Low Impedance Pickup Research

[guitician]

Maybe with some sort of instrumentation amplifier circuit.

[bbsailor]

No, there is no preamp that will give good signal to noise ratio at such a low voltage. It is remotely possible you could make one using many, many paralleled devices, but it would take amperes of current, and it would be hard to fit in the available space.

Hans,

Mike is correct about using active electronics instead of a transformer but after some personal brainstorming here are some options that might achieve your desired results of fitting a string loop, CT-based pickup into available guitar space. Consider the distance from the bridge pickup to to control cavity. We can assume that the U-shaped string loop is 5 inches to span the string width plus another 2.5 inches to get that string loop wire routed into the pickup cavity. If you use AWG 11 magnet wire which is 105 micro ohms per inch then a total string loop of about 10 inches long will be 1050 micro ohms. If you use a thinner and flexible stranded, insulated wire such as AWG 12 to 14 then you could put two loops through the 500 turn torrid CT primary to get a 250 turns ratio. However, the output voltage would be lower. Here is where you could use an FET or a surface mount, low noise op amp circuit to boost the output right after the CT to an output equal to a pickup output in the 80 mv to 250 mv range using a voltage gain of about 25. This way you can eliminate the cable capacitance effect and have a wider range of CT turns ratio selections. The challenge is mechanical with impedance implications. How to get the lowest resistance string loop around the string magnets and into the control cavity where it can be easily connected through the CT primary with a small enough amplifier circuit to buffer and boost the CT output. This will require some experimentation to find the right balance between these key factors:

1 Output level. Higher turns ratios produce more output.
2. Output impedance. This impedance is directly related to turns ratio which can be controlled by using a single loop through the CT and exploring the use of various secondary turns CT models or how many primary string loops you can put though the CT primary to lower the effective turns ratios with more turns through the primary loop space.
3. Noise. This is related to how you shield the transformer and string loop from external noise. It is always good to ground the string loop as well as try to use a copper or mu metal shield around the CT if the noise is externally induced.
4. Tone. Here is where trying various string loop wire thicknesses or multiple small wires like Litz wire can alter the tonal response. If a torrid CT has a .250 inch opening, then a single thick wire bent into a loop will work but be very hard to rout through the guitar body into the control cavity. You could use a wire bundle of multiple smaller wires that are connected as multiple parallel loops. Do this experiment outside the guitar, first by taping the CT string element under the strings, and find what sounds pleasing before attempting to internally mount it. Try listening to the sound through a guitar cable not buffered and then buffered with an active amplifier circuit. Listen for the noise level with different amounts of active gain. Where each smaller wire loop is joined together run a single ground wire to allow each smaller loop wire to be grounded to minimize noise.

Look at the Prem Magnetics SPCT-251 (a little smaller than 1 cubic inch, measures 1 X 1 X .75 inches) 2000 turn CT with two .160 inch square openings on a E-I laminated frame CT to loop the string loop through. Just use a piece of copper tubing to join the string loop to this CT in the control cavity with the CT mounted on a circuit board with the active preamp. Here is a little math that will help you. The SPCT-251 output impedance will be about 4,000,000 times (turns ratio squared) the string loop resistance. Assuming you are using AWG 11 at 105 micro ohms per inch, you will have an output impedance of about 4200 ohms, much too high for a mic input but good enough for a line input. But, if you mount a preamp in the control cavity right after this CT you can buffer the output from external loading and cable capacitance plus add enough gain to get the string loop pickup using this CT up to a standard pickup level with a gain probably less than 10 and minimize external noise pickup by getting the signal level higher at the source. You can even look for a CT with a center tap secondary so you can make a true balanced line output with the center tap being the ground. If using a non-center tap CT, you can also minimize noise by running a .125 watt resistor from each CT secondary connection point to ground. Try a resistor value between 5 to 10 times the calculated output impedance and listen for the resistor value with the lowest noise.

