Balanced Low Z pickups for XLR connections

[dfiction]

Hello,

New poster here. I've been reading a lot about low impedance pickup designs—it's fascinating work you're all doing here.

I'm wondering if anyone here could comment or point in the right direction. I'm trying to make balanced, passive pickups for XLR preamps. Currently I'm using cheap humbuckers I found on ebay, which actually work quite well, but there is still a lot of hum and there is definitely an impedance mismatch. Since my instrument uses over 18 pickups, buying pickups isn't really an option, and I'm interested in building my own anyway.

So—anyone have any ideas? I'm looking to avoid using a preamp or DI or transformer, and interested in matching the impedance of the pickup to the xlr input of the mixing console. It has to be balanced, since my cable runs are sometimes over 50 feet long!

I'm a maker of sound installations and instruments. My reason for asking about this is to further develop my instrument, Lady's Harp, which uses pickups and pressure transducers to drive piano wire.
Composing the Tinnitus Suites (2012)
(the project is here.)
The instrument is played via controls of the mixer to process and manage the feedback loop for each string. thus, why I am looking for passive pickups.

Composing the Tinnitus Suites: 2014 (excerpts) on Vimeo Video here if you're curious.

thanks
daniel

[David King]

Hi Daniel,

Welcome and glad you found us.

[charrich56]

Daniel,

Welcome and there are actually a lot of folks here who would be willing to help.

It looks from your link that you are looking to pick up single strings for the most part. In one picture you are using a 6-string pickup to pick up 1 big string. Do you need to pick up vibrations on just 18 strings? Are they going to be closely spaced together? Or would it be better to have 18 single-string pickups because of physical spacing?

If you want to match a low-Z XLR mic input, it would be good to stay in the range of 150-450 ohms output impedance for your pickups.

If you need single string pickups, it might be best to just wind your own. The advantage of winding low impedance coils is that you need far fewer turns than for a high-impedance pickup.

I am visualizing that you need single - string small pickups and ideally they should be balanced output and humbucking.

Try .... sewing machine bobbins with an Alnico rod magnet (ebay, Guitar Parts USA, or a number of other suppliers) in the middle, wound with about 200-400 turns of #34 to #36 AWG (this is just a guess on such a small bobbin but a starting point). One of the forum members here , Salvarsan has made a coil winding calculator to help you estimate the windings you need for a given DC resistance.

Coil Estimator

Here's a link which might work for you for the bobbins:

50 x Clear Plastic Style SA156 Sewing Machine BOBBINS for Brother | eBay

Make two of these, tape or glue two of these side by side , with the magnet flipped on one, and the coils wired out of phase in series, connect your balanced coax cable, and ... you have a single string, balanced, low impedance humbucking pickup.

It would be good if you had access to a bench or handheld LCR meter for measuring both the inductance (preferably at 1 KHz) and DC resistance of your pickup. Make sure you measure inductance with the magnets installed. If you have those two numbers, then you can calculate the impedance pretty easily.

Just estimating from my own experience, you are going to end up with about 25-40 millihenries inductance, and 50-100 ohms DC resistance (both coils series.) This will put you right in the ballpark for a nice mic-level output. You can probably combine several of these in series and parallel to get several multiple strings with one output, and maybe use magnet and pickup positioning to balance levels if you need to. That way you might not need so many mic inputs.

Just kind of wanted to throw this out for starters but others on this forum may also have some even better ideas.

Take care,

Charlie

[cbarrow7625]

For any of the knowledgeable folks here to give you the best suggestion we definitely need to know if you are planning on sending each pickup to separate input or if all can be combined and/or some combination. If you can put all 18 in parallel than you could just use 18 normal 10k Ohm high impedance guitar pickups in parallel to achieve about 600 ohms to match the mic input impedance. Let us know what the series / parallel limitations are.

Anyone reading this without looking at his pictures really needs to hit that link or you'll be confused. I was until I saw what he had going on. Neat stuff!

[bbsailor]

Hello,

New poster here. I've been reading a lot about low impedance pickup designs—it's fascinating work you're all doing here.

I'm wondering if anyone here could comment or point in the right direction. I'm trying to make balanced, passive pickups for XLR preamps. Currently I'm using cheap humbuckers I found on ebay, which actually work quite well, but there is still a lot of hum and there is definitely an impedance mismatch. Since my instrument uses over 18 pickups, buying pickups isn't really an option, and I'm interested in building my own anyway.

So—anyone have any ideas? I'm looking to avoid using a preamp or DI or transformer, and interested in matching the impedance of the pickup to the xlr input of the mixing console. It has to be balanced, since my cable runs are sometimes over 50 feet long!

I'm a maker of sound installations and instruments. My reason for asking about this is to further develop my instrument, Lady's Harp, which uses pickups and pressure transducers to drive piano wire.
Composing the Tinnitus Suites (2012)
(the project is here.)
The instrument is played via controls of the mixer to process and manage the feedback loop for each string. thus, why I am looking for passive pickups.

Composing the Tinnitus Suites: 2014 (excerpts) on Vimeo Video here if you're curious.

thanks
daniel

Daniel,

Check out this forum thread. http://music-electronics-forum.com/t14952/. Here I use an 8 ohm to 20 K miniature transformer to extract the voltage generated across a string vibrating in a magnetic field to a useable output. Try using alligator clips to put the 8 ohms side across a string, connect the high Z (20K side) to the amp input, hand hold a magnet near the string and pick the string. You will hear a good level signal and as you change the position of the manet the tone will change as well as changing the length of the magnetic field under the string will affect the output level of that string.

