Low Impedance Pickup Research

[bbsailor]

Joseph,

I've read the other thread on moving coil pickup: Very inspiring. I will post a reply there.
I'm planning to make some single loop pickups and will post some pictures if they're done.
Thanks for your useful information.

Hans

Hans,

If you have an oscilloscope it will help with your low Z pickup experimentation. I use a combination function generator plus a 2 channel digital USB scope: Velleman PCSGU250. You can use a driver pickup coil with a few hundered turns of AWG 30 to 32 wire to make Bode frequency response plots of your pickups and see the effect of using various gauges of wire on the string sensing loop and current transformer turns ratios.

Joseph Rogowski

[MisterBzr]

I've made a pick up out of 9 mm plywood. I've drilled three holes, in the middle hole I placed two magnets. In the other holes if put 6 mm2 (AWG 10) electric wire, making a loop with a thermoplastic connector (with the plastic removed). I've also made a pick up with 4 mm2 (AWG 13) wire and I got similar results.

Signal path: pick up - mic to line adapter - Boss gt 6 - laptop (guvcview - Linux)

0-20s only reverb, 20s - 40s eq + reverb, 40s eq + od + reverb

I'm happy with the clean sound, the OD-sound needs modification.
Next step is changing the position of the pick up relative to the bridge and the strings to fine tune the sound.

[GuyBoden]

Alex,
You can use any microphone matching brand that is similar to the Shure A95U series.

Hello, great forum.

I'm getting the parts to make this interesting low impedance single loop pickup.

Does anyone know if the SHURE A85F IMPEDANCE CONVERTER Low to high impedance adapter would be ok to use instead of the Shure A95U.

Shure Americas | A85F Line Matching Transformer

Many thanks to all who contributed to this great thread.
GuyB

[bbsailor]

Hello, great forum.

I'm getting the parts to make this interesting low impedance single loop pickup.

Does anyone know if the SHURE A85F IMPEDANCE CONVERTER Low to high impedance adapter would be ok to use instead of the Shure A95U.

Shure Americas | A85F Line Matching Transformer

Many thanks to all who contributed to this great thread.
GuyB

GuyB

The Shure A85F transformer is rated at 300 ohms for the primary XLR side and 40,000 ohms for the secondary .25" plug side. It has about a 1 to 12 turns ratio. Actually it is 11.547. 40,000 divided by 300 equals 133.333. Take the square root of 133.333 to be 11.547. If you use a current transformer with 500 secondary turns the CT output should be between 150 and 300 ohms depending on the wire gauge of the primary string loop. The string loop and the metal frame of the current transformer should be soldered to the ground wire to minimize noise. Mount the A85F at the amp end to minimize capacitance effects of high impedance guitar cables.

Share your progress with us.

Joseph Rogowski

[GuyBoden]

GuyB

Guy

The Shure A85F transformer is rated at 300 ohms for the primary XLR side and 40,000 ohms for the secondary .25" plug side. It has about a 1 to 12 turns ratio. Actually it is 11.547. 40,000 divided by 300 equals 133.333. Take the square root of 133.333 to be 11.547. If you use a current transformer with 500 secondary turns the CT output should be between 150 and 300 ohms depending on the wire gauge of the primary string loop. The string loop and the metal frame of the current transformer should be soldered to the ground wire to minimize noise. Mount the A85F at the amp end to minimize capacitance effects of high impedance guitar cables.

Share your progress with us.

Joseph Rogowski

Many thanks for the very informative answer to my question and all of the answers you have provided on these low Z pickup in various threads, I'm hoping to complete the project in the next two weeks.

Many thanks again.
Guy

[Arcadian]

Hi Everyone
Just happened on this discussion a few days ago. What a revelation, but a whole lot to get ones head around.
I'll see if this gets posted ( only just registered) then return.

Richard

[Arcadian]

I was really excited by the concept of this system as it might address several long-term problems I have had amplifying acoustic instruments.The other apect is that it would seem to allow a much wider range of aesthetic/design possibilities which is quite important to me.
I've only had a day or two to try and assimilate all this stuff but made a quick prototype from bits which were to hand. I think the CT's mentioned in the thread will be pretty difficult to get here in UK but as a starter just used a small output transformer (0.5ohm primary, 70ohm c/t secondary) and some 5mm NeoD mags.

