Low Impedance Pickup Research

[bbsailor]

Joseph J. Rogowski

your advice is always valuable, and denotes your great expertise in the field, thank you very much, if I have any news in the construction of my pickups I will post them on the forum, kind regards,

Antonio Surdo

Antonio,

I have another way to minimize noise on acoustic guitar pickups. Since the strings are not grounded and the string ends rest on a wood notch they are subject to noise pickup. There is a product called a Platemate that uses a metal plate that matches the string spread on most guitars. With this the string ends now rest on metal and do not eat away the wood over time.

I ordered one of these for my acoustic guitar that I was using for one of my early CT pickup designs and found that when I soldered a wire to the plate before installing the Platemate, and connecting this wire to the thick wire string loop in my CT pickup, the noise was reduced. With this the strings and the thick wire string loop are all grounded. This is because the ungrounded strings act as antennas and pick up magnetic induced noise and transfer this noise to the guitar pickup. This idea of grounding has become an undocumented use of the Platemate. Search the web for the words -Platemate for guitar- to find a maker of it.

Give it a try. Let me know how it works out for you.

Joseph J. Rogowski

[Helmholtz]

This is because the ungrounded strings act as antennas and pick up magnetic induced noise and transfer this noise to the guitar pickup.

Strings do not pick up magnetic noise.
Generally, grounding doesn't lower magnetic noise.

Grounding the strings helps because it grounds the body of the player, which is a major source of electrical ("electrostatic") noise.
But only works as long as grounded strings (or other grounded parts) are touched.

[bbsailor]

Strings do not pick up magnetic noise.
Generally, grounding doesn't lower magnetic noise.

Grounding the strings helps because it grounds the body of the player, which is a major source of electrical ("electrostatic") noise.
But only works as long as grounded strings (or other grounded parts) are touched.

OK but grounding the strings does reduce the noise. The Platemate is good way to add a ground connection by just soldering a wire to it and connecting this wire to a ground connection on any commercial pickup housing or wire.

On my CT pickups, connecting the ground to the string loop and/or the the 2 conductor coax shield connection to the pickup. When using the Triad CSE 186L CT with the primary removed and a heavy string loop installed, connect the ground to the string loop and the outer metal band that covers the transformer laminations.

Joseph J. Rogowski

[ncblue]

Hi I have been experimenting with pickups for a few months, looking for a clear and bright tones. I am a mechanical engineer and as such know only enough electromagnetism to be dangerous. Before finding this forum I was experimenting with low wind “active” pickups. I had some old junk ceramic single coils and wound some 42 awg (55 turns and 200 turns) coils and fed the output into a Stratoblaster style preamp. The 200 turn one sounded Ok but needs a preamp with higher gain.

When I came across this forum I was intrigued and bought a couple of cheap current transformers from Ebay. They are listed as HMCT103C, but actually marked ZHT103, 1000 turn, 55DCR just 2 leads, no taps, about $1.20ea. I'm not setup for balanced/xlr, so just regular unbalanced guitar cables into guitar amps.

Some ideas I tried:
Single loop of solid 12 awg wrapped around an Alnico strat PU. Thin sound and low output. Definitely had a Strat quality to it probably due coil shape and magnetic poles.



Double loop 16 awg stranded, around neck mid PUs with humbucker wind. A little more power and a good in-between sound. Not at all like a humbucker or two pickups in series, just phasing sounds from the two string sampling positions.


I slapped together a pickup test-bed guitar from cheep spare parts which allows me to test out different ideas quickly. I've tried several configurations of loops taped directly onto the body around a ceramic bar magnet. I can get 5 loops of 18 awg stranded wire under the strings. The output is similar to a weaker strat PU. When I tried it with 2 of the CTs in parallel it sounds even better. It can produce bright jangley sounds, clear cleans, and when sent directly into an amp a pretty good OD. It does not sound good into a typical overdrive pedal. Estimated resonant peak 3kHz. That's a lot lower than I expected, but sounds good to me.


None of the humbucking arrangements I tried seem to help with noise which is mostly goes away when grounding my fingers on the strings. I'm hoping other noise goes away with shielding and a neat installation.

