LoZ Pickups

[David Schwab]

My main question is what all the voodoo is in the shrinktube on the underneath of the alumitone. It looks like no one was willing to sacrifice one and take one apart, but who would a $100 a pop. It looks like 2 bobbins coiled with fine wire. I dont want to rip off his idea but I think understanding the concept helps.

It's a step up transformer.

David,
I was checking out your store and I see you use Neodymium in some of your pickups. What pull force is acceptable on my strings before funky things start happening? I tried using some online magnet calculatorsand it looks like the magnets I have pull with .11 lbs of force from a 3/8 inch space between the strings and the magnet. Is that too much?

I don't use neos directly under the strings because they pull too much. Maybe small ones would work fine.

[Cashmaro]

BBSailor,

I wasn't looking for resistance, thats pretty easy to calculate. The Low Z pickups aren't very resistive because there isn't much wire involved. I was looking for voltage and amperage output based on a unit stringpluck, which would be either an impulse or step function. The input doesn't matter so much, I just wanted to make sure the output was equivalent to Lace's design since they are both going to be on the same guitar. I drew up the circuits and intended to solve them using Ohm's law, but I need to add the string movement and use Faradays law as well. I'm not sure if anyone paying attention to this thread are into the engineering so much but if anyone is interested I will add it to this thread.

I have the bracket being machined now, it will look like Lace's for aesthetics but be 1/4 inch thick.

Where can I get the CSE-187L, XLR-3 and a mic mixer from for a good price?

Thanks for the input!

[bbsailor]

BBSailor,

I wasn't looking for resistance, thats pretty easy to calculate. The Low Z pickups aren't very resistive because there isn't much wire involved. I was looking for voltage and amperage output based on a unit stringpluck, which would be either an impulse or step function. The input doesn't matter so much, I just wanted to make sure the output was equivalent to Lace's design since they are both going to be on the same guitar. I drew up the circuits and intended to solve them using Ohm's law, but I need to add the string movement and use Faradays law as well. I'm not sure if anyone paying attention to this thread are into the engineering so much but if anyone is interested I will add it to this thread.

I have the bracket being machined now, it will look like Lace's for aesthetics but be 1/4 inch thick.

Where can I get the CSE-187L, XLR-3 and a mic mixer from for a good price?

Thanks for the input!

Cashmaro,

Post a photo of your bracket and tell us the type or metal you are using. If you are making it out of alumium, you will need to use "C-shaped" laminations to wind your miniature bobbins on. The best source for bobbins is: Cosmo Corporation - Coil Bobbin Catalog Main Menu .

The Triad CSE-187L current transformers are available form most large mail order electronics suppliers. The 3 pin XLR connectors are commonly availabe at Radio Shack. The Guitar Center has a single microphone channel microphone input mixer for about $40.00.

The output level is dependent on three variables. (1) Magnet strength, (2) The impedance of the primary string loop, (3) number of turns on the secondary coil or the turns ratio.

Obtain some current transformers, thick copper wire (household behind the wall wire is easy to find in many sizes from AWG 12 to AWG 8), packs of ceramic magnets available from Radio Shack, and thin copper tubing to make good solder joints between the CT primary leads and the string loop.

I have made up a special 10 foot cable (2-conductor shielded) connected to an XLR mic connector with alligator clips on the end to quickly try various current transformers. You can even use two CTs on each end of the string loop and then attach one to the mic input and the put a variable load (pot) on the other CT and change the impedance of the string loop and hear tonal variations with the second CT open, shorted or with a variable load.

Add your engineering analysis so others can jump in.

I hope this helps?

Joseph Rogowski

[Cashmaro]

BBSailor,
See pictures attached.

The bracket was made out of 6061 Aluminum bar stock. It was machined with endmills for the big cuts and the slots were made with a saw blade. I had a machinist buddy of mine make it in his spare time. The sharp corners were made with files, its rough but I haven't cleaned it up and sanded down the mill grooves yet. You can see it in comparison with the standard alumitone bracket on my guitar.

I have some aluminum weld wire I intend to use to grab current from the bracket, the same way Lace does.

My magnets are 1"x.25"x.125" thk neodymium iron boron magnets, N42. I realize I can boost my voltage output (and inversly impact amperage) by turns ratio : Vs/Vp=Ip/Is



I will refine my work in a later post.

Thanks for the help,
Cashmaro

[uneumann]

Many thanks for sharing some great info and ideas. I may have missed something in an earlier post, but it seems that a major advantage of LoZ is reduced (non-existent?) noise pickup due to electric fields. But what about magnetic fields? Don't these LoZ designs still pickup hum from transformers?

[bbsailor]

BBSailor,
See pictures attached.

The bracket was made out of 6061 Aluminum bar stock. It was machined with endmills for the big cuts and the slots were made with a saw blade. I had a machinist buddy of mine make it in his spare time. The sharp corners were made with files, its rough but I haven't cleaned it up and sanded down the mill grooves yet. You can see it in comparison with the standard alumitone bracket on my guitar.

