Low Impedance Pickup Research

[Joe Gwinn]

I can fit a single solid strand of AWG 4 wire into this Kemet CT-06-50 almost 6mm opening. Bending this very heavy with the CT in the center, without breaking it may require some design challenges to ensure that once you make the string loop and secure its single loop current generation connection that you have a low impedance pickup with an output impedance in the acceptable range of a low Z mic input. i suspect that you can use an AWG 4 wire and go up to a 750 turn CT or even an 1000 turn CT and still have something that may please your ear. An 8 inch piece of AWG 4 through a 1000 turn CT should produce an output impedance near 240 ohms depending on how well (low resistance connection) you make the string loop connection. Let your ear decide but understand the physics behind it. You can even use the effects of the skin effect to better balance the emphasis of lower frequencies using these known characteristics of the wire string loop size, current driven induction sources and current transformer theory.

There is no reason one cannot replace the AWG#4 single loop with multiple parallel loops of thinner wire, such that the aggregate cross-sectional area of the parallel loops equals that of AWG #4 solid wire. This will be electrically identical, but far easier to handle mechanically.

The parallel loops need not be connected to one another, but there is no harm if they do touch here and there. For instance, the wires could be soldered into a set of holes drilled in a single piece of brass. Alternately, the wire loops can be closed by soldering the ends of the loops into pieces of brass tubing. Squash the brass tubes slightly, or crimp the tubes lightly onto the wire, so the wire will stay put before and during soldering.

[Mike Sulzer]

Actually the other pickup of the guitar above - the tele neck - is such a pickup: 500 windings, 0.2 mm wire. It is not "neutral", "hifi" or however You may call it, but it emphasizes the mids - just as it did with its original high Z winding. And of course it feeds an XLR input nicely.

Why? The inductance is about 100 times lower than the hiZ pickup. The cable capacitance would need to be the same factor higher to keep the resonance the same. If the capacitance is the same, then the resonance frequency should be about 10 times higher, that is above 20 KHz. On the other hand, the Q is higher than the Hi Z pickup; perhaps the beginning of the emphasis extends down into the upper mids. It should be possible to flatten the response by restively loading the pickup. If I remember correctly, you can make a pickup with about 2000 turns flat to about 20 KHz by loading it. It should be easier with 500 turns.

[bea]

That's actually the same question as "what makes some HiZ pickups mid boomers and some with identical wiring and very similar construction not".

Despite the mid emphasis, which will of course become a lot more pronounced if i reduce the resonance frequency (unloaded it is well above 30 kHz), the pickup sounds fine, especially in wide bandwidth mode. It must be something with the material properties, notably the steel being used for the poles (it has an underlying bar magnet). It was, of course, also mid boomy in its original HiZ state, and that's retained through the rewinding :-( (which would , of course, imply that i would achieve better results if i use single magnets like the original tele neck)

[Mike Sulzer]

That's actually the same question as "what makes some HiZ pickups mid boomers and some with identical wiring and very similar construction not".

Despite the mid emphasis, which will of course become a lot more pronounced if i reduce the resonance frequency (unloaded it is well above 30 kHz), the pickup sounds fine, especially in wide bandwidth mode. It must be something with the material properties, notably the steel being used for the poles (it has an underlying bar magnet). It was, of course, also mid boomy in its original HiZ state, and that's retained through the rewinding :-( (which would , of course, imply that i would achieve better results if i use single magnets like the original tele neck)

Eddy currents in steel take out the high frequencies. So I guess it is not the best choice for a wide bandwidth pickup unless it happens to give a particular sound you are trying to get.

[bea]

Just some off topic:

Eddy currents in steel take out the high frequencies. So I guess it is not the best choice for a wide bandwidth pickup unless it happens to give a particular sound you are trying to get.

Imagine a mudbucker. A newer one with strong ferrite magnets either the Mid-Z epiphone version or the high-Z Artec. Replace the pole screws by a steel bar, 6 mm squared. An the Artec, connect the coils in parallel. That's mudbucker on steroids but despite the significant eddy currents not as dark as the original.

Back to topic: making multiple thinner loops might also be an option for the Talema CTs in order to fill the available space (19.6 mm^2)...

