Low Impedance Pickup Research

[bbsailor]

I've had great success making lowZ PU's with what I learned on this forum. However, they all require a transformer or an amp with a mic channel. Now I'm wanting to make a low z pu that will not need a mic transformer, and I wonder if something akin to the old Gibson Charlie Christian would work with these specs: 5000 turns x 36ga. The DC resistance calculates at around 1200 Ohms. Inductance calculation says it would have about 3.8H. I would use 6 tiny neod mags 6mm diam x 3mm thick as the poles. Before I go ahead and waste a lot of wire on this idea it would be good to know if there is any hope of it working in a common guitar amp.

Singer15,
The reason why I chose a 500 turn current transformer (CT) is to match the typical low impedance microphone input impedance rated at 150 to 250 ohms but typically has an actual input impedance of about 2400 ohms to act as a bridging impedance to not load down the mic too much, Now, if you want to target a high impedance guitar amp, do this.
1. Do a web search for a current transformer with 5000 turns or more.
2. Use the thickest string loop wire through the CT and a good low resistance string loop joint.
3. You can use two CTs per pickup either with the CTs next to each other or with one CT on each end of the pickup string loop.
4.You can wire or switch the CT outputs in series or parallel for different tones. Make sure the phase is correct or they will cancel out each other.

Since the string loop is very thin compared to a typical pickup, no pickup cut out needs to be made in the guitar body. This makes the bridge to neck connection better since there are no pickup cutouts except about 1 square inch to accommodate the CT but not in line with the neck. .Also, the pickup output from the induced vibrating string is all very close to the string loop and not distributed up and down like in a high Z pickup.. You should get about 80 mv to 100 mv using a single 5000 turn CT.

Helmholtz is correct.. If you use 5000 turns of AWG 36 you are making a bulky but traditional high Z type pickup that needs a body cutout. Order a few 5000 turn CTs and experiment!!!

Joseph J. Rogowski

[Singer15]

Singer15,
The reason why I chose a 500 turn current transformer (CT) is to match the typical low impedance microphone input impedance rated at 150 to 250 ohms but typically has an actual input impedance of about 2400 ohms to act as a bridging impedance to not load down the mic too much, Now, if you want to target a high impedance guitar amp, do this.
1. Do a web search for a current transformer with 5000 turns or more.
2. Use the thickest string loop wire through the CT and a good low resistance string loop joint.
3. You can use two CTs per pickup either with the CTs next to each other or with one CT on each end of the pickup string loop.
4.You can wire or switch the CT outputs in series or parallel for different tones. Make sure the phase is correct or they will cancel out each other.

Since the string loop is very thin compared to a typical pickup, no pickup cut out needs to be made in the guitar body. This makes the bridge to neck connection better since there are no pickup cutouts except about 1 square inch to accommodate the CT but not in line with the neck. .Also, the pickup output from the induced vibrating string is all very close to the string loop and not distributed up and down like in a high Z pickup.. You should get about 80 mv to 100 mv using a single 5000 turn CT.

Helmholtz is correct.. If you use 5000 turns of AWG 36 you are making a bulky but traditional high Z type pickup that needs a body cutout. Order a few 5000 turn CTs and experiment!!!

Joseph J. Rogowski

bbsailor & helmholtz,
thanks for the replies. When I said LoZ I was thinking lowish! I am not concerned with the bulk of the pickup, because I have built some guitars with the idea in mind of being able to mount any size pickup without making holes in the guitar top. I recently built a pair of guitars that use a projected neck, with about 20mm clearance, enough for a very fat pickup. My true LowZ pickups sound great with this by the way, and into my amp I have no problems at all, since it can deal with HiZ or LoZ. I was just hoping to recreate something like those old Charlie Christian PUs that had 38ga wire, for my own amusement. So what I am unsure of is exactly where the crossover would be between LoZ and the need to have a mic channel, and a pickup with enough native winds to drive a standard guitar amp. I don't have any 38ga wire but I do have a spool of 36. Any suggestions as to the least number of winds that would get a useful signal?

[Helmholtz]

When I said LoZ I was thinking lowish!

Well, not lowish either. With an inductance of 3.8H your PU's impedance and resonant frequency would lie somewhere between a strat PU and a PAF type humbucker.

[Singer15]

Well, not lowish either. With an inductance of 3.8H your PU's impedance and resonant frequency would lie somewhere between a strat PU and a PAF type humbucker.

