Low Impedance Pickup Research

[bbsailor]

All this will work, except for that threaded brass rod. If a 5000-turn coil produces a few volts, one turn will produce maybe a millivolt. Such a low voltage will be unable to punch through any kind of surface film. Sufficiently good mechanical joints are very hard to arrange. All joints must instead be welded, brazed, or soldered.

For all the experimenters using a CSE-187L or any current transformer, Joe Gwinn is correct about this. I can't emphasize this enough: a very low resistance connection in making the single turn string loop is very important for maximizing the output level and tonal balance with lower total string loop impedance favoring lower frequencies better.

I took many measurements with the Extech LCR meter and here are some numbers to help those without the Extech LCR meter play with this CSE-187L transformer and still obtain some good results.

1. If you bent the premounted single turn of AWG 12 wire on the CSE-187L primary into a loop, butt the ends of the wire together and the solder to make a shorted primary loop you will have an output impedance of between 78 and 80 ohms. The difference is based on the winding turns tolerance on the secondary which is specified at 500 turns plus or minus a few percent.

2. The length of the primary AWG 12 wire is 1.89" for a DC resistance of 250 micro ohms or 0.000250 times 250,000 (turns ratio squared) for a calculated output impedance of 62.5 ohms. Transformers are not perfect so in a real transformer there is leakage inductance that tends to look like a small inductance in series with the primary loop of 80 - 62.5 or about 17.5 ohms due to leakage inductance.

3. Using AWG 11 copper magnet wire is 105 micro phms per inch. Copper Magnet / Enameled & Bare Wires A string loop is typically adds about 5 to 6 inches of AWG 11 wire to the 1.89 inch long primary wire resistance. Each inch of AWG 11 wire adds 26.5 ohms to the output impedance measured at the output of the CSE-187L. A 5 inch wire added to the string loop to span the string width adds 26.5 times 5 or 132.5 ohms plus the 62.5 primary loop resistance plus 17.5 leakage inductance for a total of 212.5 ohms as the low impedance output impedance.
This is close enough to the nominal 150 ohms impedance typical for XLR low impedance microphone connections. The key to using this low impedance output is to ensure low noise.

4. Low noise using the CSE187L current transformer is simple. Use 2-conductor shielded microphone cable with XLR pins 2 and 3 going to the floating output pins on the current transformer. Attach the mic cable shield to pin 1 on the XLR and the metal frame of the CSE-187L. Also, ground one end of the low impedance string loop to the metal transformer frame. In some cases you may need to add a 2.5K ohms resistor from each transformer output pin to ground (the transformer metal frame) to ensure the lowest noise connection. Experimentation will tell you what sounds best.

5. If you locate AWG 6 square copper wire at the Surplus Sales of Nebraska web site, you can make a hairpin U-shaped loop that is about 35 microohms per inch so your output impedance will be 80 plus 8.75 ohms per inch (.000035 times 250,000). 8.75 times 5 inches adds 43.75 ohms to the 80 ohms on the primary string loop for a total output impedance of 123.75 ohms.

The lower the output impedance will indicate a lower primary string loop impedance. Here is a little truck that can help squeeze out a little more bass response out of a CSE-187L current transformer. Obtain some AWG 20 magnet wire. This AWG 20 wire will just fit between the CSE-187L transformer secondary and the laminated frame to add from one to four more single turn primary loops all connected in parallel with the installed primary AWG 12 primary wire. This will lower the total string loop impedance and distribute some current directly over the secondary winding to slightly reduce the leakage inductance.

If you have an acoustic guitar, this pickup is idea to experiment with because you can mount it directly behind the fingerboard. Make the transformer connection span the B-string with the transformer primary being vertical so the transformer connection is directly under the B-string to dip the current loop under this typically loud string to better control its output. Solder the 2-conductor shielded wire to an XLR connector and plug it directly into a mic mixer. You will have a very nice acoustic guitar pickup without the classic high impedance pickup resonant hump in the typical midrange where the ear is most sensitive. The creativity is making the mechanical size of the pickup fit your available mounting space.

