Very Low Impedance pickups...as PCBS?

[dfiction]

my question is about making a very low impedance pickup—with current transformer—on a printed circuit board. has anyone ever tried it?
these days it's so easy to prototype and fab with pcb warehouses.

[Helmholtz]

So called "low impedance PUs" should actually be called transformer pickups, as you can't extract a signal without making the sensing conductor loop the primary of a transformer.
The actual output impedance depends on the number of turns on the secondary of the transformer.
If you want to connect to guitar amplifier you will need 5000+ turns, making it a medium to high impedance output.

Good bass reponse requires extremely low primary loop resistance, hardly achievable with a PCB track.

[bbsailor]

my question is about making a very low impedance pickup—with current transformer—on a printed circuit board. has anyone ever tried it?
these days it's so easy to prototype and fab with pcb warehouses.

dfiction,

Hemloltz comments are right on.

Here is another thing to consider, the wire thickness skin effect. Many AWG wire charts include the frequency of the skin effect or the highest frequency that will reach the center of the wire. Thicker solid wires have lower frequency skin effect frequencies and will have a tonal effect on the final sound coming out of the current transformer.

The construction challenge is to make the lowest resistance copper string loop going around the magnet and also looping through or connected to the current transformer. Lower resistance string loops produce a higher induced current and thus produce a higher output. The mechanics of how you make the string loop connection is very important, lower is better. I like to us a Triad CS186L current transformer with the 3 turns of AWG 18 removed to open the current transformer, with 500 turns on the secondary, to use an AWG 8 solid copper wire In a space of about one eighth of an inch square opening. Wrap a thin piece of tape around the transformer lamination to prevent the string loop from shorting out if both sides touch the lamination.

The output impedance of this setup should be about 120 to 150 ohms based on:
1. Wire gauge of the sting loop
2. Length of the string loop in the 7 to 8 inch range of length
3. String loop connection resistance with lower being better

This will produce a low impedance microphone output level to drive an XLR connector like this.
Pin 1 Ground and a connection to the coax shield and string loop and the metal frame of the CSE186L.
Pin 2 One output pin of the current transformer.
Pin 3 Other output pin of the current transformer.

You can try to use stranded wire to minimize the skin effect but it will have a higher resistance due to the air space between strands and your ability to make a very low resistance string loop joint.

Do you have a way to measure the output impedance of the current transformer?

If you build one of these pickups, post your progress on this forum. It will be a very good learning experience to see new pickup design opportunities.

Joseph J. Rogowski

[dfiction]

Hi Mr. Joseph Rogowski! Thanks again for all the knowledge you provide.

Hi Helmholtz! Thanks for your help!

I'm still learning about transformer pickups, though I did make a successful prototype with A Triad CS187 back in 2017.

This project seems perfect for using a PCB, because it uses the CT transformer as a pcb part, and because if measured correctly in the design phase, you'd have the exact hairpin for each product you'd make.

You say extremely low impedance on the primary..how low?

I was wondering about making a PCB track rather wide, like a .5 inch trace width. You could also lower the impedance further by doing a 4 layer pcb, with traces on each layer connected with vias on each layer. Or, two PCBs stacked on each other with the loops connected via traces or headers.

dfiction,

Hemloltz comments are right on.

Here is another thing to consider, the wire thickness skin effect. Many AWG wire charts include the frequency of the skin effect or the highest frequency that will reach the center of the wire. Thicker solid wires have lower frequency skin effect frequencies and will have a tonal effect on the final sound coming out of the current transformer.

The construction challenge is to make the lowest resistance copper string loop going around the magnet and also looping through or connected to the current transformer. Lower resistance string loops produce a higher induced current and thus produce a higher output. The mechanics of how you make the string loop connection is very important, lower is better. I like to us a Triad CS186L current transformer with the 3 turns of AWG 18 removed to open the current transformer, with 500 turns on the secondary, to use an AWG 8 solid copper wire In a space of about one eighth of an inch square opening. Wrap a thin piece of tape around the transformer lamination to prevent the string loop from shorting out if both sides touch the lamination.

The output impedance of this setup should be about 120 to 150 ohms based on:
1. Wire gauge of the sting loop
2. Length of the string loop in the 7 to 8 inch range of length
3. String loop connection resistance with lower being better

This will produce a low impedance microphone output level to drive an XLR connector like this.
Pin 1 Ground and a connection to the coax shield and string loop and the metal frame of the CSE186L.
Pin 2 One output pin of the current transformer.
Pin 3 Other output pin of the current transformer.

You can try to use stranded wire to minimize the skin effect but it will have a higher resistance due to the air space between strands and your ability to make a very low resistance string loop joint.

Do you have a way to measure the output impedance of the current transformer?

If you build one of these pickups, post your progress on this forum. It will be a very good learning experience to see new pickup design opportunities.

Joseph J. Rogowski

[Helmholtz]

I just remember that the resistance of the primary had to be extremely low for good bass response.
The larger the conductor cross-section the better.
The lower corner frequency could be calculated from f = R/(2 pi L).
E.g. with a loop inductance of 3µH (strongly affected by the transformer core) and a loop resistance of 1milliOhm the corner frequency becomes 53Hz.

..because it uses the CT transformer as a pcb part..

Could you elaborate?
How many turns and what core type?

[bbsailor]

dfiction,

Here is an easy way to estimate the current transformer output impedance.

1. AWG 8 solid wire has .00005234 ohms per inch resistance. A string loop that goes through the CT and the magnets that is 8 inches long will be .00041872 ohms.
2. Square the primary to secondary turns ratio. In this case it is 1 to 500 or 250000.
3. Multiply the primary resistance by 250000 and you will get about 105 ohms.

Based on the accuracy of the published CT turns count and the low resistance quality of the string loop joint, and the inductive coupling efficiency of the string loop to the CT core laminations you will have an actual output measured impedance in the range of 105 ohms to about 120 ohms.

The minimum load on the CT output impedance should be a minimum of 10 times higher. Typical XLR microphone loads are 2400 ohms so it will put less load on the CT output voltage.

After all the measurements, let your ears be the final judge.

Joseph J. Rogowski

[Helmholtz]

dfiction,

Here is an easy way to estimate the current transformer output impedance.

1. AWG 8 solid wire has .00005234 ohms per inch resistance. A string loop that goes through the CT and the magnets that is 8 inches long will be .00041872 ohms.
2. Square the primary to secondary turns ratio. In this case it is 1 to 500 or 250000.
3. Multiply the primary resistance by 250000 and you will get about 105 ohms.

That gives you AC resistance, not impedance.
Inductance wil significantly increase the impedance at 1kHz.
Also the DCR of the CT coil adds to impedance but might be negligible.