Low Impedance Pickup Research

[Guitar fool]

Sorry for the late reply. Thanks for the positive comments. I am looking forward to trying out your RITE system. It may be a while before I get a chance as I am building a garage/workshop.

Hans, here is some photos of one that I have put together as a mock up with some old broken transformers so you can see what it looks like. Once the loop is soldered together I then use a belt sander to take off the excess solder and copper to leave the top flat.

[David Schwab]

Paul,

It just knocks me out seeing the low-Z goodness in a standard humbucker sized package. The cutout and detail work on the loops is unique, artistic, and mondo cool. Great job engineering and designing these.

-Charlie

Don't forget that you can wind a standard humbucker with low-z coils. I made bass pickups like that for years!

[charrich56]

David,

I have indeed been rewinding "normal" humbucker bobbins with #32 to #36 AWG and have made several low-Z humbuckers, Strat-sized single coils, and P-90's. Low as in less than 200 ohms impedance at 1Khz. That works great for a mic level input to most mixers, or with an onboard preamp. I think it's an excellent way to go for getting a really high fidelity signal out of the strings and into whatever signal chain you want.

I've been following this thread with Joseph's experiments and advice and really like that Paul has done the "single loop" ultra low-Z design with current transformers in that size/form factor.

You have been a good contributor to the low-Z posts on the forum and I really dig the low-Z hi-fi bass playing clips you did.

-Charlie

[David Schwab]

I've also done it with regular 42 gauge wire. That's the way Bill Lawrence did the Les Paul Signature pickups too. You get a different tone than from using heavier wire.

I used a JFET preamp, but you could also use a transformer.

[charrich56]

I've also done it with regular 42 gauge wire. That's the way Bill Lawrence did the Les Paul Signature pickups too. You get a different tone than from using heavier wire.

I used a JFET preamp, but you could also use a transformer.

The ones I built, both with standard humbucker and single coil parts were about 40-50 ohms DCR and about 35 Millihenry. For the humbucker, I targeted 100 ohms/bobbin and 70 mHenry, and put the coils in parallel.

When you get that low on output, you need about 28-32 dB gain. JFET preamps work well if you bootstrap the gain up. A transformer followed by a single stage simple JFET preamp works really well. ( but you have to make sure the transformer itself doesn't pick up hum.) I can post schematics for any of these if someone is interested in experimenting.

I like to use the larger wire partially because it's easier to wind (for me) and doesn't stretch as easily, and it makes the resistive part of the impedance lower for better matching and step-up ratio with the transformer, or bridging to a medium impedance input like a mic preamp channel on a mixer. But the 42 AWG in general should be just fine into a medium or high Z input. Will have to do the experiment for tone differences.

The Xicon miniature audio transformers from Mouser can do a good job for guitar and fit in a control cavity without much of a problem. Look at the TM series, for example the 42TM117-RC. The bass response drops off below 80 Hz or so, so one would have to go to bigger iron for this approach for bass. You can get about 17 dB voltage step-up, and then about 12 dB or so more gain from the JFET preamp, with very low battery drain and low noise.

[ptsefton]

Hi, I'm new here.

Very interesting and inspiring stuff.

I remember reading somewhere in one of these threads "make sure you have phantom power turned off". I'm keen try building one of these things to try out on a banjo and a tenor guitar but I do usually have phantom power turned on - what would happen if I had 48v going through the output side of a CSE187L?

[bbsailor]

Hi, I'm new here.

Very interesting and inspiring stuff.

I remember reading somewhere in one of these threads "make sure you have phantom power turned off". I'm keen try building one of these things to try out on a banjo and a tenor guitar but I do usually have phantom power turned on - what would happen if I had 48v going through the output side of a CSE187L?

Welcome aboard!

Read this link to see details about the potential damage. Pay attention to reference 5. https://en.wikipedia.org/wiki/Phantom_power.

If you look how power is supplied to an XLR microphone you will see that the positive voltage goes to both pins 2 and 3 with the same voltage. That means that there is no voltage added to the AC mic signal output of the current transformer if it is wired correctly. The output of the CSE187L or any current transformer (CT) should go to pins 2 and 3 of the XLR connector. Therefore, no current will flow between these pins. Pin 1 is the ground connector to ground the metal frame of the current transformer and the string loop to minimize noise. Current would only flow between pin 1 and either pins 2 or 3 but if you measure the resistance between these pins it should read open. It is a good quality control check to ensure that there is no continuity between either pin 1 to pin 3 or pin 1 to pin 2 to make sure that any phantom voltage would not affect the transformer. It is always a good thing to turn off phantom power so you can to prevent any mishaps if one of the wires in the XLR connector becomes shorted or undone.