I am not an expert, but going by an online coil inductance calculator (https://www.eeweb.com/tools/rectangle-loop-inductance) the inductance of a strat PU is supposedly 7.6H, twice my example. How low can the inductance be before it is too low for a regular guitar amp to work with?

[Helmholtz]

I am not an expert, but going by an online coil inductance calculator (https://www.eeweb.com/tools/rectangle-loop-inductance) the inductance of a strat PU is supposedly 7.6H, twice my example.

Typical strat PUs measure between 2H and 3H. Without poles, values would be even lower by around 30%.

I tested this online calculator and found that it gives wrong (way too high) results. I don't think the formula it is based on is correct.
There is a more complex formula in literature which gives quite correct results, but is a PITA to work with.
And of course these theoretical formulas only really work for air-core coils.

[bbsailor]

I am not an expert, but going by an online coil inductance calculator (https://www.eeweb.com/tools/rectangle-loop-inductance) the inductance of a strat PU is supposedly 7.6H, twice my example. How low can the inductance be before it is too low for a regular guitar amp to work with?

Singer15,

Guitar pickups evolved to provide enough output voltage to drive the early Fender amp designs that had limited amplifier gain to reduce the tubes required to build the amp. It all gets down to the signal to noise ratio. If the pickup output is too low you need to raise the volume control to hear the pickup, but you also hear the hum and all the noise that the pickup coil is also picking up. The requirement to make a pickup produce enough voltage to drive the Fender guitar amps required about 6,000 to 7000 turns of very fine to produce about 80 milli- volts (mv) with the consequence or making a coil with inductance in the 2 to 4 Henry range.

Your question is about if you can wind enough AWG 36 wire to produce a pickup that will fit your allotted space, match your amp input impedance and not affect to high frequency tonal quality that your ear would like.

Here is how this all fits together.

With inductance, when you double the turns you make the inductance (H) 4 times higher. If 1,000 turns makes a 1H pickup coil then a 2,000 turn pickup makes a 4H pickup. If I make a 1,000 turn pickup that is 1H then I add 100 turns, what is the new inductance? The 1,100 turn coil is 10 percent higher or 1.1. Now just square 1.1 to get 1.21 and that is your new coil inductance with 100 extra turns.

All coils with inductance have a reactance or an AC resistance that is frequency dependent. Reactance is XL and is calculated as XL equals 2 pi (6.28) times the Frequency(F) times the inductance or L in Henries.

Now we need to think about pickup loading to minimize how much the upper frequencies are being loaded down by these things.
1. The on body volume control.
2. The coax cable capacitance
3. The amplifier input impedance typically rated at 1 million ohms but check your amps rated input impedance.
4. The combined load of the on body volume control value in parallel with the amplifier input impedance which slightly lowers the combined value.

Here is a rule of thumb that has been in effect but little has been said about this which I call "the rule of 40". Volume pot values are typically about 40 times higher than the DC resistance of the pickup to not load down the high frequencies of the pickup too much. If you use the 1000Hz frequency to do your XL calculations you will be within a plus or minus 5 percent value that is optimum. Typically low impedance pickup have about one tenth to one twelfth the number of turns compared to a high impedance pickup.

A Shure A96U matching transformer has a bridging impedance (10 times higher than the source impedance) for low impedance microphones or pickups in the range of about 100 to 250 ohms. The turns ratio in 1 to 12 meaning that the voltage is boosted 12 times. If your high impedance pickup has 6000 turns the your low impedance pickup should have 500 turns so he output from the transformer would be near what a high impedance pickup would produce without a transformer. Here are the other things to consider. High impedance pickups will be more affected by coax cable capacitance of about 30 pf per foot of cable .A 10 foot cable is like putting a 300 pf capacitor across the pickup output. Only your ears will tell you if this affects your sound. Simple switch in a 300pf cap across the pickup and listen for any changes. Low impedance pickups bypass this effect as they are not as affected by cable capacitance at low impedance outputs. Since the Shure A95U is plugged in to the high impedance guitar amp input jack you get the voltage boot of the transformer without adding as much noise as if you added another amplifier stage. Low impedance pickups are less sensitive to hum and noise than a typical high impedance pickup would also pick up.

If you use AWG 36 to make a high impedance pickup you will make a very large coil winding if you had 6,000 to 7,000 turns on the bobbin. You might want to make two 3,000 to 3,500 turn coils like a humbucker to keep the coil wires closer to the magnets and make the coils more efficient at sensing the vibrating strings but at a tonal cost of less high frequencies.

All pickup designs are a matter of compromise, balancing the following things to consider.