This is the technique the ribbon mic designers use to allow very high turns ratio transformers to passively get the most gain out of the ribbon and then use an active FET or op amp to buffer and boost the output to match the standard mic input stages of recording equipment. Look at the full range of small current transformers that are commercially available and see how they can be used in this new way when combined with on-board active buffering and amplification to achieve your desired result. This is a design trade off between noise, tone and physical mechanics necessary to adapt this transducer to being fit onto and into a guitar.

With a little creative manipulation of the mechanics involved with making low impedance string loop pickups, you can mount these pickups in traditional guitars without needing to cut up the pickup mounting hole to accommodate the CT. I hope this gets you going in the right direction.

Joseph J. Rogowski

[MisterBzr]

Thanks Joseph for your replies.
Fortunately I don't have to worry about about standard measurements, because I build my own (prototype) instruments. I mount the single loop straight on the body and drill a hole to give room for the AS-104 CT. Also, I've build a simple fretless stick bass with the pickup build in the neck (photo), which works great.
For the sake of experimenting, I will wind some conventional pickups only with an impedance of around 200-300Ohm and see how that sounds / works (as is described here: http://music-electronics-forum.com/t38114/.
Thanks again.
Hans

[bbsailor]

Thanks Joseph for your replies.
Fortunately I don't have to worry about about standard measurements, because I build my own (prototype) instruments. I mount the single loop straight on the body and drill a hole to give room for the AS-104 CT. Also, I've build a simple fretless stick bass with the pickup build in the neck (photo), which works great.
For the sake of experimenting, I will wind some conventional pickups only with an impedance of around 200-300Ohm and see how that sounds / works (as is described here: http://music-electronics-forum.com/t38114/.
Thanks again.
Hans
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Hans,

Here is an idea that I have been thinking about and your photo of the bass pickup being built into the neck may be applied to a guitar or bass at the lower (body) end of the fingerboard. Most guitars or basses have a small portion of fingerboard that extends beyond the last fret. Make this area a few mm wider (to accommodate the string loop) and drill 6 (for guitar) or 4 (for bass) holes mount round magnets. Place the string loop around these magnets and run the ends of the string loop, where it goes through the current transformer (CT), somewhere in the body close to this string loop. This would work especially well on an acoustic guitar where the CT could be mounted directly below the string loop in the body just in front of the sound hole. This would make the pickup invisible all but for the slight widening of the fingerboard to accommodate the string loop. Magnet depths and lengths could be altered to better balance the output from each string with the B-string magnet being farther from the string or use a shorter magnet to get less output from this typically dominant string.

When using the "low-impedance pickups thin" approach, you can experiment with tonal variation by winding the coil with multiple taps on the low impedance coil such as at 200, 300, 400, and 600 turns (or more turns). This way, you can try a few wire sizes with these taps and audition each tap to listen for the effect on the sound by just changing a rotary switch to select different taps.

Most modern microphone mixers have about a 2.4K ohms actual impedance to act as a bridging input for microphones rated as 150 ohms (nominal impedance) that may actually have up to about 300 ohm outputs. Remember, impedance varies with frequency so finding the right tone requires listening to the various coil taps. As a rule of thumb, the DCR of a low Q pickup loop coils mostly matches the impedance at lower frequencies but at higher frequencies, the loaded impedance will raise about 25 percent above the DCR. That is why the bridging impedance of 10X the source impedance allows room for the maximum voltage to be transmitted from the source (pickup) to the load (mixer or transformer input). If you keep the pickup coil inductance between 20 mh and 25 mh you will have an output impedance at 1000 Hz of 157 ohms at 25 mh and 126 ohms at 20 mh.

Your current transformer-based pickup built into the neck (photo) was a nice, creative way to mount and hide the pickup.

