Low Impedance Pickup Research

[bbsailor]

The Lace Alimatone pickups caught my interest recently when I was researching some info related to pickup replacement on my Bass guitar. After reading some of these threads on constructing a Lo-Z pickup (thanks to contributing members, especially bbsailor) and being an occasional DIYer, I ordered a couple of CSE187L transformers and constructed the pickup pictured below. I physically designed the layout around a magnet I scrounged from a discarded magneto-optical drive (guessing neodymium) looped through 2 Low Z transformers with 8 awg copper wire. Output was fed to a line matching transformer with an XLR cord. Volume wise I thought the output was about the same as my passive pickups, but the tone sounded hollow and very trebly. Any suggestions on how to improve construction to enhance low end?
[ATTACH=CONFIG]21019[/ATTACH]

MoreBass,

Yes there is a simple way to increase the bass with the way that you have a CT on each end of the low impedance string loop. Simply hook the microphone wire with the XLR connector to only one CT. Then listen to the sound, then alligator clip or tack solder a short across the second CT. This will have the effect of changing the impedance of the low impedance string loop to make it look like a thicker wire. The bass response should increase. A few years ago I tried putting a 1K ohms pot across the second CT (not connected to the XLR wire) and found that by varying the pot as a variable resistor, I could achieve a tonal change. In my lab notebook I labeled this a "RITE control" standing for Reflected Impedance Tone Equalization.

Just so you and everyone else reading this knows, the output impedance of the CT is governed by the primary string loop resistance multiplied by the square of the turns ratio. Just a CSE187L with the AWG 12, 250 microohm primary bent to form a shorted loop has 79 ohms impedance, 8.4mH at 1KHz measured on the secondary (20 ohms side). When you add AWG 8 wire with 52.3 microohms per inch you can see how each inch adds .0000523 X 250000 = 13.075 ohms per inch of wire plus the impedance of the second CT. Remember, the resistane of everything in the primary loop is in series and good soldering and low resistance connections are required. Try using silver solder for a little lower resistane connection on the primary string loop. You want to keep the output of the CSE187L below about 250 ohms converting 1000 microohms (or 1 milliohm) on the primary side as measured on the secondary. This is a good match for a mic matching transformer or XLR microphone preamp impedance typically around 2000 ohms. The lower the string loop impedance the higher the output will be and the better the low frequencies will be. Shorting out the second CT will be the quickest fix for your bass pickup design. Another quick fix is to put a short between the magnets so each CT sees its own string loop, However, there will be some interaction between the CTs if you were thinking of having a balance control for each string pair.

If hum is a problem, make two short 2-string loops and wire them like a humbucker. You can switch them between series and parallel for some tonal variation. You can put a balance control/independent volume pots (try 1K) to control the balance of each string pair with no interaction between them as the primary string loops are now totally independent.

For more bass, try using AWG 6 wire that is .1620" in diameter. Simply use a drill press and make two vertical holes .078" diameter (to match the CSE187L primary turn diameter) in the vertically-held wire end about .187" deep to insert into the ends of the CTs. Use silver solder to secure.

I applaud your effort and hope these tips will make it sound better. Let us know how it works out and which alternative identified above you tried?

Joseph Rogowski

[David King]

MoreBass,

Disc drive magnets are often segmented with reversed poles (N & S) side by side so you might have a pickup that's fighting itself. I'd try a different magnet that has a known polar orientation. Neo magnets are inexpensive off the shelf.

[MoreBass]

Thanks for the quick replies. This thread has been dormant for over a year so I wasn't sure if there would be a continued interest.

bbsailor,
When testing my dual CT pickup I did try different configurations as you suggested. Shorting out the second CT increased volume significantly and placing a 1K pot across the the CT as you suggested did result in a variable tonal changes, but not enough to enhance the low end of my Bass and eliminate the hollow sound. If I understand correctly, to enhance low frequencies I should reduce resistance with a larger cross sectional loop (6 awg or larger such as ¼ bar stock as you suggested in a previous post), and silver solder joints. Understanding you were looking for cheap off the shelf items for this project, would it be relatively easy to wind a coil/CT to better integrate the the pickup loop with less connections?

