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Thread: Low Impedance Pickup Research

  1. #351
    ToneOholic! big_teee's Avatar
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    This is the most watched thread on the Pickup Forum.
    With over 100,000+ views!
    Because of it's popularity, stuck the thread to the top.

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    Last edited by big_teee; 05-08-2019 at 09:19 AM.
    It's only Rock and Roll, but I like it!

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  2. #352
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    Quote Originally Posted by big_teee View Post
    This is the most watched thread on the Pickup Forum.
    With over 100,000+ views!
    Because of it's popularity, stuck the thread to the top.
    Big Teee and other forum members,

    I have been sharing my private research using current based guitar pickups for a few simple reasons. (1) easy construction (2) less electric guitar typical sound due to resonant points being near where the ear is the most sensitive and (3) targeting a low impedance mic input typically 2400 ohms (actual input impedance) being a good load for microphones in the 150 to about 250 ohm range (4) simple access to over the counter current transformers (CT) to make experimentation very easy.

    Here is a little more detail. I initially used a Triad CSE-187L 500 turn CT which works very well but found that the pre-installed AWG 12 primary CT turn tends to restrict the current even when a thick string loop wire is used because the 1.8 inch primary is in series with the thicker string loop using 2 solder joints. When I tried the Triad CSE- 186L current transformer rates at 166 turns, i simply removed the three turns of AWG 16 primary wire that is pre-installed to open a large square area to accommodate an AWG 8 wire going through the transformer opening and continue for a few inches to make a string loop to span about 2.25 inches with some room to bend the transformer 45 degrees to 90 degrees to easily fit an acoustic guitar round hole opening or to slide the pickup under an electric guitar with the transformer bent up. This way you can build a low impedance pickup attach the 2 conductor shielded mic wire and run it directly into a mic XLR input or put a Shure A95U matching transformer directly into the guitar amp input and play the guitar without any major modifications.

    Once you have a working CT based pickup, you can easily experiment with magnet, (1) strength, (2) type, (3) covering, (4) size and (5) layout to listen to sound changes as well as string loop wire gauge sizes and stranding to hear the effect on the sound.

    Solid AWG 8 at 52 micro ohms per inch makes a good pickup in the 104 to 120 ohm range for a length of 8 inches making the hairpin string loop to fit directly in the CSE-186 primary opening with tape on the transformer lamination, on each side, to prevent the string loop from shorting against the transformer lamination. Working with this thick wire and making a good low resistance joint to finish the string loop can be a challenge. then I discovered this wire bundle:https://www.goldmine-elec-products.c...?number=G22583. This fine stranded wire has less skin effect than the solid wire and is much easier to work with, especially on making an acoustic guitar pickup. I use a copper crimping sleeve by Garden Bender, part number 10-301C for wire gauges 18 to 10. First, I squeeze the round sleeve into an oval and push the two ends of the string loop through the sleeve. Clean the sleeve with contact cleaner to make the lowest resistance joint. Compress the sleeve on the wire and use silver solder on the end to fully secure of the strands to the sleeve. I get a reading of about 180 ohms on my Extech LCR meter using this stranded copper wire.

    With my pickup using an epoxy coated magnet 2.25 long by .375 wide and .25 high with a .125 inch wide groove on each side to hold the wire better to the magnet (K&J Magnetics). With the pickup .5 inches below my low E electric guitar string I get about 10mv and about 5mv from the high E string at the same location below the strings. On an acoustic guitar with a thin magnetic string core on the low E string, the output would be closer to the same.

    I ordered a 100 epoxy coated .25 inch diameter by 1/8 inch thick Neo magnets from K&J Magnetics part number D42E. I chose epoxy coated to eliminate any eddy currents that would be generated in the metal coating of some of their models. I use ice cream pop sticks to make test pickups.

    Stype 1 places 6 magnets to fit directly under the string near the end of the fingerboard on an acoustic guitar. Then the string loop goes around these magnets and the XLR cable hot wires are soldered to the two end pins on the CSE-186L CT. The shield goes to the string loop and to a jumper wire from the string loop to the CSE-186 metal transformer frame. Clean the glue or other stuff off the transformer frame before soldering to ensure a good joint. This will ensure a good low noise pickup.

    Style 2 places the strings between adjacent magnets spaces about .125 inches apart. Use 7 magnets so the string is sitting exactly in between the magnets. This allows the magnets to be a little closer to the magnets as it sits in a slightly null zone. This setup will emphasize the 2nd harmonic of the initial pluck or strum as it generates a pulse at each end of the string sideways swing. Then you can slide the pickup sideways to put the string directly over the magnets and hear the effect of this magnet position relative to the strings. Fender did this on their bass guitars to minimize the fundamental tone called the the muddy sound on their early electric bass guitars.

    Things to consider when making CT based pickups. The output impedance is very close to the total resistance of the string loop wire times the CT turns ratio squired. Using AWG 8 at 52 micro ohms per inch makes an 8 inch string loop of 416 Micro ohms. Just move the decimal point 6 places left to do the math. .000416 ohms times 500 squared is 250,000 times .000416 or 104 ohms as the theological lowest resistance, minimum output with the highest current and highest output. Since the joint that completes the string loop will add some resistance the output will have a slightly higher resistance with slightly less current and a little less output. Now, look at the area of the transformer primary space that is occupied by wire and you have leakage inductance that also raises the output impedance some. Once you tinker with this stuff enough you will discover these things for yourself and hear how it all affects the sound you hear from your own creations.