If you want to try using higher turn current transformers or toroids you may need to use two or more turns on the string loop primary to keep the impedance in the nominal 150 ohm range (100 to 400 ohms). A 1000 turn current transformer multiplys the primary string loop impedance by 1,000,000 (1,000 squared) while a 500 turn current transformer increases the string loop impedance by 250,000 (500 squared).

This is just some stuff I have learned by playing with current transformers and guitar pickups for several years now. The CSE-187L is a good current transformer to begin your experimentation to obtain a good, low noise, higher fidelity output.

Remember, with current based guitar pickups, the low impedance of the primary string loop controls the current transformer output impedance and the tonal voicing.

I hope this helps.

Joseph Rogowski

[bea]

If you have an acoustic guitar, this pickup is idea to experiment with because you can mount it directly behind the fingerboard. Make the transformer connection span the B-string with the transformer primary being vertical so the transformer connection is directly under the B-string to dip the current loop under this typically loud string to better control its output. Solder the 2-conductor shielded wire to an XLR connector and plug it directly into a mic mixer. You will have a very nice acoustic guitar pickup without the classic high impedance pickup resonant hump in the typical midrange where the ear is most sensitive. The creativity is making the mechanical size of the pickup fit your available mounting space.i

But this will be also the case for traditional LowZ designs with just a few 100 windings around the magnets, won't it? You can achieve micophone level output and impedance with these as well quite easily, and resonance peaks outside the important frequency range are also easy to achieve. So how do the two concepts compare?

BTW: controlling the treble resonance with RC-filters may necessitate to large values of the capacitors which will start to become uncomfortable to handle.



A remark on microphone impedances: dynamic microphone are used as voltage sources with ideally no, actually very small currents. This means that the output impedance of 150 or 200 Ohms (German standards) is roughly the maximum permissible impedance. Accordingly the input impedance of the amplifier is about 10 times larger, often about 2.2 kOhms. Lower is always good if You are able to maintain a sufficiently large output voltage. Somewhat larger as well as long as the source will be able to maintain the larger currents and the larger internal voltage drop caused by the output impedance of the source. This will give us some degree of freedom in pickup design.

[bbsailor]

But this will be also the case for traditional LowZ designs with just a few 100 windings around the magnets, won't it? You can achieve micophone level output and impedance with these as well quite easily, and resonance peaks outside the important frequency range are also easy to achieve. So how do the two concepts compare?

BTW: controlling the treble resonance with RC-filters may necessitate to large values of the capacitors which will start to become uncomfortable to handle.



A remark on microphone impedances: dynamic microphone are used as voltage sources with ideally no, actually very small currents. This means that the output impedance of 150 or 200 Ohms (German standards) is roughly the maximum permissible impedance. Accordingly the input impedance of the amplifier is about 10 times larger, often about 2.2 kOhms. Lower is always good if You are able to maintain a sufficiently large output voltage. Somewhat larger as well as long as the source will be able to maintain the larger currents and the larger internal voltage drop caused by the output impedance of the source. This will give us some degree of freedom in pickup design.

You are correct about other ways to make low impedance pickups. If you plan on using 100 turns I suggest that you use an 8 ohm to 20K miniature output transformer used backwards with the 8 ohms side being the input to the 100 turn coil. If you use 50 ft (which is about 100 turns) you will have 1.28 DCR ohms using AWG 24; 1.62 DCR ohms using AWG 25; and 2.05 DCR ohms using AWG 26. This transformer will have about a 50 to 1 turns ratio as calculated by dividing 8 into 20K for 2,500 and taking the square root of 2500 to equal 50 or the turns ratio this transformer. Roughly speaking, you want the equivalent turns to be near 5000 turns typical of a high Z pickup but scaled for this transformer turns ratio. 100 turns times 50 gets your voltage boosted and the transformer output impedance range will be near 12 to 15K depending on the resistance of your chosen primary wire gauge. This means that you want to use an input impedance about 10 times higher and this puts you into the 120K to 150K ohm range. If you make a dual coil pickup using a humbucking design with a separate transformer on each coil and put the two transformers in series, you will now be in the 240K to 300K range similar to the Fender single coil pickup range.