David King,
I choose the magnet because it was free and happened to measure .437" x 2.75" and covered all 4 strings on my Bass. I wasn't quite sure which way to orientate it in the loop, but with the magnet loose and trial & error with positioning, I settled on my present configuration. I'll try some different magnets and see what the differences are. Thanks again for the great feedback. I'll keep experimenting and post new results when I get some time.

MoreBass

[David King]

MoreBass,

I would check the magnet against another magnet to see if and where the divisions are located. You could post a photo of the magnet and mark where the divisions lie and the rest of us would be better informed in the future. You are not the first to attempt a pickup using a scavenged magnet.
RE the thin tone, how close is the pickup to the bridge?

[bbsailor]

MoreBass

Submit a photo of your bass, show the location where the pickup will be located. Iindicate the string width over the pickup location and the size of the available space for a replacement pickup. I can recommend the Prem Magnetics SPCT-251 CT (do a web search) which will allow a .160" wire diameter and square wire to be looped through the open primary. Only one is needed. The output impedance will be about 1000 ohms but will be about 4 times higher in output voltage than the CSE187L. Then you only need to make one joint to form the string loop around the magnet. Go to some hobby shops on the web and look for model making copper tubing to make the joint on the string loop (for using round wire). Surplus Sales of Nebraska (do a web search) has some nice square AWG6 wire that will nicely fit the SPCT-251 or can be drilled to work with the CSE187L

I do not recommend winding a coil for a CT pickup design as a single loop, very low resistance loop works best. If you want to wind a pickup like a Les Paul Triumph bass low Z pickup, that is another design issue that we can also offer some help.

Joseph Rogowski

[MoreBass]

bbsailor
Bass is a P/J style as pictured below. I temporarily placed the Low Z pickup on both sides of the split pickup (tape and foam padding), ~ 3" and 7" from the bridge. String width is 2.125" at the pickup. My plan was to build a new body for the guitar with 2 low Z pickups. Next I'll try 6 awg wire (or square wire) and silver solder as you suggested. I searched the web for the Prem Magnetics SPCT-251 CT, about $13, so that sounds inexpensive enough I may give it a try if not satisfied.

David King,
Pictured below is the magnet I used (out of the frame) for the Low Z pickup. Placed some paper and iron filings on top of the magnet, pattern indicated one continuos rectangle. I'll look for other magnets and give them a shot.

MoreBass

[bbsailor]

MoreBass,

See this thread for more good low impedance pickup information. http://music-electronics-forum.com/t3737/. Here is a good rule of thumb for sizing the value of a volume pot for pickup or CT output resistance. Make the volume pot 40 times the DCR of a low Q pickup and about 50 times for a higher Q pickup or a current transformer with a higher core inductance. For the CSE187L at 20 ohms DCR X 50 or 1000 ohm volume pot, for the SPCT-251 at 300 DCR X 50 or 15K ohms pot. Note that most Fender stype single coil pickups that are about 6000 ohms use a 250K volume pot to tame the resonance level somewhat. To the ear it is a tradeoff between pickup output level, tone, noise and what happens to the sound when going from the instrument to the amp through the cable.

Magnets and pickup window width: You can quickly experiment with a wide magnet window of 1" or a narrow window of .25" by trying some commonly available magnets. Radio Shack has a 5 pack of ceramic magnets 1" X .75" X .187 thick, magnetized through the thickness with a hole in the center, part number 64-1879. Some hardware and craft strores carry retail packs of ceramic magnets from www.magnetsource.com , part number 07001, an 8 pack of .25W" X .875" L X .187" thick, magnetized through the thickness. Only your ear can tell which ones sound best and damp the strings the least. K&J Magnetics has a good selection of stronger neo magnets. Try looking at ring magnets between .25" and .375" in diameter that can be mounted with a screw on a foam pad (weather strips) to vary the height of each pole piece inside the string loop.