I found some scrap Aluminum to make an Alum-a-tone style one loop PU. The hard part was connecting the transformer. Two steel tabs are screwed to the Aluminum. Then a 12 awg copper wire thru the ZHT103 transformer was soldered to the tabs. It works great with a fairly flat response and decent output, how about that! Noise is low too. I'm surprised that the output is almost as hot as the 5 loops. Estimated resonant peak 14kHz!


Drawbacks of the pickups I've made:

• The bright tones can be a bit harsh and fatiguing. This should be tamed by adjusting the load resistance and capacitance.
• While I am getting some nice overdrive tones when using the preamp in my amps. My overdrive pedals sound bad. It's not just a gain problem. Typical OD pedal midrange hump and treble cut need to be redeveloped to suit flatter low-z PU frequency response curves and lower output. Might experiment with passive or active eq on the guitar.
• Signal to noise ratio can be poor when using higher gain amplification. Need to tidy up wiring, and explore shielding.

I plan to keep experimenting and build a better quality guitar for real world testing. I've been trying to get a measure the PU's resonant frequency rather than relying on my subjective feel/ hearing. Thanks to everyone here for the inspiration.
Nick

[bbsailor]

Hi I have been experimenting with pickups for a few months, looking for a clear and bright tones. I am a mechanical engineer and as such know only enough electromagnetism to be dangerous. Before finding this forum I was experimenting with low wind “active” pickups. I had some old junk ceramic single coils and wound some 42 awg (55 turns and 200 turns) coils and fed the output into a Stratoblaster style preamp. The 200 turn one sounded Ok but needs a preamp with higher gain.

When I came across this forum I was intrigued and bought a couple of cheap current transformers from Ebay. They are listed as HMCT103C, but actually marked ZHT103, 1000 turn, 55DCR just 2 leads, no taps, about $1.20ea. I'm not setup for balanced/xlr, so just regular unbalanced guitar cables into guitar amps.

Some ideas I tried:
Single loop of solid 12 awg wrapped around an Alnico strat PU. Thin sound and low output. Definitely had a Strat quality to it probably due coil shape and magnetic poles.



Double loop 16 awg stranded, around neck mid PUs with humbucker wind. A little more power and a good in-between sound. Not at all like a humbucker or two pickups in series, just phasing sounds from the two string sampling positions.


I slapped together a pickup test-bed guitar from cheep spare parts which allows me to test out different ideas quickly. I've tried several configurations of loops taped directly onto the body around a ceramic bar magnet. I can get 5 loops of 18 awg stranded wire under the strings. The output is similar to a weaker strat PU. When I tried it with 2 of the CTs in parallel it sounds even better. It can produce bright jangley sounds, clear cleans, and when sent directly into an amp a pretty good OD. It does not sound good into a typical overdrive pedal. Estimated resonant peak 3kHz. That's a lot lower than I expected, but sounds good to me.


None of the humbucking arrangements I tried seem to help with noise which is mostly goes away when grounding my fingers on the strings. I'm hoping other noise goes away with shielding and a neat installation.

I found some scrap Aluminum to make an Alum-a-tone style one loop PU. The hard part was connecting the transformer. Two steel tabs are screwed to the Aluminum. Then a 12 awg copper wire thru the ZHT103 transformer was soldered to the tabs. It works great with a fairly flat response and decent output, how about that! Noise is low too. I'm surprised that the output is almost as hot as the 5 loops. Estimated resonant peak 14kHz!


Drawbacks of the pickups I've made:

• The bright tones can be a bit harsh and fatiguing. This should be tamed by adjusting the load resistance and capacitance.
• While I am getting some nice overdrive tones when using the preamp in my amps. My overdrive pedals sound bad. It's not just a gain problem. Typical OD pedal midrange hump and treble cut need to be redeveloped to suit flatter low-z PU frequency response curves and lower output. Might experiment with passive or active eq on the guitar.
• Signal to noise ratio can be poor when using higher gain amplification. Need to tidy up wiring, and explore shielding.