I have some aluminum weld wire I intend to use to grab current from the bracket, the same way Lace does.

My magnets are 1"x.25"x.125" thk neodymium iron boron magnets, N42. I realize I can boost my voltage output (and inversly impact amperage) by turns ratio : Vs/Vp=Ip/Is

[ATTACH=CONFIG]18494[/ATTACH][ATTACH=CONFIG]18495[/ATTACH][ATTACH=CONFIG]18496[/ATTACH]

I will refine my work in a later post.

Thanks for the help,
Cashmaro

Cashmaro,

If you want to grab the current from the alumium bracket, you will need to obtain some "C-shaped" transformer laminations and feed them through two bobbins (about .75" long) connected in parallel. The resistance of each bobbin will need to be twice the rated Alumitone pickup resistance since the two coils are in parallel. I suspect you will need about 10,000 turns of AWG 48 to make a high impedance version to match other pickup output. If you go for a low resistance output, you will need between 50 and 70 ohms DC on each bobbin to match a low-Z input impedance.

Your biggest challenge will be to locate long "C-shaped" laminations to fit the thickness of the common node point and coil bobbins where the current from both low alumium coils passes through the interleaved laminations. Check the on-line catalog from Cosmo Corporation for bobbins.

Please share photos of how you design is progressing.

Thanks

Joseph Rogowski

[David Schwab]

Joseph, can't you connect the two coils in series?

[bbsailor]

Joseph, can't you connect the two coils in series?

David,

Yes, the coils can be connected in series but I believe the patent says that parallel is the preferred connection. Also, since we are dealing with a current transformer, a series connections that work well with voltage-based pickups are reversed (parallel connection) with current-based pickups. Current transformers with 500 up to 1000 turns ratios work well with impedances up to about 500 ohms (low-Z inputs). Generally, high impedance pickups work in an impedance range about 100 times (turns ratio squared) higher than low-Z pickups. This means that high impedance pickups have about 10 times more turns than low-Z pickups. These conclusions come from my experimentation and the application of transformer theory to current-based vice voltage-based guitar pickups.

If you have an Alumitone available, please measure the physical size and resistance of each coil or look at the coils and confirm how they are connected, series or parallel? Also, see if you can estimate the gauge of the wire being used. I suspect that they use AWG 46 to 48 wire to fit enough turns on the small bobbins.

Current transformer output impedance is closely related to the turns ratio squared times the DC resistance (impedance) of the primary loop. Assuming a 10,000 turn secondary, the impedance will be 100,000,000 times a few hundred microohms, the resistance of the primary alumium loop.

I hope I have answered your question?

Joseph Rogowski

[Mike Sulzer]

David,

Yes, the coils can be connected in series but I believe the patent says that parallel is the preferred connection. Also, since we are dealing with a current transformer, a series connections that work well with voltage-based pickups are reversed (parallel connection) with current-based pickups.

Joseph Rogowski

A current transformer is designed to sample the current in a circuit, affecting the circuit as little as possible. This means that it should have a very low impedance. This should be compared to some thing that samples voltage, which should have as high an input impedance as possible for the same reason: not to affect the circuit. You can use current transformers for other purposes than sampling current; they are useful when you are working at a very low impedance and want to step it up many times.

To use a current transformer as a current sampler you want to make sure that the impedance including that reflected back from the load on the secondary is very low compared to the impedance of the circuit connected to the primary. I do not think that you are doing that with the single turn pickup because its impedance is so low, and therefoe not much higher than the load placed on it by the transformer. I believe that you are just using it as a step up transformer, and so it is not necessary to conform to the connection topology that you would use when using it as a true current sensor.

[madzub]

Joseph,

I took some photos of the Alumitone guts with my webcam, not great shots but i'll post one. I'll also measure the bobbin size, tho the pickup is currently down at the shop so i'll get around to it tomorrow perhaps. I've rewound both my Alumitones, but I'm pretty sure the originals were wired in series, not parallel. The coil wire was very thin, felt like a fine fur after slicing it open, much finer than 42 awg, tho i don't think i could measure it accurately. I'll also be wiring one of the Lo-Z Alumitones up again soon -- they've been out of use for a while now waiting for a new guitar to be built -- and will try out a series-parallel switch, and let you know how the sound differs.

[David Schwab]

I've rewound both my Alumitones...

Was there a reason for that? Is that photo after it was rewound with larger gauge wire?

[bbsailor]

A current transformer is designed to sample the current in a circuit, affecting the circuit as little as possible. This means that it should have a very low impedance. This should be compared to some thing that samples voltage, which should have as high an input impedance as possible for the same reason: not to affect the circuit. You can use current transformers for other purposes than sampling current; they are useful when you are working at a very low impedance and want to step it up many times.

To use a current transformer as a current sampler you want to make sure that the impedance including that reflected back from the load on the secondary is very low compared to the impedance of the circuit connected to the primary. I do not think that you are doing that with the single turn pickup because its impedance is so low, and therefoe not much higher than the load placed on it by the transformer. I believe that you are just using it as a step up transformer, and so it is not necessary to conform to the connection topology that you would use when using it as a true current sensor.

