XLR mic inputs are typically 2400 ohms. That means that you want the current transformer to be around 240 ohms to not load down the pickup output and cut the high frequencies. A shorted CSE187L is 80 ohms measured output impedance. Then, just add the string loop resistance. AWG 11 is 105 micro ohms per inch so a 6 inch string loop is 630 micro ohms. Multiply this by turns ratio squared or 250'000 and get 157.5 ohms. Now add this to 80 and get a total output of 237.5 ohms. This is right near the upper limit to feed a mic mixer without affecting the sound too much. It would be very close to what you would measure with an Extech LCR meter at 120Hz.
Lace manufactures these pickups by stamping them out of 3/32 to 1/8 inch thick aluminum to match the footprint for either single coil pickups, humbucker or P90 pickups. Then, they are bent into the final shape to allow a space below for two transformer bobbins.
Look closely and see that they put interlocking U-shaped transformer laminations about 1.25 to 1.5 inches long around the pickup frame where the center aluminum strip joins the lower strip. This location is the place where the currents from each loop join together and where the current transformer passes the current through the transformer laminations, and two coils are placed on this U-shaped lamination, one on each side of the lamination bundle. The output resistance is listed to be about 2500 ohms, but these coils are connected in parallel so each coil is individually about 5000 ohms. The wire gauge is between AWG 44 to AWG 46 to put enough turns on the bobbins to provide an output in the range of a high impedance guitar pickup. Although I have not measured the number of turns, I can only estimate that each bobbin could have between 15,000 to 20,000 turns of very fine wire.
By balancing the thickness of the aluminum stock and the width of the side strips, Lace can control the string loop resistance and its relationship to the number of turns on the two coils on the transformer secondary under the pickup frame the same way you can use different wire gauges and current transformer turns ratios to adjust the sound or voice of your pickup. Once you find the pleasing sound with the right combinations, you can easily mass produce the frames and coils under the frames and have a very consistent product. Science does not tell you what will sound good but once your ears say, «this sounds good», science can help you analyze the characteristics and then help you replicate it.
Higher current transformer turns ratios produce more output but not near what a high impedance guitar pickup puts out. This is why I chose to target the commonly available range of low impedance microphone because commonly available commercial parts could be designed to work in this range. Unless you can find current transformers that have 5000 to 10,000 turns in a small size you are better off working in the 150 to 250 ohms output range. This way you can make a very low noise pickup with a very broad frequency range without the typical resonant hump that makes the typical electric guitar pickup sound.
Prem Magnetics makes the SPCT-251 2000 turn current transformer with a primary square opening of 0.160" which will allow a bundle of smaller wires to fit or a single piece of square AWG 6 copper wire to fill the space. The output impedance will be the calculated resistance of the string loop times the turns ratio squared or 4,000,000. If AWG 6 wire has 30 micro ohms per inch and the total string loop, including the approximate 2 inches that goes through the SPCT-251 primary for a total of 8 inches the output impedance is 8 x 30 x 240 micro ohms. Multiply .000240 times 4,000,000 and you get 960 ohms as the absolute minimum impedance not taking into account leakage inductance and mechanical resistance of joining the ends of the wire to form a low resistance string loop. What ever wire size you used on a 500 turn transformer you now need 16 times more wire area to have the same impedance as the 500 turn transformer. The 2000 turn transformer impedance is much to high to target a microphone XLR input but could drive a 10,000 to 20,000 ohm line input on a mic mixer but only have an output 4x that of the 500 turn current transformer at best. It could drive a guitar amp but the level would be low and you might be subject to more noise. To find a good online resistance calculator look up «Salvarsen resistance calculator». You can even change the metal type to see how using copper, aluminum, brass and other metals affects the resistance of the string loop that produces the current that ultimately drives the output voltage.
Once you understand the theoretical math of working with current based transformers, turns ratios, string loop resistance and how that translates into output impedance and ultimately how it all affects the sound you hear, you can begin to appreciate how you can see cheap electronic stuff on the web based electronic stores and get stuff to do your own experiments and have a high degree of success. Go to «The Electronics Goldmine», «All Electronics» and «Surplus Sales of Nebraska» and see their magnets, copper strips, copper wire, transformers, coils and inductors to get some inexpensive stuff to play with when they are on sale.
Bottom line, let your ears be the fine judge of what sounds good. As you change one variable like wire gauge and type (single strand versus stranded), current transformer turns ratios (1:500, 1:750, 1:1000, 1:2000, 1:2500, 1:5000, 1:????), magnet type and strength and the final interface (Shure A95U matching transformer or mic mixer) to your amplifier you can then tell that by doing this or that I get more bass or treble sounds, more or less noise, higher or lower output signal. The theory of what I am sharing just helps you better understand how all the pieces fit together. There is not too much published information about this stuff. You guys seem to like tinkering and I hope that the results of my tinkering can help.