    I am glad I have stimulated many readers to get me over the 100,000 mark. Thanks to all of you that have read, supported and experimented based on my "Low Impedance Pickup Research" postings.

    Thanks

    Joseph J. Rogowski

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    Last edited by bbsailor; 05-09-2019 at 01:51 PM.

  3. #353
    Twobie
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    Many thanks for sharing your knowledge Joseph and all folks! It's really nice that you still reply our questions up today! I've learnt so much by reading the posts on this forum.

    Here are some of my progress moths ago.
    While I don't put my hands on a better transformer... I'm thinking to do some tests with the one that I got.

    I'm using a 5 mm (0.19 in/ ~4 AWG) copper bar to make the primary. It still needs some cuts to do. (picture).
    I pretend to do the lower cut to fix its height, then use the cut leg to make the connection between the primary legs, closing the primary circuit. The connection is shaped to fill the gap using just pressure.

    So I think it will work, but not sure if it will increase or decrease the resistance, when compared with a solder joint.

    Anyway, Ill post the results here.


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  4. #354
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    Diego,

    Try this low current Kemet Current Transformer. https://content.kemet.com/datasheets/KEM_SE0203_CT.pdf

    The most important task is to make a very low resistance connection by fitting the CT in the slot and putting a piece of copper tubing in the CT hole and then placing a screw sideways to clamp the metal on the slot sides against the copper tube to make a good low resistance connection. The copper tube should be just a little longer than the current transformer thickness so you do not crush the CT when you add the screw. Try both the 500 turn CT and the 1000 turn CT and use either a mic mixer XLR connection or use the Shure A95U type transformer. Make sure you clean the spots where the metals connect to get the lowest resistance connection.

    Your design is a very innovative way to make a string loop with a current transformer located on the side and under the magnet(s). If you get the CT too close the the magnet(s), you may want to use a piece of ferrous metal to shield the CT from the magnetic field.

    Joseph J. Rogowski

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    I have built a few Lo Z pickups with the help of forum members. They are about 5mm thick and use a neod magnet 50mm long, and 500 to 750 turns of 36 ga. This will give a very clean sound into my Henricksen Jazz amp, through the XLR or the regular jack (with a mic xformer). The xformer increases the signal but is not strictly necessary. However, once I introduce a volume pot, it only works well thru the xfromer. Otherwise the pot introduces horrible static. I don't know why that is. Maybe someone can explain it? I also wonder why Gibson used resistors in the Les Paul Lo Z guitar circuits?Click image for larger version. 

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    I am very pleased with these thin pickups, and they can be simply attached to a guitar top with blue tack. They have no hum and are very even sounding, perfect for acoustic guitar. I plan to install one permanently on an archtop guitar however, with volume and tone controls, but this static is a puzzle.
    Don Nathan

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  6. #356
    Member bea's Avatar
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    What do You mean by static?

    I am running a 500 turns LoZ pickup with a 2.5-k-Pot (and a C-Switch), into an assymetric cable. No probs. But it was also ok, when i sent the signal through a symmetric cable into my mixing console. So i do net get the problem.

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    Using the volume pot creates the static, which is very loud. My cable is not balanced, as I use a single conductor wire with shield. Is it possible this is coming from the 48v phantom power the amp puts out on the xlr? I don't have a schematic and the manufacturer, Henriksen, did not reply to my request for help. I use a 10k pot, and this seems to work ok through the xformer side.

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    The correct way to wire this is to make sure the XLR ground goes only to shielding on the guitar, and not any part of the signal path. If the volume control is connected only to the XLR (+) and (-), there should be no noise because the +48V is present on both of them.

    Quote Originally Posted by Singer15 View Post
    Using the volume pot creates the static, which is very loud. My cable is not balanced, as I use a single conductor wire with shield. Is it possible this is coming from the 48v phantom power the amp puts out on the xlr? I don't have a schematic and the manufacturer, Henriksen, did not reply to my request for help. I use a 10k pot, and this seems to work ok through the xformer side.

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    On my amp there is no switch for the 48v phantom power, which comes on the #1 pin. Without connecting that pin there is no signal. No problem with xlr on an amp without phantom power. Does anyone have a way to switch the 48v? Henriksen Jazz amp 310.

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    Quote Originally Posted by Singer15 View Post
    On my amp there is no switch for the 48v phantom power, which comes on the #1 pin. Without connecting that pin there is no signal. No problem with xlr on an amp without phantom power. Does anyone have a way to switch the 48v? Henriksen Jazz amp 310.
    How did you check for voltage? If you saw voltage between pins 1-3, you should also see voltage on 1-2. Phantom power should appear on pin 2 and 3, pin 1 is common. If your wiring is non-standard, that is the source of your problem. Again, the voltage between 2-3 should be negligible, and won't affect any microphone that doesn't use the common except as an electrostatic shield (such as passive mics). You should do the same with your guitar. Use a balanced connection to the guitar circuit and your problem won't exist.

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    Quote Originally Posted by Singer15 View Post
    Using the volume pot creates the static, which is very loud. My cable is not balanced, as I use a single conductor wire with shield. Is it possible this is coming from the 48v phantom power the amp puts out on the xlr? I don't have a schematic and the manufacturer, Henriksen, did not reply to my request for help. I use a 10k pot, and this seems to work ok through the xformer side.
    You can decouple it with a DC blocking capacitor in the guitar.

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