You can also use this backwards and feed a signal generator into the high Z side of the transformer, place a magnet under the string, sweep the generator and when you hit the primary frequency of the string or a harmonic, the string will start to vibrate like a speaker. This seems to be another alternative for your multitude of single string transducers. This is fun stuff to play and learn with.

Joseph Rogowski

[charrich56]

Daniel,

I envy you the time and space to work with (and play with) this stuff in an artistic context.

Joseph's solution would work fine both ways, both for picking up and transducing back into the string.

With strings that long and big, you could get some amazing harmonic feedback effects with some experimentation with positioning on nodes and antinode points.


-Charlie

[dfiction]

Hello!

Thanks for the kind words.

The limitation is that each pickups needs an independent channel—so, all 18 separate inputs on the mixer are used and manipulated independently. That means series/parallel configurations won't work as a way of changing impedance.

thanks,
daniel!

For any of the knowledgeable folks here to give you the best suggestion we definitely need to know if you are planning on sending each pickup to separate input or if all can be combined and/or some combination. If you can put all 18 in parallel than you could just use 18 normal 10k Ohm high impedance guitar pickups in parallel to achieve about 600 ohms to match the mic input impedance. Let us know what the series / parallel limitations are.

Anyone reading this without looking at his pictures really needs to hit that link or you'll be confused. I was until I saw what he had going on. Neat stuff!

[dfiction]

Joseph,

thanks for your reply. your posts are deep with lore and wisdom, and i've been trying to decipher them. i'll try the moving coil approach next. is there any reason i couldn't run this moving coil configuration balanced?

the backwards magnet trick is something i've played around with, using solenoid relays and an amplifier. but i was never really excited by any of the results i got.

xo
daniel

Daniel,

Check out this forum thread. http://music-electronics-forum.com/t14952/. Here I use an 8 ohm to 20 K miniature transformer to extract the voltage generated across a string vibrating in a magnetic field to a useable output. Try using alligator clips to put the 8 ohms side across a string, connect the high Z (20K side) to the amp input, hand hold a magnet near the string and pick the string. You will hear a good level signal and as you change the position of the manet the tone will change as well as changing the length of the magnetic field under the string will affect the output level of that string.

You can also use this backwards and feed a signal generator into the high Z side of the transformer, place a magnet under the string, sweep the generator and when you hit the primary frequency of the string or a harmonic, the string will start to vibrate like a speaker. This seems to be another alternative for your multitude of single string transducers. This is fun stuff to play and learn with.

Joseph Rogowski

[bbsailor]

Joseph,

thanks for your reply. your posts are deep with lore and wisdom, and i've been trying to decipher them. i'll try the moving coil approach next. is there any reason i couldn't run this moving coil configuration balanced?

the backwards magnet trick is something i've played around with, using solenoid relays and an amplifier. but i was never really excited by any of the results i got.

xo
daniel

daniel,

Yes, you can run this balanced but you need to use a small transformer with an output impedance that will match the typical actual input impedance of a microphone mixer. Try using a 3.2 ohm to about 500 to 600 ohm secondary for a good match on a balanced mic input. Look at the Calrad miniature audio transformer 54 series and the Xicon 42TL series but if you use a transformer with an 8 ohm rated input you can use an output value of about 1200 ohms as the low impedance of the string will lower the output impedance in this case to be near a good microphone input match. If you use a transformer with about a 1 to 12 turns ratio with 1 mv generated on the string (and across the transformer low Z input side) will generate about 12 mv as an output which is good enough for a mic input range. Use the high Z output side of the transformer as the balanced line output or pins 2 and 3 of the XLR connector. Ground the shell of the transformer to the 2 conductor mic cable shield (XLR pin 1) and try running a ground from the string to the shield also to get the lowest noise.

The output level will be directly related to length of the magnetic field under the string.

I hope this helps?

Joseph Rogowski

[charrich56]

Joseph,

Using the moving coil approach, even with the larger diameter of the piano wire strings, wouldn't the resistance of the strings be an issue for the 4-8 foot (guessing) lengths shown in Daniel's link?

Daniel, could you share the string gauge or diameter, and length range of the strings you are using, or possibly a resistance reading of a typical string? Even if you have this licked already you have piqued some curiosity.

Thanks,

-Charlie

[bbsailor]

Joseph,

Using the moving coil approach, even with the larger diameter of the piano wire strings, wouldn't the resistance of the strings be an issue for the 4-8 foot (guessing) lengths shown in Daniel's link?

Daniel, could you share the string gauge or diameter, and length range of the strings you are using, or possibly a resistance reading of a typical string? Even if you have this licked already you have piqued some curiosity.

Thanks,

-Charlie

Charlie,

As a rule of thumb, when the string impedance matches the transformer input impedance the voltage generated on the string by induction will be about one half the unloaded voltage as measure by a 1 meg ohm oscilloscope probe across the string. If you obtain a transformer with a center tap on the input side (low Z side) of the transformer, you can try either the full winding or the center tap winding to listen for the best match or observe the output on an oscilloscope.

Joseph Rogowski