Low Impedance Pickup Research

[surdopickups]

I agree with your analysis, but the customer who mounted the pickups on an electric bass, who sent the picture, wants to have volume and tone.


Concordo con la tua analisi, tuttavia il cliente che ha montato il pickups su un basso elettrico, che ha mandato la foto, desidera avere volume e tono.

[J M Fahey]

I agree with your analysis, but the customer who sent the pickups on a bass electric, wants to have volume and tone

Un vero rompe coglioni.
Suggerisci un preamplificatore attivo con tutti i controlli che lui vuole.

A real "balls crusher"
Suggest an active preamp with all controls he wants.

[MarkusBass]

Can you give some more details of the pickups (I'm thinking what kind of preamp can be used with them)? You have two independent coils with low signal. It seems that the output is not symmetric so you have to mix two signals, add some gain, add tone controls. This may be quite difficult since the signal is very low. Have you thought about any specific circuit? Do you need a symmetric output, or just typical output like in a bass guitar?

Mark

[rjb]

I'm guessing the pickups were designed to produce mic level, balanced output.
So, if you want to plug into a guitar amp, you need a mic preamp with EQ and an unbalanced output jack.
Yes, no?

-rb

[MarkusBass]

I'm guessing the pickups were designed to produce mic level, balanced output.
So, if you want to plug into a guitar amp, you need a mic preamp with EQ and an unbalanced output jack.
Yes, no?
-rb

This is exactly the problem. Looking at the photo it seems to me that the output is not balanced. I see two independent pickups. XLR may be misleading. So the main question is: is the output balanced, or not? Depending on whether the output is balanced or not, you need different preamps.

Mark

[rjb]

Uninformed Kibitzing

...Looking at the photo it seems to me that the output is not balanced. I see two independent pickups. XLR may be misleading....

Oh, now I see the Y-cable on the string bass. And although the four coils might be wired in series to send a balanced signal directly to a mixing console, the XLR might alternately be wired with two unbalanced signals and a common ground.

And who knows how the electric bass is wired? Probably like a J-Bass, I bet. Was the owner expecting to plug straight into a bass amp?

Since I've really got nothing to contribute, I'll shut up now.

Later,
-rb

[surdopickups]

There are 2 coil
400 ohms for coil
Neodymium magnets
Screen
xrl output

Sono 2 bobine
400 ohm per bobina
Magneti Neodimio
Schermo
xrl output

[MarkusBass]

Sono 2 bobine
400 ohm per bobina
Magneti Neodimio
Schermo
xrl output

Cerchiamo di scoprire se il segnale è simmetrico, o hai due bobine indipendenti?
Sulla base della descrizione, hai due bobine indipendenti. Ho ragione?

Marco

[surdopickups]

Yes
two independent coils
balanced signal

[bea]

May i continue with that old thread? My target guitar for that PU is still not stable. Therefore i did not build anything. But i am having a new project...

Checking my materials (i need to find the magnets...): copper wire 6 mm^2, diameter 2.7 mm. CT Talema AX500mm. Central hole 5 mm. My soldering iron can cleanly solder the wire, but not much more. (Meanwhile i have an old cable with 5x10 mm^2 left, maybe i could try to solder that as well)

To my understanding, the CT is just 500 windings on a ferrite coil. Which brings up the question if it might be possible to use just 500 windings of wire directly on the primary conductor (well, on a thin insulating layer. Either paper or nail polish.) Without ferrit core. I have plenty of 0.2-mm wire for traditional LoZ pickup experiments which could be used.

What Do You think? Can the ferrite core be omitted?

BTW: a link to the datashett of the CT: http://static.mercateo.com/c1/88ab5e...ries.pdf?v=330

[bea]

regarding the diy aspect of the transformer itself: it was 4 o'clock in the night. meanwhile i noticed my error...

[Mike Sulzer]

May i continue with that old thread? My target guitar for that PU is still not stable. Therefore i did not build anything. But i am having a new project...

Checking my materials (i need to find the magnets...): copper wire 6 mm^2, diameter 2.7 mm. CT Talema AX500mm. Central hole 5 mm. My soldering iron can cleanly solder the wire, but not much more. (Meanwhile i have an old cable with 5x10 mm^2 left, maybe i could try to solder that as well)

To my understanding, the CT is just 500 windings on a ferrite coil. Which brings up the question if it might be possible to use just 500 windings of wire directly on the primary conductor (well, on a thin insulating layer. Either paper or nail polish.) Without ferrit core. I have plenty of 0.2-mm wire for traditional LoZ pickup experiments which could be used.