The advantage of using the SPCT-251 is that the wire acting as the primary turn can be the same heavy gauge wire used for the string loop up to .160" in diameter if round or a square wire to fill the available space. Otherwise, the thinner prewired primary turn, like on the CSE187L with an AWG12, puts about a 250 microohm wire in series with even the thickest wire you can find to make the string loop.

The output impedance of the SPCT-251 (2000 turns ratio secondary) is very sensitive to the single primary string loop resistance as its output impedance is calculated by multiplying the string loop resistance in microohms by 2000 squared or 4,000,000. Assuming that your chosen wire has 32 microohms per inch, then each inch of string loop wire equals 128 ohms. Then you can add between 100 to 200 ohms for leakage inductance, since no transformer is perfect. Filling the primary wire window minimizes the leakage inductance. Use this handy wire resistance calculator written by Daniel, one of our moderators http://www.salvarsan.org/pickups/ResistivityCalc.html .

The Extech 380193 LCR meter is very helpful in doing experiments with current transformers and is a valuable learning tool.

How are you planning to boost the output of the CT to a high impedance level, mic matching transformer before a high impedance bass amplifier, or microphone preamp?

Some preamps have line inputs rated at between 10K ohms and 20K ohms that will allow the SPCT-251 to directly feed them with a good match. To minimize any noise, plan on grounding the low impedance string loop to the output going to the mic transformer or mic mixer. You can even reduce noise further by wrapping a layer of .25" wide copper tape around and over the outside of the transformer core soldered and then grounded. Do not wrap this tape inside the core, as it will look like a shorted turn and reduce the output level. Make sure the copper tape does not short out the output pins on the SPCT-251 on the side with the dot.

Remember, low Z pickups do not have the resonant hump typically found in high Z pickups where the amplitude in the resonance area can be up to 4 times higher than the primary bass string frequencies below about 200Hz. EMG make some nice active bass equilization modules BTC Control (stacked treble and bass) and the one with an added variable midrange control, BQC.

I hope these tips help. Keep us posted about your progress.

Joseph Rogowski

[MoreBass]

bbsailor,

Thanks for all the great info. Looking forward to experimenting with these options you've mentioned and will post results.

MoreBass

[MoreBass]

bbsailor:

I've had a little time to do some preliminary tests following some of your suggestions. After receiving the SPCT-251 CT, I bought 1' of solid 4 AWG at Ace Hardware, hammered it square and looped it around the transformer with one scarf joint silver soldered. I cut pole pieces from a ¼" bolt and placed them through the loop with the bar magnet underneath. To my ear the sound was more broad, even and detailed! I plan on obtaining other types of magnets (cylindrical for pole pieces and various bar types) to experiment with different configurations. Volume output appeared to be considerably less than the standard P/J configuration going through a standard guitar cord, improved a little through LoZ-HIZ transformer. I currently have an onboard JFET preamp I built (Kreuzer - Onboard JFET Preamp) and installed on this guitar which I don't really use because it starts to overdrive with guitar volume at about 3, and the tone change is not significant, but my next thought is to utilize it with this Loz pickup. I currently use a graphic EQ through my rig, but I'm considering building a parametric, something like a Rod Elliott design - Parametric and Sub-Woofer Equaliser . Limited time with holidays approaching, but will post any significant progress.

Thanks again -- you provoke as many questions as you answer, but I enjoy the education and challenge.