Mike,

Thanks for jumping in. Generally, current transformer topology uses a low resistance burden resistor that must stay attached to the secondary to prevent a very high voltage from developing and arching out the secondary wire insulation. However, in a guitar pickup the current developed in the primary string loop is between 0.1A to about 0.3A and only a voltage of about a few hundred millivolts is developed on the secondary output. With a 250K volume pot load on the secondary and assuming a 10,000 turn secondary, reflects 2.5 milliohms back into the primary. The resistance of an alumium loop 0.125" thick by 0.25" wide and 7.5" long (about the Alumitone dimensions) equals 247.6 microohms using this calculator: Resistivity Calc .

The patent shows both series and parallel connections and notes a tonal variation with different coil connections. Generally, you want the volume pot value to be about 10 times the pickup output impedance. With a 10,000 turn secondary coil and about a 250 microohm string coil primary loop, you have a 25,000K ohm output impedance.

The thing that makes current transformer pickups interesting and different is the tonal variation or voicing you get when altering the resistance of the low impedance string loop that goes directly around the magnet. I reverse engineer my desired output impedance range by selecting the current transformer turns ratio, 500 to 1000 for low impedance pickups and 5000 turns for higher impedance outputs along with choosing the wire gauge that goes around the magnets.


Mike, do you feel that using thick alumium or copper as the primary loop creates eddy currents or skin effects that attenuates higher frequencies? Do you think that using Litz wire as the primary loops will improve the high frequency components?

Joseph Rogowski

[bbsailor]

Joseph,

I took some photos of the Alumitone guts with my webcam, not great shots but i'll post one. I'll also measure the bobbin size, tho the pickup is currently down at the shop so i'll get around to it tomorrow perhaps. I've rewound both my Alumitones, but I'm pretty sure the originals were wired in series, not parallel. The coil wire was very thin, felt like a fine fur after slicing it open, much finer than 42 awg, tho i don't think i could measure it accurately. I'll also be wiring one of the Lo-Z Alumitones up again soon -- they've been out of use for a while now waiting for a new guitar to be built -- and will try out a series-parallel switch, and let you know how the sound differs.

[ATTACH=CONFIG]18685[/ATTACH]

Madzub,

Thanks for the photo. When you measure the coil bobbins, try to measure the size of the wire window that is occupied by wire so that we can more accurately estimate the turns on each coil and reverse engineer the electrical characteristics of the pickup.

Bobbin inner length: ?
Bobbin inside square opening size for lamination to fit: ?
Bobbin outer flange size: ?
Bobbin wall thickness: ?


Joseph Rogowski

[madzub]

Was there a reason for that? Is that photo after it was rewound with larger gauge wire?

I rewound the first one after breaking a wire (it was very fine!) while trying to partly take it apart to see what was inside... I then rewound it following the Lo-Z info in another thread, and liked it so much that i intentionally rewound another working Alumitone. The originals didn't sound amazing to me, not bad, but like a regular pickup with less resonance, more clarity in the highs, but nothing more. The Lo Z version seems to have a lot more going on, greater dynamic range being the most important. I'll get it wired up again soon and do an A-B comparison with a regular pickup to compare accurately.

Yes. the photo is of the rewound pickup with 34 awg wire.

[Cashmaro]

BBSailor,

I see where you were going with the off the shelf transormers, an easy way to get a voltage boost (1000X with one, 2000X in series and some less in parallel) without adding a lot of resistance. This would not kill the high end, resulting in a very bright pickup.

I tried looking at the math, but from the looks of it it is a complex circuit that perhaps someone with an elictrical engineering degree could solve, however I am just not that great with electronics math. Bajaman had shown a circuit in one of the other threads, and its quite complex to derive a frequency response from. If I had access to some electrical engineering tools such as PSpics I would maybe be able to get what I was looking for with some help.

Lace is getting some 4000 ohms resistance, doing some math i found it would take about 8000 turns of 42 AWG wire with a mean turn diameter of .5 inch to get that resistance with both coils in series. I think it was about 111 ft of wire on each bobbin.

I am going the route of Madzub, I bought 100 Ft of AWG 30 wire and I planned on wrapping 2 bobbins with 50 ft each, which is about 350 turns each. I had one bobbin machined and wound it already. I attached an image of what i have so far and a 3D rendered image on my winding technique, it worked fairly well and I had little scatter winding. I have some steel weld wire which I planned on using in place of the C-shaped lamination. Since it is drawn steel it should have the grain directionality I am looking for but I will probably get some losses from eddy currents. This is a potential problem point.

All I need to do now is make another bobbin, wrap it and assemble and hopefully it sounds good. I was going to try wiring in series and parallel and see the differences. I will keep posting my progress as it comes.

Madzub,
Compared to the original alumitone, are you getting a better high end frequency response? Was the pickup quiter?

Thanks for more imput everyone,
Cashmaro