Joseph J. Rogowski

P.S. If you do not have an LCR meter like the Extech 380193 you can try doing this to measure the output impedance of any pickup coil or CT output. You will need an oscilloscope or a software based oscilloscope to do this accurately. Plunk a low, medium and high frequency note on your instrument. Measure the unloaded output. Put a variable resistor load (pot) on the output of the pickup and vary the pot until the output voltage on the oscilloscope is exactly one half the unloaded value. The value of the pot is the impedance of the pickup at the frequency of the note struck to make this test.

[MisterBzr]

I've build a prototype of a thin pickup (http://music-electronics-forum.com/t38114/#post384974) and the first results are promising.
I have a LCR-meter at work so I can do some measurements.
I want to vary the diameter of the wire and the number of turns, but want to stay under 300 ohms of resistance.
Hans

[KhzDonut]

Hello, new to the forum, but I've been reading this thread with much amazement and interest for the last week or so (took quite awhile to get through the whole thing!) as well as some over at Talkbass (specifically this one Low impedance, Low noise pickups Research. What's the Skinny? | TalkBass.com)

I've started making some prototypes using many of the design ideas and parts (CSE187L, Calrad 45-715, etc...)

Just wanted to say thanks so much to everyone, especially Mr. Rogowski for all the information. The thoroughness and open-sourceness of this thread is really intriguing and inspiring. It makes it really easy to get involved, and that's making it easy to get excited about the whole endeavor.


Thanks so much for the excellent read!

[MisterBzr]

Popsicle Pickup

I made a simple pickup to fit my modified baritone guitar (tuned it in fifths, and adjusted it accordingly).
My wife made me promise that it would be a "non-destructive" pickup, so no drilling allowed.

I came up with this:



I sandwiched two 25x5x3mm strong magnets between two popsicle sticks. I made a single loop around the magnets using some electric wire which I had lying around, keeping it in place with two tie wraps. I coupled the single loop with a 1:500 current transformer (Talema AS-104). Finally I soldered a microphone cable to the transformer.
The result is a low Z pickup with a balanced output suitable for a microphone input.
The distance between the two sticks is just a bit wider than the thickness of the top, with the use of two screws the pickup is clamped in the soundhole.

I've made two short soundclips, one with only a wee bit of reverb, and a second one with the highs rolled off around 2kHz.

I'm still pondering over a better way to retain the cable (besides using black tape instead of grey )

Merry Christmas!

Hans

[bbsailor]

I made a simple pickup to fit my modified baritone guitar (tuned it in fifths, and adjusted it accordingly).
My wife made me promise that it would be a "non-destructive" pickup, so no drilling allowed.

I came up with this:



I sandwiched two 25x5x3mm strong magnets between two popsicle sticks. I made a single loop around the magnets using some electric wire which I had lying around, keeping it in place with two tie wraps. I coupled the single loop with a 1:500 current transformer (Talema AS-104). Finally I soldered a microphone cable to the transformer.
The result is a low Z pickup with a balanced output suitable for a microphone input.
The distance between the two sticks is just a bit wider than the thickness of the top, with the use of two screws the pickup is clamped in the soundhole.

I've made two short soundclips, one with only a wee bit of reverb, and a second one with the highs rolled off around 2kHz.

I'm still pondering over a better way to retain the cable (besides using black tape instead of grey )

Merry Christmas!

Hans

Hans and anyone tinkering with this type of pickup

Here are some things to try that will help improve your design.

1. When using a toroid current transformer (CT) try to fill the opening with the largest wire that will snugly fit into the opening. This will minimize any potential microphonics caused by the string loop wire and transformer from vibrating against each other.

2. The output impedance of the CT is determined by the DCR of the string loop which also included the resistance of the wire connection making the loop or connecting to a pre-installed CT primary turn (as in using the CSE-187L). Obtain some copper tubing with an inside diameter near the outside diameter of the chosen wire. Join the string loop wire together with about 6 mm length of copper tubing (typically a length twice the diameter of the chosen wire). Clean the inside of the tubing with fine sandpaper, insert the wire, crimp if the wire is not snugly set and the solder using silver solder. Copper tubing a little too small to fit the wire can be expanded by using the tapered shaft of a nail set (or similar tool) to slightly expand the tubing to snugly fit the wire.