I plan to keep experimenting and build a better quality guitar for real world testing. I've been trying to get a measure the PU's resonant frequency rather than relying on my subjective feel/ hearing. Thanks to everyone here for the inspiration.
Nick

Nick,

When tinkering with current transformer based pickup design keep these things in mind.
1. The input impedance and loading that the amp input circuit places on the pickup output impedance.
2. The output impedance of a current transformer(CT) is based on the CT turns ratio.
2a. Using about 8 inches of AWG 8 solid wire will make a pickup with about 100 to 125 ohms output impedance using a 500 turn CT.
2b. To get a little more output level try using a 750 turn CT and you will get an output impedance 1.5 X 1.5 or 2.25 time higher in the range of 225 to 281 ohms.
2c. Since low impedance XLR mic inputs are 2400 ohms you want an output impedance about one tenth of 2400 ohms.
2d. If you use a 1000 turn CT now the output impedance still using an AWG 8 solid wire is in the 400 to 500 ohm range. This is too high for a typical XLR input. However, there is a way to make the output impedance lower…use a thicker string loop wire.
3. Using a thicker AWG wire like AWG 6 or AWG 4 will make the output level higher and the output impedance to be lower. However there is one more effect that you can only measure by listening to the pickup. This is the skin effect or the highest frequency that will cause a current flow to the core of the wire. Here is where using different wire such as stranded wire minimizes this skin effect but now making a very low resistance string loop joint becomes the main challenge.

If you are able to build or modify a guitar neck, I have an idea to use each string as an individual pickup which allows electronically balancing the output rather than mechanically by adjusting magnet height. If interested let me know in your next reply to this MEF topic.

Joseph J. Rogowski

[Helmholtz]

Hi I have been experimenting with pickups for a few months, looking for a clear and bright tones.

If you want the linear reponse of a low impedance PU (without the resonance peak and early HF drop of a high impedance PU), just wind a typical Strat style PU with 500-1000 turns and connect to a microphone input. Wire gauge is insignificant.
Of course the coil/bobbin can be much flatter and the magnets can be shorter.
This avoids all the drawbacks from having to use a transformer.
There's absolutely no benefit from using a single turn PU with a transformer.

[ncblue]

I've wanted to try pickup building for some time, but have been put off due to having to wind thousands of winds of fragile wire. Even 500 turns would be a lot. The single loop or low count turns with a transformer is appealing to me as I can very easily experiment with different configurations. The transformer is nice because I wont need to have an on guitar preamp. This is fun for me and I have a lot of ideas, and as Micha points out there are lots of design possibilities. I even built a new pickup this morning from some copper tubing, kind of meh.

[bbsailor]

I've wanted to try pickup building for some time, but have been put off due to having to wind thousands of winds of fragile wire. Even 500 turns would be a lot. The single loop or low count turns with a transformer is appealing to me as I can very easily experiment with different configurations. The transformer is nice because I wont need to have an on guitar preamp. This is fun for me and I have a lot of ideas, and as Micha points out there are lots of design possibilities. I even built a new pickup this morning from some copper tubing, kind of meh.

Nick,

What is the thickest single turn string loop wire that you have used? What is the string loop wire length?

What is the most turns in any current transformer that you have used?

Do you have any noise issues when using a current transformer for a pickup?

Do you have a way to measure the output impedance of the current transformer when loaded by the low resistance string loop?

Have you connected the current transformer output to a three pin XLR mic input on a mic preamp?

When using thick solid wire to make the string loop, look up the resistance and calculate the resistance per inch of wire. A typical string loop will be about 8 inches long so you multiply the total string loop resistance times the current transformer turns squared to get a close estimate of the output impedance. The key variable is the quality of the string loop solder and or mechanical joint to make a low resistance string loop joint. Remember, a lower resistance string loop increases the current in the string loop and increases the current transformer output voltage.

Keep experimenting and post what you discover.

Joseph J. Rogowski

[ncblue]

Joseph,

-Thickest was the 12awg gauge. About 7 inches. Although the Aluminum one has a lot more material.

- I just have the 1:1000 turn CT. I've tried 2 of them on the same loop with the secondary in series or parallel. In series didn't sound much different than a single. The parallel was brighter and seemed stronger too.

- Noise issues: The Aluminum PU is pretty quiet with clean or with gain. The 5 loop one is noisy with gain. The lower output ones are just noisy due to poor signal to noise ratio.

- I can't measure impedance.

- I don't have any XLR gear. I would like try a balanced line with a mic transformer at the amp in the future.

- I have been struggling to get good solder joints with my 25 watt iron, so I must have a point of high resistance there.