1. Magnet size and strength
2. Pickup distance from strings
3. Magnetic damping on the strings
4. Noise that the pickup also picks up
5. Coil size or compactness to fit allotted space, be visually appealing, or be efficient with the chosen magnet(s)
6. Ability to properly feed the input impedance of your amp or transformer mounted on or near the amplifier.

The best way to learn is to tinker with what you have and discover things that please you. I only offer the above things to help you better understand some fundamental ground rules that have governed pickup evolution since Leo Fender built his first electric guitars and amps. The best current transformer that I have discovered so far is using a Triad CSE186L with the a three turn primary winding of AWG16 removed leaving a .125 square inch space to mount an AWG8 string loop. An eighth inch round file will help makes space for the AWG8 wire to fit the space. Place electrical tape around the laminated frame in the wire window to prevent the AWG8 wire from being shorted out against the lamination frame. The output impedance is in the 100 to 150 ohms range and perfect for feeding an XLR low impedance mic input or a Shure A95U style mic matching transformer.

Joseph J. Rogowski

[AlexShroyer]

Long time lurker, first time poster, finally made something I can contribute to this thread. I took a lot of inspiration from the designs posted by micha70 and the first CAD rendering posted by alexoest.
I wrote a post about it here: https://alexshroyer.com/posts/2023-0...ce-Pickup.html (there's also a rough sound test video and CAD model code there).

Aluminum rods connect aluminum plates as rivets as in this video: https://www.youtube.com/watch?v=FnBZltkXqT0 (although I didn't have a tapered reamer).

Thanks to all the contributors here for sharing their ideas over the years, it's satisfying to build these things and I'm looking forward to the next one!
-Alex

[bbsailor]

Long time lurker, first time poster, finally made something I can contribute to this thread. I took a lot of inspiration from the designs posted by micha70 and the first CAD rendering posted by alexoest.
I wrote a post about it here: https://alexshroyer.com/posts/2023-0...ce-Pickup.html (there's also a rough sound test video and CAD model code there).

Aluminum rods connect aluminum plates as rivets as in this video: https://www.youtube.com/watch?v=FnBZltkXqT0 (although I didn't have a tapered reamer).

Thanks to all the contributors here for sharing their ideas over the years, it's satisfying to build these things and I'm looking forward to the next one!
-Alex

Alex,

Nice work!

Try to obtain an LCR meter to measure the output from the current transformer. The lower the output impedance means a better or lower resistance string loop going through the current transformer. I use an Extech LCR 380193.

Here is a design you can try. Get an epoxy coated neo magnet that is 2.25 long by .25 to .375 wide by .25 thick.
Get a triad CSE 186L CT and remove the three primary turns. Get an .125 thick piece of copper that is .562 wide and about 4 inches long. Cut a long notch in the center to slide the copper plate into the CT square openings. Put thin tape on the transformer laminations to prevent the copper plate from shorting out. You should get good low noise output in the low impedance microphone range or around 100 ohms. connect the CT output to pins 2 and 3 of an XLR connector. Solder a wire to the copper string plate and solder it to the CT metal plate around the lamination and pin 1 of the XLR connector. You will need a creative way to close the open notch to finish making the low resistance string loop.

Prem Magnetics makes a nice 2000 turn CT SPCT251. It has an opening that will allow an AWG 6 solid wire or a thicker notched copper plate to be used. The output impedance should be about 16 times higher than the 500 turn CT. 2000 divided by 500 is 4, then square 4 to get 16 as the approximate increase in the output impedance compared to the 500 turn CT.

Traditional pickups wound with AWG 42 or 43 wire span the height of the bobbin. This has the consequence of the induced vibrating string voltage to be much less in the lower turns. However, with a thick copper plate or aluminum plate the bulk or the CT turns is located much closer to the strings being induced as a higher current.

The other advantage of this CT pickup design is not needing to cut out the guitar body wood under the string path. This could have some advantage in improving the string sustain.

Another advantage of the CT pickup design is to use solid copper wire as the string loop. Leave about 1 inch space from the end of the magnet and the CT to prevent magnetizing the CT laminations and to allow the CT to be bent down between 45 to 90 degrees to be easily mounted on an acoustic guitar. If you use AWG 8 solid wire in a hairpin loop to go through the CT you will need to use a copper plate that is .125 inch thick by .75 inch long and .25 inch high with two holes drilled to close the hairpin loop with a good low resistance string loop joint.