I am attempting to add some photos to this post to show you different views of a CSE-187L current transformer, a piece of flexible magnetic door jam scrap, AWG 11 solid magnet wire, a few pieces of thin wall copper tube, a Shure matching transformer.

The slot in the door jam magnetic material allows me to press fit the AWG 11 single turn string loop wire into the grove on each side. I choose to place the current transformer mechanical mounting spot directly below the B-string to reduce the output from that string, and locate an easy place to mount the transformer on an acoustic guitar just behind the neck with two conductor shielded cable attached directly to the CSE-187L either soldered or connected with alligator clips to test many pickup configurations. A little double stick tape and hand bending the AWG 11 string loop to match the arc of the fingerboard allows quick and easy experimentation with current-based low impedance pickups. Shielding the transformer with a copper cover tape will help reduce noise but in extreme situations use a mu metal shield around the transformer.

On one view you will see another added primary wire over the secondary coil to add another parallel primary and experiment with the voicing of the output tone. The AWG 12 installed primary can be supplemented with from one to five additional primary loops. AWG 20 wire can fit in between the secondary coil and the transformer laminated frame. As you add more primary supplemental loops you will reduce the output impedance by about 8 ohms per wire added in parallel with the installed primary wire.

Using these quick-to-make current-based pickups with common, low cost off-the-shelf parts allows guitarists and technicians to reorient their thinking from high output, high Z pickups to focusing on the low-z common microphone impedances; mixer and matching transformer impedances with a broader bandwidth that would be better suited for a more acoustic sound with less emphasis on an accentuated mid range in the 3Khz to 5Khz range. The 1 to 500 turns ratio seems to work well for output impedances from 150 ohms by using a thicker primary winding loop (AWG 10 or 8) to near 200 ohms by using AWG 11 wire. You can use mostly any set of scrap magnets you might have laying around. Just make the loop size fit the magnets and tape the magnet to the wire loop and then mount behind the neck with tape to test, listen and enjoy. Add more primary loops on the CSE-187L to lower the impedance and increase the low end response somewhat.

Each pickup, in addition to being a voltage or current generator is an LRC filter with many of the C values being hidden inside phantom values of capacitance in the coil turn-to-turn winding capacitance, coax capacitance, loading by on board controls, and sensitivity to external noise pickup. When you ground the AWG 11, 10, or 8 string loop wire to the metal frame of the CSE-187L and attach the mic cable shield to the metal frame you will have a very loise noise pickup with an output range well into the mic output range around about 5mv to 10mv.

Obtaining a string balance from acoustic guitar strings where only the core is made of magnetic material offers a challenge to any pickup maker. This design allows quick fabrication of different physical design ideas to accomodate unique space and mounting restrictions.




Joseph Rogowski

[bea]

If i use, say, 100 turns on some pickup i would probably not need to use any current transformer at all (i can actually achieve crunching of the output stage of my Epiphone Valve Junior when i drive it with my 500 turns LowZ tele pickup...). I just need a damping resistor to control the treble resonance and possibly a capacitance to adjust the treble resonance according to my needs.

My impression from the tele pickup is that You cannot take just some crap magnets and some thick wire: my tele pickup sounds nearly as crappy as it did with its original HiZ wiring, just a bit cleaner. And the geometry of coils and magnets will keep their importance on the tone, especially in humbucker configurations.

I also remember that guitars like the les paul studio or the Jack Casady bass lose lots of their versatility by using LoZ pickups with output transformers. IMO the better approach is a thoroughly designed (yet possibly still simple) amplification stage in the guitar. This will allow You to adjust the treble response by either an external RC circuitry or by a state variable filters (or more than one like in the present WAL basses). I actually believe it will be easier to come close to the response of HiZ pickups with that approach.

That will be probably my approach. Beside the above i feel it simpler to make some 200-500 windings with thick wire (e.g. 0.2 mm), than fiddling around with current transformer and struggling with the additional resistance of sub-optimal soldering connections. Especially as a beginner in pickup making.