3. Ground the string loop at the joint to the XLR connector pin 1, ground to minimize the noise. If using a CSE-187L CT then also ground the metal frame of the transformer.

The output impedance using a 500 turn CT can be easily calculated by looking up the resistance of the chosen wire per inch and then multiply that value by the chosen length of wire. AWG 11 is 105 micro-ohms per inch, thus a 7 inch length through a toroid CT will be 105 X 7 or 735 micro-ohms. Multiply this value by the turns ratio squared or in this case 250,000. Since 250,000 is one fourth or 1 million simply divide 735 by 4 and get about 184 ohms. Add about 17 ohms for leakage inductance and you get 201 ohms. This is well below the maximum impedance of about 300 ohms that will work into an XLR mic input. If you choose to use a 1000 turn CT you will need to use a thicker string loop wire that will fill the toroid opening, as the output impedance will be about 4 times higher if using the same size wire a previously noted. Use a wire table to obtain the resistance of the chosen wire to do this calculation.

Here is a little trick that will help you mount the pickup lead wire more easily. Wrap a piece of bungi cord (stretchy rubber cord) abound the heel of the neck, up toward the head and bend down under the heel of the neck and make tight. Attach about a 1 foot length of 2 conductor shielded mic wire to the pickup and the other end to a stereo miniature female in line connector. Secure this in line connector to the bungi cord right after the bungi cord bends down 90 degrees from the guitar top on the upper side of the neck. Make a custom male stereo plug and wire length to an XLR connector to allow you to easily attach the pickup to the amp with no modifications to your guitar.

You will find that you can add extra parallel wire turns to fill the toroid opening space. Each wire should be soldered to itself to form a string loop and then grounded with the other grounds to keep the noise very low. Each additional added wire will lower the total string loop resistance and lower the output impedance. Lowering the string loop resistance also has the effect of shaping the voicing of the pickup to favor lower frequencies.

This CT pickup design provides a wide variety in the ways that can be adapted to an acoustic or even electric guitars as you have demonstrated in your creative work.

Happy Holidays!

Joseph J. Rogowski

[MisterBzr]

Thanks Joseph

I've made a sideview shot, to get a better view of how it is build.



I've indeed grounded the single loop.

Hans

[rjb]

I made a simple pickup to fit my modified baritone guitar (tuned it in fifths, and adjusted it accordingly).
My wife made me promise that it would be a "non-destructive" pickup, so no drilling allowed....

I'm still pondering over a better way to retain the cable (besides using black tape instead of grey )

For a more permanent installation, you could use a 1/4" stereo endpin jack instead of the bungi cord arrangement.
You would need to enlarge the existing endpin hole with a hand reamer- but, "legally speaking", that wouldn't be drilling a new hole.
Then use a standard TRS to XLR cable to your balanced input.

[MisterBzr]

Why didn't I think of that myself?! That's a great idea, I was stuck on using a XLR connector, but this is far better.
Thanks!

Hans

[Guitar fool]

Hi all, hate to dig up an old thread but I have found these low impedance threads very helpful and thought I might try to contribute to one.
After trying a few prototypes and finding them to be far superior than the pickups on my collection of cheap guitars and basses I invested in some waterjet cut copper and laser cut plastic. The first pair are finished and have transformed the old Samick UM-1. I am using two CSE187L transformers with one shorted out. They both have the primary loop replaced with 1.6mm thick copper U shapes that fit into slots cut into the top of the pickup. The two halves of the tops are glued onto a peace of acrylic that has holes cut to recess the neo magnets into and also holes to pass the primary loop through. Soldering the primary loop together was tricky without melting the acrylic beneath. I then linished the tops flat on a belt sander. They are shiny now but will dull with age very quickly.