Moritz Voegeli
#3
XLR mic inputs are typically 2400 ohms. That means that you want the current transformer to be around 240 ohms to not load down the pickup output and cut the high frequencies. A shorted CSE187L is 80 ohms measured output impedance. Then, just add the string loop resistance. AWG 11 is 105 micro ohms per inch so a 6 inch string loop is 630 micro ohms. Multiply this by turns ratio squared or 250'000 and get 157.5 ohms. Now add this to 80 and get a total output of 237.5 ohms. This is right near the upper limit to feed a mic mixer without affecting the sound too much. It would be very close to what you would measure with an Extech LCR meter at 120Hz.
Apr 21, 2017
Moritz Voegeli
#4
Lace manufactures these pickups by stamping them out of 3/32 to 1/8 inch thick aluminum to match the footprint for either single coil pickups, humbucker or P90 pickups. Then, they are bent into the final shape to allow a space below for two transformer bobbins.
Look closely and see that they put interlocking U-shaped transformer laminations about 1.25 to 1.5 inches long around the pickup frame where the center aluminum strip joins the lower strip. This location is the place where the currents from each loop join together and where the current transformer passes the current through the transformer laminations, and two coils are placed on this U-shaped lamination, one on each side of the lamination bundle. The output resistance is listed to be about 2500 ohms, but these coils are connected in parallel so each coil is individually about 5000 ohms. The wire gauge is between AWG 44 to AWG 46 to put enough turns on the bobbins to provide an output in the range of a high impedance guitar pickup. Although I have not measured the number of turns, I can only estimate that each bobbin could have between 15,000 to 20,000 turns of very fine wire.
By balancing the thickness of the aluminum stock and the width of the side strips, Lace can control the string loop resistance and its relationship to the number of turns on the two coils on the transformer secondary under the pickup frame the same way you can use different wire gauges and current transformer turns ratios to adjust the sound or voice of your pickup. Once you find the pleasing sound with the right combinations, you can easily mass produce the frames and coils under the frames and have a very consistent product. Science does not tell you what will sound good but once your ears say, «this sounds good», science can help you analyze the characteristics and then help you replicate it.
Higher current transformer turns ratios produce more output but not near what a high impedance guitar pickup puts out. This is why I chose to target the commonly available range of low impedance microphone because commonly available commercial parts could be designed to work in this range. Unless you can find current transformers that have 5000 to 10,000 turns in a small size you are better off working in the 150 to 250 ohms output range. This way you can make a very low noise pickup with a very broad frequency range without the typical resonant hump that makes the typical electric guitar pickup sound.
Prem Magnetics makes the SPCT-251 2000 turn current transformer with a primary square opening of 0.160" which will allow a bundle of smaller wires to fit or a single piece of square AWG 6 copper wire to fill the space. The output impedance will be the calculated resistance of the string loop times the turns ratio squared or 4,000,000. If AWG 6 wire has 30 micro ohms per inch and the total string loop, including the approximate 2 inches that goes through the SPCT-251 primary for a total of 8 inches the output impedance is 8 x 30 x 240 micro ohms. Multiply .000240 times 4,000,000 and you get 960 ohms as the absolute minimum impedance not taking into account leakage inductance and mechanical resistance of joining the ends of the wire to form a low resistance string loop. What ever wire size you used on a 500 turn transformer you now need 16 times more wire area to have the same impedance as the 500 turn transformer. The 2000 turn transformer impedance is much to high to target a microphone XLR input but could drive a 10,000 to 20,000 ohm line input on a mic mixer but only have an output 4x that of the 500 turn current transformer at best. It could drive a guitar amp but the level would be low and you might be subject to more noise. To find a good online resistance calculator look up «Salvarsen resistance calculator». You can even change the metal type to see how using copper, aluminum, brass and other metals affects the resistance of the string loop that produces the current that ultimately drives the output voltage.
Once you understand the theoretical math of working with current based transformers, turns ratios, string loop resistance and how that translates into output impedance and ultimately how it all affects the sound you hear, you can begin to appreciate how you can see cheap electronic stuff on the web based electronic stores and get stuff to do your own experiments and have a high degree of success. Go to «The Electronics Goldmine», «All Electronics» and «Surplus Sales of Nebraska» and see their magnets, copper strips, copper wire, transformers, coils and inductors to get some inexpensive stuff to play with when they are on sale.
Bottom line, let your ears be the fine judge of what sounds good. As you change one variable like wire gauge and type (single strand versus stranded), current transformer turns ratios (1:500, 1:750, 1:1000, 1:2000, 1:2500, 1:5000, 1:????), magnet type and strength and the final interface (Shure A95U matching transformer or mic mixer) to your amplifier you can then tell that by doing this or that I get more bass or treble sounds, more or less noise, higher or lower output signal. The theory of what I am sharing just helps you better understand how all the pieces fit together. There is not too much published information about this stuff. You guys seem to like tinkering and I hope that the results of my tinkering can help.
Apr 21, 2017
Southern Ray
A few years later... Thank you.
Apr 14