Nick

[bbsailor]

Joseph,

-Thickest was the 12awg gauge. About 7 inches. Although the Aluminum one has a lot more material.

With a 1000 turn CT you should use a thicker string loop wire. To get the CT output impedance low enough to feed an XLR typical input impedance you will need to use AWG 6 solid copper wire that is 0.162 inch diameter with 0.000032925 ohms per inch resistance. A 7 inch U shaped string loop will be 0.000230475 ohms. Multiply this number by the CT turns squared or 1,000,000 and you will get 230.475 ohms, in theory. It will be a little higher due to the conductor resistance going through the CT and the total string loop resistance.

You cannot place this thick wire through the CT and bend it to form a loop as you will damage the plastic CT case. You also do not want to apply a lot of soldering heat near the CT either. Here is where a little creative design comes into play. Make the AWG 6 wire into a U shaped string loop just as wide as the CT thickness plus the thickness of two round hole wire lugs that will smugly fit the AWG 6 wire. Clean the wire before clamping the lugs to the ends of the AWG 6 wire. Find a thick wall copper tube that fits the CT opening and cut the tube just as long to match the CT thickness. Place the string loop with the lugs on just over the copper tube inside the CT. Use a matching nut and bolt to clamp the ends of the string loop to the CT and finish the low resistance loop. You can even apply some solder to the wire lugs before putting it on the CT. Again I say, a low resistance joint will contribute to keeping the output impedance of the CT low enough to drive a typical XLR mic input impedance.

- I just have the 1:1000 turn CT. I've tried 2 of them on the same loop with the secondary in series or parallel. In series didn't sound much different than a single. The parallel was brighter and seemed stronger too.

- Noise issues: The Aluminum PU is pretty quiet with clean or with gain. The 5 loop one is noisy with gain. The lower output ones are just noisy due to poor signal to noise ratio.

To keep the noise low XLR pins 2 and 3 attach to the CT. Pin 1 or ground should attach to the string loop.

- I can't measure impedance.

A good LCR meter to consider getting is the Extech 380193 LCR meter. This takes measurements at 120 Hz and 1000Hz.

- I don't have any XLR gear. I would like try a balanced line with a mic transformer at the amp in the future.

Consider getting this…iRig Pre. They have many models that accept an XLR input and provide a headphone output and other iphone or ipad connections.

- I have been struggling to get good solder joints with my 25 watt iron, so I must have a point of high resistance there.

If you get a more powerful soldering iron or gun in the 100 watt plus range consider using silver solder for a little better connection.

Nick

I hope my comments above get you going in a good creative direction. If you have an acoustic guitar a CT based pickup mounted near the neck in the round hole is a low risk way to make a good sounding low Z pickup. Use an 1/8 inch tip, ring, sleeve female connector attached to a thin plastic card mounted under the guitar strap stud that is typically mounted on the heel of the neck. Then use a male connector with 2 wire shielded cable to go to the XLR connector. Here is where using different stranded wire can offer some different sounds to compensate for the lower frequency skin effect from using thick solid wire.

Joseph J. Rogowski

[ncblue]

Joseph,
I have calculated the impedance using wire length resistance and turns squared (1,000,000), of some of my tests and get as high as 24kohm! I find impedance to be a bit confusing, first this value let's say 24kohm for my 5 turn PU, would this be at all relatable to the DCR values guitarist often quote?

Second impedance is AC, so it would vary with frequency right and what frequency would be relevant to this calc?

Trying to understand. Nick

[Helmholtz]

Joseph,
I have calculated the impedance using wire length resistance and turns squared (1,000,000), of some of my tests and get as high as 24kohm! I find impedance to be a bit confusing, first this value let's say 24kohm for my 5 turn PU, would this be at all relatable to the DCR values guitarist often quote?

Second impedance is AC, so it would vary with frequency right and what frequency would be relevant to this calc?

Trying to understand. Nick

If your transducer resistance is 24mOhm, a 1:1000 transformer will step this up to 24k. This assumes that only one of your turns is coupled to the transformer.
The result is AC resistance - not impedance. AC resistance doesn't vary with frequency if skin effect can be ignored.
Actual impedance at say 1kHz will be considerable higher because of inductance. The inductive part increases with frequency.