Running this as a balanced line straight into a mic channel of a small mixer proved a complete shock, -amazing! - quiet, incredibly clear and bright, a bit of eq'ing needed but all very encouraging. I've tried lots of commercial systems and all seem to have drawbacks, not least of which is price, so this does look to be something with huge potential (for me anyway) plus the fact that it seems so versatile in the way it can be constructed, especially useful for experimental instruments.

I did find the usual acoustic guitar problem (I think bbsailor mentioned this) of uneven string balance, rather powerful 2nd(especially in DADGAD), weak 3rd (wound) but I'm hoping that a bit of experimentation stacking different strength magnets might cure this. I'm reluctant to use adjustable poles as this will compromise what is to me one of the major pluses, the thinness of the assembly.

As mentioned , I cant find the CT's recomended over here. There are plenty of the 'through-the-hole' types but they all seem to be specifically 50/60Hz or 20kHz/200kHz operating range.Is it likely these would operate sufficiently well in this application and if so,would the tiny surface-mount devices be appropriate?

Finally, Major thanks to bbsailor for sharing his knowledge,time and patience opening-up these interesting possibilities.

Richard
ps. Happy to post some images but havent figured that yet!

[bbsailor]

I was really excited by the concept of this system as it might address several long-term problems I have had amplifying acoustic instruments.The other apect is that it would seem to allow a much wider range of aesthetic/design possibilities which is quite important to me.
I've only had a day or two to try and assimilate all this stuff but made a quick prototype from bits which were to hand. I think the CT's mentioned in the thread will be pretty difficult to get here in UK but as a starter just used a small output transformer (0.5ohm primary, 70ohm c/t secondary) and some 5mm NeoD mags.

Running this as a balanced line straight into a mic channel of a small mixer proved a complete shock, -amazing! - quiet, incredibly clear and bright, a bit of eq'ing needed but all very encouraging. I've tried lots of commercial systems and all seem to have drawbacks, not least of which is price, so this does look to be something with huge potential (for me anyway) plus the fact that it seems so versatile in the way it can be constructed, especially useful for experimental instruments.

I did find the usual acoustic guitar problem (I think bbsailor mentioned this) of uneven string balance, rather powerful 2nd(especially in DADGAD), weak 3rd (wound) but I'm hoping that a bit of experimentation stacking different strength magnets might cure this. I'm reluctant to use adjustable poles as this will compromise what is to me one of the major pluses, the thinness of the assembly.

As mentioned , I cant find the CT's recomended over here. There are plenty of the 'through-the-hole' types but they all seem to be specifically 50/60Hz or 20kHz/200kHz operating range.Is it likely these would operate sufficiently well in this application and if so,would the tiny surface-mount devices be appropriate?

Finally, Major thanks to bbsailor for sharing his knowledge,time and patience opening-up these interesting possibilities.

Richard
ps. Happy to post some images but havent figured that yet!

Richard,

Welcome to the forum. Many folks here will freely share technical information. Try to do some research and experimentation to try to narrow down your questions for a better chance of getting an answer that will help you.

Acoustic instruments only have the core of plain strings being the magnetic portion of the string that stimulates the generation of induction in the coil around the magnet(s). If you ue a single heavy loop of wire around the magnet and connected to the primary of a current transformer you can bend or dip the wire a few mm lower under the B-string while raising the wire a little closer to the G-string to attempt to obtain a better balance. As an alternative you can place small neo magnets under the primary string magnet and increase the magnetic field on the strings you want louder and skip using it under the B-string. I use wood craft stricks both 9mm and 17mm wide to fabricate my current transformer (CT) based pickups on.

Craft ceramic magnets from Magnets - Master Magnetics, Inc. part number 07001 contains 8 magnets 5.8mm X 22mm that allow you to make a single CT pickup spanning 6 strings or using two string loops under 3 strings with a CT for each set of 3 strings. Wire the CT outputs in series and place a variable resistor (500 ohm miniature pot) across them to balance the output.