MoreBass

[bbsailor]

bbsailor:

I've had a little time to do some preliminary tests following some of your suggestions. After receiving the SPCT-251 CT, I bought 1' of solid 4 AWG at Ace Hardware, hammered it square and looped it around the transformer with one scarf joint silver soldered. I cut pole pieces from a ¼" bolt and placed them through the loop with the bar magnet underneath. To my ear the sound was more broad, even and detailed! I plan on obtaining other types of magnets (cylindrical for pole pieces and various bar types) to experiment with different configurations. Volume output appeared to be considerably less than the standard P/J configuration going through a standard guitar cord, improved a little through LoZ-HIZ transformer. I currently have an onboard JFET preamp I built (Kreuzer - Onboard JFET Preamp) and installed on this guitar which I don't really use because it starts to overdrive with guitar volume at about 3, and the tone change is not significant, but my next thought is to utilize it with this Loz pickup. I currently use a graphic EQ through my rig, but I'm considering building a parametric, something like a Rod Elliott design - Parametric and Sub-Woofer Equaliser . Limited time with holidays approaching, but will post any significant progress.

Thanks again -- you provoke as many questions as you answer, but I enjoy the education and challenge.

MoreBass

MoreBass,

The output will be low by high impedance pickup standards, but the noise will be lower. Make sure you ground the low impedance string loop to minimize the noise. The SPCT 251 output impedance should be about 2K ohms to about 2.5K ohms. This is too high for mic matching tranformers that like to see about 250 ohm inputs but is in a good range for line inputs in a mic mixer input of about to 10K to 20K ohms. You can also try putting a ferrous metal strip under the magnet and the low impedance string loop to increase the output by about 25 percent.

Try using ceramic magnets to minimize the inductance of the primary string loop.

I am glad you are starting to enjoy this stuff.

Joseph Rogowski

[MisterBzr]

I've been experimentating with the single loop pickup concept.
My primary goal is to make a easy, cheap, but good sounding pickup from off the shelf products.

I'm very suprised by the sound of my recent (scrappy) prototype of a humbuckerstyle pickup. It gives a very clear sound. Attached to my Boss gt 6 (using a shure mic to line transformer) I can get some pretty decent guitar sounds out of it. There's almost no noise. I think the input volume is almost the same as a single coil pickup (I don't have equipment to measure the impendance).
I've also hooked it up to my zoom B3 playing a low E bass string and with some eq it gives a usable bass sound.
I want to make the distance between the magnets and string a few mm bigger because now I can hear the magnets "pull" the strings (slight e-bow sound).

I used a piece of 9 mm plywood and drilled 5 holes lengthwise. I've pressed 4 bar magnets (2 in each hole) in holes nr. 2 and 4 and used the other holes for the wire. The wire is connected with a AS-104 1:500 coil.

I used plain electric wire (2,5 mm2 (awg 14?) and 4mm2 (awg 13?) and strong neody. magnets.
I've used both solid and multi strand wire, I've connected them by soldering or using a thermoplastic connector (dutch: kroonsteen).
Al the different setups give more or less the same sound.

Using multi strand wire is easier to use, but what could be the disadvantages?
Are there others who get good results?

Hans Bezemer

[bbsailor]

MisterBzr,

Welcome to the forum. You will find a wealth of technical information freely shared here. Learn to use the search feature to locate specific information that you seek.

You have entered the world of low impedance pickups or should I say very, very, very low impedance string sensing loops typically measured in microohms. With a 1 to 500 turns ratio you will have a transformer output impedance that will be the actual DC resistance of the string loop times 250,000 (500 squared) plus about 30 ohms for leakage inductance. Just use the following web link, enter the wire diameter and length and obtain the resistance of your string sensing loop. Resistivity Calc You want the output impedance to be in the range of 150 to 300 ohms to match the input range of your mic matching transformer.

Ensure that you use a good method to obtain a very low resistance connection of the string sensing loop. I use copper tubing and silver solder to join the ends together. Solid copper wire can be bent into a pretty tight loop but stranded wire will be easier to bend but will be a little thicker due to the air space between the strands. I have used AWG 6 0.160" square wire and can make a .375" "U-shaped" ID bend to fit through the current transformer primary. However, round wire will be easier to work with when using matching copper tubes to make the joint.