To my ears they sound very clear and almost like an acoustic but very usable. I have to clean up my playing because every buzz and rattle comes though the amp clear as a bell. I am still experimenting with cap and pot values.
A P-bass set I made for my bass also sounds great but a tiny bit distorted on the bottom string.

A big thanks to all who have contributed to these threads especially Joseph Rogowski. My Korean guitars and I are most grateful for the information shared by you all.

[bbsailor]

Hi all, hate to dig up an old thread but I have found these low impedance threads very helpful and thought I might try to contribute to one.
After trying a few prototypes and finding them to be far superior than the pickups on my collection of cheap guitars and basses I invested in some waterjet cut copper and laser cut plastic. The first pair are finished and have transformed the old Samick UM-1. I am using two CSE187L transformers with one shorted out. They both have the primary loop replaced with 1.6mm thick copper U shapes that fit into slots cut into the top of the pickup. The two halves of the tops are glued onto a peace of acrylic that has holes cut to recess the neo magnets into and also holes to pass the primary loop through. Soldering the primary loop together was tricky without melting the acrylic beneath. I then linished the tops flat on a belt sander. They are shiny now but will dull with age very quickly.

To my ears they sound very clear and almost like an acoustic but very usable. I have to clean up my playing because every buzz and rattle comes though the amp clear as a bell. I am still experimenting with cap and pot values.
A P-bass set I made for my bass also sounds great but a tiny bit distorted on the bottom string.

A big thanks to all who have contributed to these threads especially Joseph Rogowski. My Korean guitars and I are most grateful for the information shared by you all.
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Paul,

Thanks for the kind words. You certainly took the use of current transformer based guitar pickups to a new level with your forming of the string loops, integrating the magnets and fitting the current transformers in a traditional pickup footprint.

See this web link for your on-board volume and tone controls. Les Paul Personal 2 The output impedance of the CSE-187L pickup is in the same range as the Les Paul Recording guitar. One of the other things you can try is to experiment with un-shorting the second current transformer and placing a variety of capacitors and loads across it to make what I call a Reflected Impedance Tone Equalization (RITE) system. You can also try using a DPDT switch to place the two current transformer outputs in series or parallel.

The other thing you can try is to make a dual three string pickup with a very low string loop impedance and then wire the two transformers in series like a humbucker pickup. With this setup you want to keep the 3-string string loop resistance near 280 micro-ohms (or lower) so the total 3-string output impedance is near 150 ohms. Then, you can EQ high or low string sets individually. You can poke out the existing U-shaped CSE-187L primary turn and insert a little thicker wire to get the impedance a little lower. I have even fit two additional U-shaped AWG 20 magnet wires in the space next to the existing U-shaped turn to lower the impedance about 10 to 15 ohms. Just poke out the glue to fit the wire.

A good trick to know is that a shorted out CSE-187L primary bent inward and soldered to form a shorted loop makes an impedance of about 80 ohms which includes the primary leakage inductance. The resistance of the string loop (in micro-ohms) is then multiplied by 250,000 (turns ratio squared) and then added to the 80 ohms of the primary shorted which provides a pretty accurate indication of the total impedance without needing an LCR meter. But, having an Extech LCR meter is a handy tool for experimentation. The quality of the solder joints will affect the total string loop resistance which is reflected in the output impedance. Make sure that you let the joints and transformer cool before taking any measurements.

I hope this helps?

Nice Job!

Joseph J. Rogowski

[charrich56]

Paul,

It just knocks me out seeing the low-Z goodness in a standard humbucker sized package. The cutout and detail work on the loops is unique, artistic, and mondo cool. Great job engineering and designing these.

-Charlie

[MisterBzr]

That's indeed a cool way to make them.
could you post some pictures of the back / inside of the pickups?

Hans