You will find that the gauge of the wire used for the string loop that connects to the primary of the CT will determine the output impedance of your pickup. A 300 turn CT (http://www.viteccorp.com/data/af1285.pdf) will provide an output impedance in the 50 ohm range while a 500 turn CT (CSE-187L) will make the impedance in the 150 ohm range. Why is this important? The rule of thumb is to make the source impedance (the CT pickup) have between one tenth to one fifth of the load impedance and work as a bridging impedance rather than a matched impedance (as in the old days of radio and telephone systems). That is why today most 150 ohm rated microphones require a mic mixer input impedance of at least 1500 ohms to have the best signal to noise ratio.

Try to order a few Triad CSE-187L metal frame, laminated E-I core type current transformers as these are very easy to make into acoustic guitar pickups using wood craft sticks. Use the CT output as balanced line for an XLR connetor and ground the metal frame and the low impedance single turn string loop to pin 1 to minimize noise. Putting a ferrous metal plate under the magnet of the weaker strings is a good way to obtain a better string balance. I make the string loop go around all the strings but I terminate the connection point centered under the B-string rather than on the end of the loop. This way the CT forms a deep U-shaped dip under the B-string to minimize it's loudness. The spacing of the U-shaped primary of the CSE-187L is almost the same spacing as the string spacing. The creativity in building these things comes in arranging commercial-off-the-shelf CTs to make a high quality guitar pickup and requires the ability to bend heavy wire and make low impedance solder connections.

If you use smaller toroid CTs look to fill the center opening with the largest wire that will fit. AWG6 will fill the Vitec 57P series CTs pretty well. Over in the UK you may be using another wire gauging system. Obtain copper tubing the same opening as the wire diameter to make a good, low impedance solder joint to form the primary string loop.

Your .5 ohm to 70 ohm DCR transformer looks like an 8 ohm impedance to 1K ohm impedance output transformer used backwards with the .5 ohm side being the input or primary. If you want to play with this transformer to learn this stuff better, try this. Wind your primary coil with magnet wire in the AWG 24 to AWG 26 range and make coil taps at 50 turns, 100 turns and the full 200 turns coil. The transformer that you are using has a turns ratio of about 1 to 10 and makes the 200 turns loop like 2000 turns, the 100 turn tap look like 1000 turns and the 50 turn tap look like 500 turns. Switch between the taps and listen to the sound that pleases you ear and keeps the level clean within the range of the mic mixer.

Study a little transformer theory and pay attention to learning about these following terms to help you better understand what is involved with your experiments.

Turns Ratio
Impedance Ratio
Leakage Inductance
Reflected Impedance
Transformer Loading
Eddy Currents
Current Transformer Theory

Since CT have a 1 to 100 up to a 1 to 5000 turns ratio and the size of the single primary string loop is measures in microohms or millionths of an ohm per inch of wire this will have a primary effect on determining the CT output impedance. To see the effect of using various wire gauges, square the CT turns ratio and mulitply that number by the total resistance of the string loop. If you are using the CSE-187L the prewired primary is about 200 microohms then wired to an extended U-shaped loop to span the strings. If the total string loop is 500 microohms then the output impeance will be about 125 ohms or .000500 (500 microohms) times 250,000 (500 squared), then add another 25 ohms for leakage inductance and you get an output impedance in the 150 ohm range. If you use a CT with too much more than 500 turns you are in a CT output impedance range that exceeds the mic mixer input range or low to high microphone matching transformer range (Shure A95U).


My Triad CST-1005 toroid 1000 turn CT with an oval loop about 80mm long and 20mm wide has an output impedane of about 290 ohms using AWG6 wire for the string loop going through the center of the CT.


Once you play with this stuff for a while, you will get a feel for what CT turns ratios and what wire gauge primary string loops produce a convenient range of impedance outputs to match the mic mixer input impedance.

Try any CTs you can get your hands on, 50/60HZ and higher frequency CTs will work. Let your ear be the final judge. The tips above should get you close to what works.

Post photos of your results to stimulate others to delve into low impedance pickups.

Joseph Rogowski

[Arcadian]

Many thanks for all that. It will take a little while to sink in !

Was unable to upload images earlier, I'll try again to give an idea where I'm looking to go with this.

Ah, seems to work now. This is the first prototype in brass and then with ebony cover on one of my little Florentine guitars.