What Do You think? Can the ferrite core be omitted?

BTW: a link to the datashett of the CT: http://static.mercateo.com/c1/88ab5e...ries.pdf?v=330

You need a ferromagnetic core to get enough inductance so that you can develop a voltage across the primary in order too induce voltage in the secondary, just like any other transformer.

A current transformer is one intended to be placed in series with a circuit. its impedance is low enough compared to the overall impedance of the circuit so that the voltage drop across the transformer is small compared to the voltage in the circuit, and so the circuit works very much as if the transformer were not there. That is, the CT is used a sensor that monitors the current without changing the circuit very much.

In a low impedance pickup circuit, a current transform is used as a voltage transformer. That is, the impedance of the pickup is low enough so that the inductance of the primary of the "CT" is high enough so that it does not load down the pickup. Then you can get the full output of the pickup transformed to a higher voltage across the secondary.

[bbsailor]

You need a ferromagnetic core to get enough inductance so that you can develop a voltage across the primary in order too induce voltage in the secondary, just like any other transformer.

A current transformer is one intended to be placed in series with a circuit. its impedance is low enough compared to the overall impedance of the circuit so that the voltage drop across the transformer is small compared to the voltage in the circuit, and so the circuit works very much as if the transformer were not there. That is, the CT is used a sensor that monitors the current without changing the circuit very much.

In a low impedance pickup circuit, a current transform is used as a voltage transformer. That is, the impedance of the pickup is low enough so that the inductance of the primary of the "CT" is high enough so that it does not load down the pickup. Then you can get the full output of the pickup transformed to a higher voltage across the secondary.

Mike is correct but there is one more important characteristic that you can control and that is the output impedance of the pickup you are making using various CTs with different secondary turns and the wire gauge of the string loop sensing the vibrating strings with the magnet in the center of this loop.

If you want to target an XLR mic input, the actual input impedance is about 2400 ohms which then limits the pickup output impedance to be about 240 ohms so the there is minimal loading which can affect the frequency response. Using the thickest string loop wire tends to generate more current but this includes making a very good low resistance joint to complete the string loop.

Trying to make a coil directly on the string loop is very much like a “Rogowski Coil” which is no relation to me. This would not be a very efficient way to convert the current in the string loop to a higher output voltage.

Joseph J. Rogowski

[bea]

Anyway, and although i know from Hans' example earlier in this thread that it basically works and although sounds nice: how important is the gap between the coil of the CT and the wire?

[bbsailor]

Anyway, and although i know from Hans' example earlier in this thread that it basically works and although sounds nice: how important is the gap between the coil of the CT and the wire?

In transformer theory there is the a thing called "leakage inductance" which can limit the development of the full current in the primary string loop turn which tends to reduce the output level and increase the output impedance. The rule of thumb to minimize leakage inductance is to more fully occupy the space with the current carrying wire, for the area allocated for the primary turn going through a toroid transformer center opening or the primary wire U-shaped loop going through a E-I laminated frame current transformer. Leakage inductance can also raise the output impedance by about 10 to 20 percent and create more losses than if a thicker wire occupied more primary turn current transformer area.

In transformer theory there is such a thing as the ideal transformer which shows how the output impedance of a 500 turn transformer would be about 250,000 (turns squared) times the resistance of the primary string loop. If you use AWG 8 wire which is about 52 micro-ohms per inch, an 8 inch total string loop would be 416 micro-ohms. 416 times 250,000 is 104 ohms as an ideal output impedance with no losses. Transformer theory also describes "real transformers" with all the losses due to resistance, magnetic coupling and skin effect of current generation of higher frequencies in thick solid wire not penetrating to the center of the wire. An increase of output impedance of between 5 and 10 percent is a low leakage inductance loss based on my measurements. Use copper tubing to join the string loop with a very low resistance connection. Clean the copper tubing inside and wire with fine sandpaper to clean the connection area. Use silver solder and a good mechanical connection. This will maximize the current induced in the string loop and optimize the output level.

When I compare the output impedance results using two E-I laminated frame CTs I found that using a Triad CSE-186L with a three turn pre-wired AWG 16 primary works better than a CSE-187L with a pre-wired AWG 12 single primary turn because when I remove the three turns in the CSE-186L CT it allows me to make about an 8 inch string loop using an AWG 8 wire and better use a larger primary transformer area of the CSE-186L to make a pickup with less leakage inductance and a higher output level.

I hope this helps?

Joseph J. Rogowski


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