But nevertheless the idea of using a current transformer and a single loop is a highly attractive one. But like the alumitones it will always have its peculiar sound.

BTW: do You know how the nanomag pickups are built?

[Mike Sulzer]

You are correct about other ways to make low impedance pickups. If you plan on using 100 turns I suggest that you use an 8 ohm to 20K miniature output

Transformers are imperfect devices. If you want a pickup that matches a mic. input, why not just wind one that has the correct number of turns to work without a transformer?

[Mike Sulzer]

If i use, say, 100 turns on some pickup i would probably not need to use any current transformer at all (i can actually achieve crunching of the output stage of my Epiphone Valve Junior when i drive it with my 500 turns LowZ tele pickup...). I just need a damping resistor to control the treble resonance and possibly a capacitance to adjust the treble resonance according to my needs.

But the point is not whether you have enough gain, but whether the signal to noise ratio is as good as it should be. There are much quieter tubes than 12AX7, and there are very quiet FETs, although they like higher current than some. Why put the preamp in the guitar? A low impedance pickup can run a very long way without much noise pickup.

[bbsailor]

Transformers are imperfect devices. If you want a pickup that matches a mic. input, why not just wind one that has the correct number of turns to work without a transformer?

Mike,

Yes, just wind about one tenth the number of turns and increase the wire diameter to make the coil DCR lower about 12 to 20 ohms for about a 550 turn bobbin.

The whole purpose of using the current transformer in close proximity to the string loop generating element is to voice the tone of the pickup knowing how transformer imperfections can be manipulated by the gauge of the primary string loop. Lower total string loop DCR and impedance increases the lower end frequency response. That is why this Triad CSE -187L current transformer works well when making a low Z acoustic guitar pickup. Make it using two current transformers, one for the lower 4 strings and the other for the higher 2 strings and use a mini pot to adjust the blend of the output.


Mike, try making one of these pickups and plug it directly into a mic mixer XLR input and listen to the wider bandwidth, low noise output.

Joseph Rogowski

[bea]

But the point is not whether you have enough gain, but whether the signal to noise ratio is as good as it should be.

Yes, indeed, thats true. Although a single 12AX7 and an ECC83 will not make a noisy gain monster.

Why put the preamp in the guitar? A low impedance pickup can run a very long way without much noise pickup.

I am aware of that, and the Guitar with the Tele pickup is actually (still) passive. You must decide between a guitar that can be simply plugged into a standard amp and an instrument needing a breakout box. If You need plug and play compatibility to passive guitars or basses with HiZ pickups, You'll need the amp or the transformer in the guitar.

[David King]

Would it be worth having Cinemag/Reichenbach design and wind a proper 500:1 transformer for us to use? Someone who knows more precisely what we'd need could contact them and get a price point.

[Mike Sulzer]

If You need plug and play compatibility to passive guitars or basses with HiZ pickups, You'll need the amp or the transformer in the guitar.

If you put the electronics in the guitar there is no reason to make the impedance low (no cable effect), and every reason to keep it reasonably medium to high for good signal to noise ratio. Preamps with low current draw for good battery life do better with the higher voltage of a higher impedance.

[charrich56]

But this will be also the case for traditional LowZ designs with just a few 100 windings around the magnets, won't it? You can achieve micophone level output and impedance with these as well quite easily, and resonance peaks outside the important frequency range are also easy to achieve. So how do the two concepts compare?

BTW: controlling the treble resonance with RC-filters may necessitate to large values of the capacitors which will start to become uncomfortable to handle.



A remark on microphone impedances: dynamic microphone are used as voltage sources with ideally no, actually very small currents. This means that the output impedance of 150 or 200 Ohms (German standards) is roughly the maximum permissible impedance. Accordingly the input impedance of the amplifier is about 10 times larger, often about 2.2 kOhms. Lower is always good if You are able to maintain a sufficiently large output voltage. Somewhat larger as well as long as the source will be able to maintain the larger currents and the larger internal voltage drop caused by the output impedance of the source. This will give us some degree of freedom in pickup design.