Cant wait to get some of the 'proper' CT's to develop this.

Thanks again
Richard

[bbsailor]

Many thanks for all that. It will take a little while to sink in !

Was unable to upload images earlier, I'll try again to give an idea where I'm looking to go with this.
[ATTACH=CONFIG]24993[/ATTACH] [ATTACH=CONFIG]24992[/ATTACH]
Ah, seems to work now. This is the first prototype in brass and then with ebony cover on one of my little Florentine guitars.

Cant wait to get some of the 'proper' CT's to develop this.

Thanks again
Richard

Richard,

When you obtain a CSE-187L place the joint slit, where the CT is connected, not in the center, as in your photo, but place it in line with the B string. This will place the current path on that side of the coil down lower adjacent to the B-string while it goes through the CT and should reduce the B string level somewhat.

Noice job using available space in a creative way.

Joseph Rogowski

[Steve Conner]

Very cool pickup!

We regularly order stuff from Mouser or Digikey whenever we need components that aren't stocked by the usual UK suppliers. Triad are an American brand, not so common over here. Mouser have the CSE-187L in stock.

cse-187l Current Transformers | Mouser

[GuyBoden]

Many thanks for all that. It will take a little while to sink in !

Was unable to upload images earlier, I'll try again to give an idea where I'm looking to go with this.
[ATTACH=CONFIG]24993[/ATTACH] [ATTACH=CONFIG]24992[/ATTACH]
Ah, seems to work now. This is the first prototype in brass and then with ebony cover on one of my little Florentine guitars.

Cant wait to get some of the 'proper' CT's to develop this.

Thanks again
Richard

Richard,

Very nice pickup, I'm still experimenting with this pickup design to get more low frequency, I'm also in the UK and I've been using these Nuvotem AS-104 1:500 turns from RS Components:

Buy Current Transformers Current sense transformer 15A 20-200kHz Nuvotem AS-104 online from RS for next day delivery.

[Arcadian]

Thanks for the helpful suggestions.

Do Mouser/Digikey and RS have minimum order quantities? -couldnt find info on their sites. Presumably there is vat on imports?

I'll follow bbsailors advice to reduce b-string power on the next version, and the plan was to use 1mm thick magnets stacked to give different height/power combinations.
Not sure if NeoD is ideal, on Mk1 version they seem to be too strong, my usual supplier doesnt do ceramic in the smaller sizes so I guess its a trial-and -error job.Maybe the stacking will do the trick.

I found the bass response on this first one really good, my small body guitar is a bit 'plunky' in the bass and this seemed to get smoothed out a bit. I'm playing through a small mixer and lowish quality amp and monitors, might be different through a guitar cab.

Thanks, Richard

[GuyBoden]

Thanks for the helpful suggestions.

Do Mouser/Digikey and RS have minimum order quantities? -couldnt find info on their sites. Presumably there is vat on imports?

I'll follow bbsailors advice to reduce b-string power on the next version, and the plan was to use 1mm thick magnets stacked to give different height/power combinations.
Not sure if NeoD is ideal, on Mk1 version they seem to be too strong, my usual supplier doesnt do ceramic in the smaller sizes so I guess its a trial-and -error job.Maybe the stacking will do the trick.

I found the bass response on this first one really good, my small body guitar is a bit 'plunky' in the bass and this seemed to get smoothed out a bit. I'm playing through a small mixer and lowish quality amp and monitors, might be different through a guitar cab.

Thanks, Richard

RS have a £4.95 handling fee on orders below £20, but you can pickup the items for free if you can get to one of their local branches. It's simple to just create an account, to avoid the non-account fee charge.

RS Info link:
Service Centre
"A handling fee of £4.95 applies to orders of £20 or below (excl. VAT) and all non account holders"

[Arcadian]

Thanks for that Guy, I'll give them a try.

Hope youre having some success with your experiments. Is this for electric guitar?
I still have a '64 Burns short-scale jazz which has three of the low-impedance 'split-sound' pickups (about 30 Ohms if I remember right, plus two matching transformers).The wiring for them is unbelievable (complex, that is) .I've also got copies of some of the original Jim Burns patents which I must dig-out.

Richard