This will make a very large impedance PU with considerable HF roll-off with a typical guitar cable connected.

[ncblue]

This assumes that only one of your turns is coupled to the transformer.

So if I can fit 2 or 3 loops through the CT I can effectively lower the ratio to 500 or 333 and dramatically lower the impedance. Interesting.

[bbsailor]

So if I can fit 2 or 3 loops through the CT I can effectively lower the ratio to 500 or 333 and dramatically lower the impedance. Interesting.

Nick,

To effectively design a guitar pickup you need to know the input impedance of the device that you are plugging it into. Here is an example. A Fender Strat type pickup will use a volume control that is about 40 times higher than the DC resistance of the pickup. Single coil pickups typically use 250 K value pots for volume control. Humbucker pickups will typically use 500 K pots for volume because they typically have a higher resistance than single coils.

It gets a little more complicated because the volume pot value is in parallel with the amp input impedance and the capacitance of the coax cable connecting the guitar to the amp. 250K in parallel with 1 meg will look like a 200K ohm load on the pickup plus the effect of about 300 pf of coax capacitance for a 10 foot cable. This capacitance tends to shift any high frequency pickup resonance lower. This is why the ear is the best judge of pickup quality.

If you look up the Fletcher Munson curve you will see the human ear threshold of hearing with the ears most sensitive region being in the 1Khz to 3Khz region. That is why any pickup with a resonant point in this region will sound bright.

Once a pickup is said to sound good, then there is motivation to measure it as it would be typically connected to the amp load, coax capacitance and volume pot load so that you can find ways to replicate it by selecting:
1. The right gauge wire
2. The insulation type and thickness
3. Pickup coil winding style…machine wind or hand space wind… as this affects coil capacitance
4. Magnet size and type
5. Pickup bobbin height and width
6. Noise compared to signal level

Doing a current transformer base pickup now brings in these things that affect the sound.
1. The right gauge wire to obtain the desired output impedance…thicker produces more output level and a lower output impedance
2. The amount of turns on the current transformer with one string loop conductive turn going through the CT
3. The input impedance of the device that the Current Transformer is feeing should be about 10 times higher than the CT output impedance
4. The choice of solid wire or stranded wire to minimize the skin effect as it affects the pickup tone
5. Noise compared to signal level
6. The mechanics of using uninsulated thick copper wire to make the string loop and grounding the important parts.

Getting a Signal Generator, LCR meter, and an Oscilloscope will help you optimize all the variables to better adapt
the pickup design to what sounds good and provides a good physical construction.

Joseph J. Rogowski

[ncblue]

Ok guys I'm starting to get a better handle on these CT pickups. It's doesn't take many, more than two, winds to blow up the AC resistance and drag down the resonant freq. Case in point I took a ceramic strat PU and wrapped 7 winds of 16 awg stranded around the bobbin. The result was very powerful, very mid rangy. If it hadn't be extremely noisy it may have been useful. I took this as a sign to work on humbuckers. Using the sketch in Joseph's first post I made a version with 12awg and two center legs. This arrangement appears to me to be a parallel humbucker which would lower the resistance by 1/4, I calculate it to be around 528ohm. The output is a bit low with a pretty flat eq. Very quiet.


Next I reconfigured to have a series humbucker. As expected the power went up as the tone got some what less bright. Resistance about 2100ohm. Also very quiet. It's a little hard to see, but the electric tape is there since the wires cross to give a reverse wind.



I wanted to try the 2 winds through the CT idea. I made a parallel humbucker with 18 awg stranded, two loops of 2 winds. Calculated resistance 1200ohm. Good power and good highs. I changed it to one wind through the CT and the power was stronger and the highs slightly less, so this gives us another option to shape the tone.




Contrary to my early experiments, which used a single magnet, these humbucker arrangements do work very well. The balance of power to tone₁ isn't quite there yet. Looking at the Fletcher Munson curves has got me wondering where I want to get the resonant freq for my taste.
Next up I plan to build a parallel humbucker with heavy gauge wire or Aluminum. I've been designing the guitar to mount these pickups and should start cutting wood soon. You guys know of any good inexpensive magnets on Ebay or even at craft stores?


1. "Power to Tone Ratio", a registered trademark of NC Inc.