Bea,

I am building low impedance magnetic pickups with the characteristics you describe. My target impedance was and is about 200-250 ohms at 1 KHz. This is still a good match for microphone preamps and allows a good signal to noise ratio. All your points above are very relevant in my own experience and well taken. I typically use about 20X the pickup output impedance at 1 KHZ for an input impedance and find that this gives the best frequency response, for a number of reasons. So, for a 250 ohms at 1 KHz pickup, I will build the preamp with an input impedance of from 4.7 to 6.8K ohms.

Just to compare, the ultra-low impedance single loop pickup has a huge degree of design flexibility and can be even integrated to be an attractive visual design element of the guitar, or be easily fitted to unusual or experimental instruments.

I give Joseph (bbsailor) a huge amount of appreciation for stimulating creativity and experimentation in this online community.

-Charlie

[Mike Sulzer]

I give Joseph (bbsailor) a huge amount of appreciation for stimulating creativity and experimentation in this online community.

-Charlie

I'll second that! And also to Mr. flatpup for his stimulating ideas and practical creations. Now, why not combine two threads? If you need a flat pickup for your acoustic instrument, and want to do some really high quality recording, wind a low impedance flatpup with a few hundred or so turns on each coil, using a larger wire size. Then you have something that you can run right into a mic. input, and it will cover the full audio range, and more.

[bea]

I give Joseph (bbsailor) a huge amount of appreciation for stimulating creativity and experimentation in this online community.

Me too, of course. Otherwise i would not discuss here.

I just do not see the "absolute universatility" of the ultra low Z approach. Like with any other good idea there must also be weaknesses, and it belongs to research to figure out these as well.

[charrich56]

Me too, of course. Otherwise i would not discuss here.

I just do not see the "absolute universatility" of the ultra low Z approach. Like with any other good idea there must also be weaknesses, and it belongs to research to figure out these as well.

Bea,

Some musicians would regard having to use an XLR balanced mic cable into a mic preamp, instead of a 1/4" mono cord into a guitar amp, a weakness.

Les Paul himself certainly didn't think so, even though his ideas did not catch on well in the market in the early 1970's.

There are also a lot of guitarists who are vintage purists and cannot stand the idea of having an onboard preamp with replaceable batteries in their instrument. Most bass players are over that thinking now. 8-)

So "strengths" and "weaknesses" have to be judged by what you are trying to accomplish and what kind of tool you envision putting into the hands of the musician, and even by the musician's style or genre of music. For example, I don't see ultra-high fidelity pickups making a big impact on the thrash and heavy metal musicians.

So I didn't think Joseph was putting his low-Z pickup research out there to be a universal solution. It is, however, a very good way to get a very high fidelity signal from the strings, where you have a lot of design freedom to put magnetic fields and pickup positions on the strings where you want them instead of being constrained by a conventional outline and position on the instrument.

A single loop ultra low Z pickup isn't very susceptible to E-field noise but M-field noise can be an issue. Joseph did address this with a "humbucking" design with two loops however, if I remember the long thread correctly.

Mike, I like the flat low-Z pickup idea a lot. I can see an archtop jazz instrument with something like that integrated in. Even under the pickguard of an Esquire.



-Charlie

[bea]

Charlie,

i have the impression that our positions are fairly similar.

The flat LoZ pickup at the end of a fingerboard - really a nice idea. Hide it invisibly under some thin veneer, perloid, MOP or the like. The Nanomag design is something like that, isn't it?

Back to Joseph's ideas and hence to our main topic: where could we hide the current transformer when using a single loop pickup on, say, an archtop fingerboard? We would have to solve the question where to go with the cable and with the plug - it is a sacrileg not to leave the top of an acoustically usable archtop intact.

BTW: although electrically not optimal i would prefer a stereo jack connection in the guitar instead of the XLR. Smaller, lighter. Possible to mount under the pickguards of a Jazz archtop (not of all - i need to use a 3.5 mm jack on my guitar).

And something regarding the market: would You think that Jazz Archtop players are more on the conservative side or more open to modern developments? And the archtop luthiers? I actually have never noticed an active Jazz archtop other than my own guitar.