Some of my latest research shows that I can increase the output by using a thicker primary string loop wire. Typically with a CSE187L using the AWG 12 pre-installed CT primary winding connected to a 6 inch long string loop of AWG 11 wire at 105 micro ohms per inch adds 105 X 6 or 630 micro ohms. Since the 500 turn CT equals an impedance ratio of 500 squared or 250,000 or one quarter of a million. Then just divide 630 micro ohms by 4 to see how much the string loop adds to the output impedance. 630/4 equals 157.5 ohms. Now all we need to do is add that to the impedance of the CSE187L shorted out at 80 ohms. So the output impedance of this setup will be 80 plus 157.5 or 237.5 ohms just shy of the upper limit of feeding a typical mic input bridging impedance (typically 2400 ohms actual input impedance) for mics rated at 150 ohms that can be anywhere from 100 ohms to 250 actual impedance measured in ohms.

If you obtain a CSE186L which is a 1 to 166 turns ratio CT and remove the 3 turn primary, you can install a single U-shaped turn of AWG 8 wire and only need to make one connection joint to form a full string loop. To remove the AWG 16 installed primary do this. The secondary still has 500 turns on it. Use copper tubing to make the joint in the string loop to ensure a good low resistance joint which controls the maximum current that can be developed in the string loop by the vibrating string in a magnetic field.

1. Remove the yellow tape using a pick and/or a needle nose pliers.
2. Cut a rectangular hole 7/16 inch by 9/16 inch about 1/8 inch deep in a piece of wood to hold the transformer flat to put in a vice to press the AWG 16 primary wires out from the glued in state.
3. Once the primary is free of the glue, use a pliers to push the wires out to fully remove from the CT.

Now you will have square openings to allow a single loop of AWG 8 at 52 micro ohms per inch to be placed into the CT. Use electrical tape on the outside walls of the transformer laminate to keep the string loop wire from shorting out to the metal frame. A 7.5 inch long total string loop including the part that goes through the CT will make a string loop of 7.5 X 52 or 390 micro-ohms. Add about 10 ohms for leakage inductance and the output impedance for this setup should be very near 100 ohms. The output will be about twice as high (10 mV p-p) as the setup using the AWG 12 primary (5mV p-p).

Run the output of this CT to pins 2 and 3 of the XLR connector and clean the glue from the metal transformer frame to ground the frame and the string loop to the mic cable shield and connect to pin 1 of the XLR connector.

Do a loud strum, just make sure that you set the mic input level to not clip showing the red flashing LED.

Now you can enjoy your new pickup without worry.

Joseph J. Rogowski

[ptsefton]

Welcome aboard!

Read this link to see details about the potential damage. Pay attention to reference 5. https://en.wikipedia.org/wiki/Phantom_power.

If you look how power is supplied to an XLR microphone you will see that the positive voltage goes to both pins 2 and 3 with the same voltage. That means that there is no voltage added to the AC mic signal output of the current transformer if it is wired correctly. The output of the CSE187L or any current transformer (CT) should go to pins 2 and 3 of the XLR connector. Therefore, no current will flow between these pins. Pin 1 is the ground connector to ground the metal frame of the current transformer and the string loop to minimize noise. Current would only flow between pin 1 and either pins 2 or 3 but if you measure the resistance between these pins it should read open. It is a good quality control check to ensure that there is no continuity between either pin 1 to pin 3 or pin 1 to pin 2 to make sure that any phantom voltage would not affect the transformer. It is always a good thing to turn off phantom power so you can to prevent any mishaps if one of the wires in the XLR connector becomes shorted or undone.

Some of my latest research shows that I can increase the output by using a thicker primary string loop wire. Typically with a CSE187L using the AWG 12 pre-installed CT primary winding connected to a 6 inch long string loop of AWG 11 wire at 105 micro ohms per inch adds 105 X 6 or 630 micro ohms. Since the 500 turn CT equals an impedance ratio of 500 squared or 250,000 or one quarter of a million. Then just divide 630 micro ohms by 4 to see how much the string loop adds to the output impedance. 630/4 equals 157.5 ohms. Now all we need to do is add that to the impedance of the CSE187L shorted out at 80 ohms. So the output impedance of this setup will be 80 plus 157.5 or 237.5 ohms just shy of the upper limit of feeding a typical mic input bridging impedance (typically 2400 ohms actual input impedance) for mics rated at 150 ohms that can be anywhere from 100 ohms to 250 actual impedance measured in ohms.

If you obtain a CSE186L which is a 1 to 166 turns ratio CT and remove the 3 turn primary, you can install a single U-shaped turn of AWG 8 wire and only need to make one connection joint to form a full string loop. To remove the AWG 16 installed primary do this. The secondary still has 500 turns on it. Use copper tubing to make the joint in the string loop to ensure a good low resistance joint which controls the maximum current that can be developed in the string loop by the vibrating string in a magnetic field.

1. Remove the yellow tape using a pick and/or a needle nose pliers.
2. Cut a rectangular hole 7/16 inch by 9/16 inch about 1/8 inch deep in a piece of wood to hold the transformer flat to put in a vice to press the AWG 16 primary wires out from the glued in state.
3. Once the primary is free of the glue, use a pliers to push the wires out to fully remove from the CT.