I use a .125 inch tip ring sleeve female connector that I attach to a thin plastic plate like a cut up credit card mounted under the strap holder on the heel of the neck. This way the pickup can be mounted on an acoustic guitar with no holes being drilled. Then I feed the pickup output into a microphone input. This low impedance balanced line output does not affect the high frequencies like a traditional high impedance pickup using a guitar cable and amp input.

Thanks for posting your own research. Keep posting your projects.

Joseph J. Rogowski

[Guitar fool]

Nice one Alex.
I too have been tinkering over the Christmas break. I have made these pickups before but this time I made a guitar to suit them. My original plan was to put a large pickup that extended from the neck to the bridge but inside the (hollow) body. The tone or position could be changed by moving magnets on the outside under the strings. A thin metal plate went on top of the pickup to give the magnets something to hold on to. It worked well but the output was very low. I was unable to tune using an electronic tuner and noise was an issue with the amp turned up. I gave up on the magnet idea and found it was better sounding with a larger stack of neo magnets on the inside but spaced off the pickup to be closer to the center of the guitar body. I then found a spot that sounded best to my ears and glued the magnet on. I was happy with it then but yesterday the magnet came loose and hit one of the transformers with enough force to break the winding.
I put another pickup together but this time a humbucker sized one and placed it where the magnet was. The result was a very similar sound but higher output.
I am using a 15 nf cap on the pickup then running it straight out to the output jack, a Swamp Industries impedance transformer and the using an EQ patch on the effects pedal to tame down both the highs and lows. A compressor also helps as I find these pickups very dynamic.
I am happy with the end result. It has a nice semi-acoustic sound to it which is what I wanted. I would still like to boost the output a lot. If anyone knows of a suitable on board pre amp schematic I would be grateful but I assume it would have the same effect as a booster patch on the effects pedal.


The guitar, pictured next to the very stump from whence it was hewn. Second photo, The position of the magnets surprised me. Just sounds best there. Also note the excessive amounts of hot melt glue to protect the CTE187L transformers.
Thanks again to all who have contributed to this thread!

[AlexShroyer]

That's a lovely guitar, nice work! Building a full instrument is on my todo list. Your magnet placement is interesting, but I'm even more surprised that a single magnet is enough. Are your transformers in series?

[AlexShroyer]

[...]
Thanks for posting your own research. Keep posting your projects.

Joseph J. Rogowski

Cheers Joseph, I wouldn't have thought to attempt this if not for you starting this thread. Thanks for the suggestions, that is a lot of food for thought!

[bbsailor]

Alex,

Go to this forum subject for more food for thought. String based current induced pickup shared idea

You can have individual and independent string output with electronic string output balancing. The key is having a very low resistance connection to the nut end string common ground and each string tail end through a matching transformer, 8 ohms to 10K up to 50K.

This will get your mind going.

Joseph J. Rogowski

[Guitar fool]

That's a lovely guitar, nice work! Building a full instrument is on my todo list. Your magnet placement is interesting, but I'm even more surprised that a single magnet is enough. Are your transformers in series?

Thanks, the magnet is a stack of two neodymium magnets. Very strong and are hard to pull apart. The two transformers are in series, one with the output shorted out.

[AlexShroyer]

Alex,

Go to this forum subject for more food for thought. String based current induced pickup shared idea

You can have individual and independent string output with electronic string output balancing. The key is having a very low resistance connection to the nut end string common ground and each string tail end through a matching transformer, 8 ohms to 10K up to 50K.

This will get your mind going.

Joseph J. Rogowski

Funny story, I remember that thread, and I actually originally bought the 2" aluminum bar stock from the hardware store with the intent of using it as fretboard material for the "low resistance return path". But since then my guitar building ambitions have changed a bit so I'm now planning to make a more conventional wood neck.

[bbsailor]

Funny story, I remember that thread, and I actually originally bought the 2" aluminum bar stock from the hardware store with the intent of using it as fretboard material for the "low resistance return path". But since then my guitar building ambitions have changed a bit so I'm now planning to make a more conventional wood neck.

Alex,

Try to use the thick metal truss rod as the ground return connected to a brass or copper nut to bring the common ground end of the strings back into the body area. The rule of thumb should be to make the total resistance of the ground return to be about one tenth of the string resistance to minimize resistive connection losses to the string transformer.

To start tinkering with this new pickup concept just use alligator clip wire to connect a single guitar string behind the nut and the bridge to the 8 ohm side of the transformer. Just make sure that no other strings are shorting out your test string as it will vastly reduce the output level.

Have fun!

Joseph J. Rogowski