You will find that the sound of these low impedance pickups is a departure from the tradtional high impedance pickups for a few simple reasons.

1. Less wire in the transformer core places the resonance point higher in the audio spectrum or possibly above the audio spectrum. There is no resonance in the typical mid range of about 2KHz to 5KHz as is found on traditional high impedance pickups.
2. With the low impedance string loop grounded, there is less noise pickup.
3. No loss of high frequencies by the cable capacitance in a typical 10 to 20 foot guitar cable. Putting the Mic transformer right next to the amp input allows very long wire runs with little to no audible effect. Use 2 conductor shielded mic cable between the current transformer and the mic transformer.

In your experiments try to stay close to the 1 to 500 turns ratio current transformers as the output will be in the range of a microphone matching transformer. You may be able to go up to 1 to 750 or 1 to 1000 but you risk making the output impedance too high to properly match the mic transformer input range. If you do try higher turns ratio current transformers, you would will need to use very, very thick string sensing loop gauges like AWG 4 or 6. The maximum output impedance, in this case, of the current transformer should be no higher than about 600 ohms or about 600 microohms on the primary string loop.


Keep on experimenting and let us know about your progress.

Joseph Rogowski

[MisterBzr]

bbsailor,

Thank you for your reply. I've been following the several low Z threads for a while and find the information very useful.
When I have some new pick ups ready I'll will post some info and sound clips.

One of the things I noticed when I was playing with the pick up was that the position relative to the bridge wasn't making that much difference in sound.
Does anyone has the same (or other) experience?
Is it common for these pickups to be height adjustable? Most designs I see are mounted on the guitar body.


Hans

[bbsailor]

bbsailor,

Thank you for your reply. I've been following the several low Z threads for a while and find the information very useful.
When I have some new pick ups ready I'll will post some info and sound clips.

One of the things I noticed when I was playing with the pick up was that the position relative to the bridge wasn't making that much difference in sound.
Does anyone has the same (or other) experience?
Is it common for these pickups to be height adjustable? Most designs I see are mounted on the guitar body.


Hans

Hans,

The position of this low Z pickup should behave just like other high Z pickups relative to the string position relative to the bridge. Try using some flat ceramic magnets and see what happens.

You are pretty much free to try any variety of magnets. I have used small ring magnets .25" dia with a .125" center hole neo type. I use a piece of rubber or weather stripping under the magnets then use a screw in the center to make an adjustable pole piece. If you have an acoustic guitar you can use craft sticks (like icecream pop sticks). They come in .275" wide and .687" wide. They allow you to make the pickup on the craft stick and then bend down the wire under the stick to mount the current transformer and finally easily mount it to an acoustic guitar. You will get a nice more acoustic-like quality as the higher frequencies are picked up that are not normally heard in a high Z pickup. The challenge here is to obtain a nice string balance as the B-string tends to dominate the sound because it is one of the thicker solid cores. I normally make a string loop with about a .25" dip under the B string then I bend down the wire feed about .375" wide centered under the B string to mount the current transformer. This helps reduce the output level of the B string. Magnet size and shape mostly define the options you can use in the string sensing loop shape.

Since you are not winding many turns for fine wire, the design of the pickup is mostly a mechanical issue to fit the constraints of the space you have to work with.

Read my thread "Moving coil pickups for the technically curious" to see how to use moving metal strings in a magnetic field as the actual pickup by using some 8 ohm to 20K ohm transformers.

You will start learning about another side of pickup design here with these current-driven pickups.


Joseph Rogowski

[MisterBzr]

Joseph,

I've read the other thread on moving coil pickup: Very inspiring. I will post a reply there.
I'm planning to make some single loop pickups and will post some pictures if they're done.
Thanks for your useful information.

Hans