[bea]

Here a sound sample from the pickup. No lack of bass anymore; apparently i did not mount it correctly:



and here a quick sound sample. Microphone input of an A&H ZED10, everything linear:

https://www.ritzert.net/~beate/Stromwandler_PU_EB11.mp3

[jakez]

Here a sound sample from the pickup. No lack of bass anymore; apparently i did not mount it correctly:

[ATTACH=CONFIG]50500[/ATTACH]

and here a quick sound sample. Microphone input of an A&H ZED10, everything linear:

https://www.ritzert.net/~beate/Stromwandler_PU_EB11.mp3

Bea,

It's a very nice result.
Your post and the others before made me want to make some new measurements more precisely thanks to the new oscilloscope I purchased recently. Previously I got my measurements from the mixing board, after AD conversion and with the computers tools. This is not a real measurement of the microphone output.
So, with the oscilloscope I monitored the output of my guitar microphone loaded by a 2,2 kOhm resistance. (the microphone description is some posts before or here : http://violons.jsld.fr/index.php/ele...asse-impedance )
I plucked as equally as possible each open string and I got a peak output between 15 mV and 25 mV.
I think it's a very good level to feed a mixing board. Actually when I compare my guitar microphone and a cardioîd dynamic microphone located at 20 cm from the guitar soundhole, the guitar microphone has a stronger output. To get the same level for both I have to set +5dB more on the mixing board on the dynamic microphone input.
The oscilloscope shows also very well the huge improvement of a balanced input in reducing the noise level.

What is interesting is to see the output of each string. Low E,A,D and G strings stay within the 15mV range with the A string a little bit weaker. 25mV are reached by the two last strings B and high E.

So, it seems that the CT bandwidth is not the main cause of the unbalanced output but much more the strings (even if the CT I'm using is not the best reference !). Indeed on my guitar, E,A,G,D are bronze wound and B,E are silvered steel. And there is no significant difference between E,A,G,D strings outputs (Low E should be weaker than A in case of a CT banwidth with bass cut off).
So, whatever magnetic pickup I will use, with the same kind of strings, I will probably have to deal with this issue. To some extent a thicker wire may help, as written previously in this thread. I'm not really sure for the CT influence, I will see. This may also be solved, for now, by the tone controls or, better, with an equalizer.
For strings like the Daddario chrome you are using , I thing there is no such question.

I go on thinking that this low impedance pickup concept is great.

Thanks again to Joseph and all folks who contributed to this thread .

Jakez

[bbsailor]

Bea,

It's a very nice result.
Your post and the others before made me want to make some new measurements more precisely thanks to the new oscilloscope I purchased recently. Previously I got my measurements from the mixing board, after AD conversion and with the computers tools. This is not a real measurement of the microphone output.
So, with the oscilloscope I monitored the output of my guitar microphone loaded by a 2,2 kOhm resistance. (the microphone description is some posts before or here : http://violons.jsld.fr/index.php/ele...asse-impedance )
I plucked as equally as possible each open string and I got a peak output between 15 mV and 25 mV.
I think it's a very good level to feed a mixing board. Actually when I compare my guitar microphone and a cardioîd dynamic microphone located at 20 cm from the guitar soundhole, the guitar microphone has a stronger output. To get the same level for both I have to set +5dB more on the mixing board on the dynamic microphone input.
The oscilloscope shows also very well the huge improvement of a balanced input in reducing the noise level.

What is interesting is to see the output of each string. Low E,A,D and G strings stay within the 15mV range with the A string a little bit weaker. 25mV are reached by the two last strings B and high E.

So, it seems that the CT bandwidth is not the main cause of the unbalanced output but much more the strings (even if the CT I'm using is not the best reference !). Indeed on my guitar, E,A,G,D are bronze wound and B,E are silvered steel. And there is no significant difference between E,A,G,D strings outputs (Low E should be weaker than A in case of a CT banwidth with bass cut off).
So, whatever magnetic pickup I will use, with the same kind of strings, I will probably have to deal with this issue. To some extent a thicker wire may help, as written previously in this thread. I'm not really sure for the CT influence, I will see. This may also be solved, for now, by the tone controls or, better, with an equalizer.
For strings like the Daddario chrome you are using , I thing there is no such question.

I go on thinking that this low impedance pickup concept is great.

Thanks again to Joseph and all folks who contributed to this thread .

Jakez

Jakez, and all forum members

We have had high impedance pickups since about 1935 using the magnet types that were available then and the amount of pickup turns needed to make a high enough output to feed the amplifiers of that time. Most pickup designs have evolved over the years as magnets types and sizes have evolved and the ear has favored certain pickup sounds, noise levels and output levels.

Typical high impedance pickups were made with the requirement to make a guitar body cutout to allow the pickup the be placed below the strings. These cutouts are inline with the mass of the guitar body between the neck and bridge and reduce the continuous mass in this critical body location. Using one or two current transformer on the ends of the string loop eliminates the need to remove a large body mass under the strings and you now only need to make a space to accommodate the size of the current transformer. Since the fingerboard height typically puts the strings close to the body, a string loop and magnet height no thicker than the fingerboard does not require removal of any wood to accommodate this new style pickup.