Now you will have square openings to allow a single loop of AWG 8 at 52 micro ohms per inch to be placed into the CT. Use electrical tape on the outside walls of the transformer laminate to keep the string loop wire from shorting out to the metal frame. A 7.5 inch long total string loop including the part that goes through the CT will make a string loop of 7.5 X 52 or 390 micro-ohms. Add about 10 ohms for leakage inductance and the output impedance for this setup should be very near 100 ohms. The output will be about twice as high (10 mV p-p) as the setup using the AWG 12 primary (5mV p-p).

Run the output of this CT to pins 2 and 3 of the XLR connector and clean the glue from the metal transformer frame to ground the frame and the string loop to the mic cable shield and connect to pin 1 of the XLR connector.

Do a loud strum, just make sure that you set the mic input level to not clip showing the red flashing LED.

Now you can enjoy your new pickup without worry.

Joseph J. Rogowski

Thanks for all that Joseph, sounds great. I'm waiting on some parts - I'll let you know how I go.

Peter

[surdopickups]

I ask for help.
Do you come up with a volume and tone circuit on a pickup of a very low impedance?
Which enhancement values and what capacitor do you use?
400 ohm impedance pickups
Thank you all

[bbsailor]

I ask for help.
Do you come up with a volume and tone circuit on a pickup of a very low impedance?
Which enhancement values and what capacitor do you use?
400 ohm impedance pickups
Thank you all

If your pickup is 400 ohms, I assume that you have a pickup with about 400 to 500 turns. Please state the actual number of turns.

The rule of thumb is that the impedance ratio is equal to the square of the turns ratio difference. Lets say that a single coil pickup pot is 250K ohms with about 6000 turns. Then, a low impedance coil with about one tenth the number of turns will use a pot 10 squared or 100 times lower in value or about 2.5K ohms. See this web link for a schematic. Les Paul Personal 2

I hope this helps?

Joseph J. Rogowski

[surdopickups]

hello

Joseph J. Rogowski

number of turns 460
awg 41


Thank you for the reply

[bbsailor]

hello

Joseph J. Rogowski

number of turns 460
awg 41


Thank you for the reply

surdopickups,

Here is a way to confirm that the pot load is not affecting the pickup sound. Obtain a few metal film resistors of values common to pot values such as 2.5K, 5K, 10K, 15K, 20K, 25K, 50K and use alligator clips to put each resistor across the pickup output while listening to the sound. When a resistor/pot value affects the high frequency tonal balance, go to the next higher value where you do not hear any upper frequency change. Another rule of thumb is that the pot value should be between 35 to 40 times higher than the pickup DC resistance to minimize affecting the high frequency tonal balance. For pickups with a lot of metal in the core you can go up to 50 times the pickup DC resistance. Let your ears be the final judge.

Joseph J. Rogowski

[surdopickups]

Mille grazie Joseph,
I vostri consigli sono molto utili.
Grazie

surdopickups,

Here is a way to confirm that the pot load is not affecting the pickup sound. Obtain a few metal film resistors of values common to pot values such as 2.5K, 5K, 10K, 15K, 20K, 25K, 50K and use alligator clips to put each resistor across the pickup output while listening to the sound. When a resistor/pot value affects the high frequency tonal balance, go to the next higher value where you do not hear any upper frequency change. Another rule of thumb is that the pot value should be between 35 to 40 times higher than the pickup DC resistance to minimize affecting the high frequency tonal balance. For pickups with a lot of metal in the core you can go up to 50 times the pickup DC resistance. Let your ears be the final judge.

Joseph J. Rogowski

[surdopickups]

4 coil cm 3x2.2 x 0.3 height
Neodimio magnete 2x1x0.2
AWG 40
Turner 430
100 ohm for coil
Totale 400 ohms
Xrl outuput

[J M Fahey]

What instrument s that?
Closely cropped picture does not give an idea of scale.

In any case, do NOT add volume or tone controls, just wire them to the XLR connector, send proper signal to the mixing desk, and use *those* controls.

Electric guitar controls are *crude* and used basically for historical reasons, with old style specs amps, are very limited and cut only , to boot most guitar pickups are very Hot so "just attenuating" is acceptable in that context, your pickups are very weak to begin with, any attenuation will worsen signal to noise ratio.
Electric guitar players of course use them for other purposes, such as volume swells, going from distorted to clean, from full range to wooly/dark, etc. , but I guess that does not apply much to a bowed Classical Instrument.

Same reason vocal microphones do not include tone controls and only an insignificant quantity include simple volume controls.

Only useful add on to yor low output passive pickups would be a mute switch, so Musician can kill his instrument output when/if needed (tuning/during long pauses, etc.)
Same as with voice microphones.