steel flat-wound outside) and the pickup.

Does any of this help?

l0calh05t · #8 02-20-2009, 09:04 AM

A pic would be helpful. Anyways, the number of windings and the DC resistance sound about right. Am understanding correctly that by "end-on-end" you mean that one of the poles points towards the strings and the other pole away from the strings? This would be correct as well... How much output did you get btw?

Allen_VA · #9 02-21-2009, 02:10 PM

Quote:

Originally Posted by l0calh05t

A pic would be helpful. Anyways, the number of windings and the DC resistance sound about right. Am understanding correctly that by "end-on-end" you mean that one of the poles points towards the strings and the other pole away from the strings? This would be correct as well... How much output did you get btw?

I think I attached a picture...hopefully it'll show up.

After a little more work I realized, I think, that the poles are not on the ends, but rather, on the long wide faces. I took an extra magnet and if I hold the end of it near another magnet, they'll attract. If I spin the magnet over about its long axis (using the same "end" of the magnet), they repel. I believe the problem is simply that the magnets are the wrong kind for making a pickup like this! I need the N and S poles at the two ends of the magnets, not on the two long faces.

SOOOO.....I should be able to push the magnets out and replace them with iron or steel bars of the same dimensions as the magnets, and lay a magnet long-way, flat on the bottom of the pickup instead, right? Do the steel bars need to physically touch the magnet on the bottom, or is it sufficient to just be close?

l0calh05t · #10 02-22-2009, 12:21 AM

Quote:

Originally Posted by Allen_VA

I think I attached a picture...hopefully it'll show up.

I can't see any pic, sorry.

Quote:

SOOOO.....I should be able to push the magnets out and replace them with iron or steel bars of the same dimensions as the magnets, and lay a magnet long-way, flat on the bottom of the pickup instead, right? Do the steel bars need to physically touch the magnet on the bottom, or is it sufficient to just be close?

Should work, but I think they will need to physically touch the magnet.

BruceWane · #11 02-22-2009, 03:56 AM

Quote:

Originally Posted by Allen_VA

1.) Last: when I say "low output", what I mean is, I held the pickup near the strings of one of my electric bass guitars and plucked the B and E strings very near the pickup. I measured the AC voltage using a digital voltmeter - The fundamental should be strong, and the AC meters generally respond well between 40 and 400 Hz but I'm getting very little output.

I've no idea how exactly how much voltage you should be seeing, but it's not going to be much - millivolts. Have you tried hooking your voltmeter up to the pickups in the electric bass to see what kind of output they produce? That way you'll have a baseline to compare to.......

Allen_VA · #12 02-22-2009, 06:09 AM

Try the pic again...

Allen_VA · #13 02-22-2009, 06:18 AM

Quote:

Originally Posted by BruceWane

I've no idea how exactly how much voltage you should be seeing, but it's not going to be much - millivolts. Have you tried hooking your voltmeter up to the pickups in the electric bass to see what kind of output they produce? That way you'll have a baseline to compare to.......

No, I haven't checked one of my passive basses yet, but I am pretty sure I should be seeing somewhere around 100 to 300 mV RMS for a strong output. I really need to find out how it sounds, but I don't have a finished EUB yet to test it on. Might have to put this baby on hold for a while until I get the instrument finished.

l0calh05t · #14 02-24-2009, 02:21 AM

The pic worked this time, but you already discovered that the magnets are not pointing in the right direction after all. Anyways, when you do rewind it with correct magnet alignment, it would be nice if you could post the output voltages of the resulting pickup, and of a regular pickup for comparison.