Obtaining a balance between wound strings and solid strings can be a challenge due to magnet strength, closeness to string and magnetic sting mass. Acoustic wound strings typically have magnetic cores that are close to one half the outside diameter of the string. Lower resistance string loops tend to favor lower frequencies so you can use this to attempt to strike a balance between string sizes and frequencies. Here is a link to show the maximum frequency of various solid wire gauges. https://www.solar-electric.com/learn...pper-wire.html

Given that most XLR inputs are near 2400 ohms the target output impedance of the loaded Current Transformer with a string loop should be near 240 ohms. Here are two ways to make string loops that can be adjusted to balance wound or plain strings.

Make a 3 +3, E,A,D and G,B,E or even a 4 + 2 , E,A,D,G and B,E loop with a CT on each loop and the output impedance of each string loop being made individually with different size style wire and the output of the CTs being wired in series. To fine tune the balance, you can even put a miniature pot used as a variable resistor across the CT output and adjust the balance.

Another way is to make a single string loop with 2 CTs, one on each end of the string loop. Then, put a low resistance bridge between either the D-G strings or the G-B strings to allow each CT to be individually balanced while being put in series to feed an XLR input.

The early users of this new CT pickup technology will evolve just like the high impedance pickups have evolved. Here are some variables to consider and report on when you let your ears and measurements tell you what sounds good.

1. CT style, toroid and laminated EI frame
2. CT core permeability
3. CT turns, 500, 750, 1000, 1500, 2000, 2500, up to about 5000 turns
4. CT output impedance when loaded by a string loop
5. String loop wire type: solid or stranded, round or square
6. Single or dual string loops under the strings
7. Leakage inductance due to more fully using primary transformer turn wire area
8. Magnet style, type and strength
9. Method used to ensure a good string loop very low resistance joint
10. Working with very heavy wire to efficiently use available space

I hope those early developers of making these CT pickups post their findings and share their results to this recent evolution of guitar pickups and sound with other forum members.

Thanks

Joseph J. Rogowski

[Mike Sulzer]

Jakez, and all forum members

Using one or two current transformer on the ends of the string loop eliminates the need to remove a large body mass under the strings and you now only need to make a space to accommodate the size of the current transformer. Since the fingerboard height typically puts the strings close to the body, a string loop and magnet height no thicker than the fingerboard does not require removal of any wood to accommodate this new style pickup.

If you replace a single turn pickup with two turns in series occupying the same space, then you have twice the voltage output, four tines the the resistance and four times the inductance You can continue this to many turns, and you see that the transformer plays no essential role if you can solve the mechanical problems associated with making, for example, a 500 turn coil on the same space as a single turn coil made with a large conductor. If you can solve these mechanical problems, then you can make a pickup that requires no space for the transformer, and so less space total. You also avoid potential problems with a transformer, such as loss of bass. For example, making the transformer as small as possible might compromise bass response.

Edit: When I said to replace one turn with two in series, I mean that the total cross section of copper is the same. This is difficult to achieve in practice, but my main point is that in theory, given a certain amount of available space for the "coil", and a suitable transformer, you can use any number of turns, including the number of turns that would give 1:1 transformer, which you then would not need to use.

[Helmholtz]

....then you have twice the voltage output...

Not quite sure, if you talk about a CT PU here. Just for clarification: While primary voltage doubles with 2 primary turns, the output voltage of a CT PU will stay the same because the voltage transformation ratio decreases with the number of primary turns .

I completely agree that a "normal" PU with 500 turns should be able to produce the same (or better) results as a CT PU with one primary loop and a CT having 500 (secondary) turns.

The general issue of any transformer is the limitation of bass transfer caused by effect of the magnetizing current.

[Mike Sulzer]

Not quite sure, if you talk about a CT PU here. Just for clarification: While primary voltage doubles with 2 primary turns, the output voltage of a CT PU will stay the same because the voltage transformation ratio decreases with the number of primary turns .

I completely agree that a "normal" PU with 500 turns should be able to produce the same (or better) results as a CT PU with one primary loop and a CT having 500 (secondary) turns.

The general issue of any transformer is the limitation of bass transfer caused by effect of the magnetizing current.

I agree. I did not clearly say that when I said the voltage changes I was referring to the output of the pickup itself before any transformer. (The job of the transformer is to give the same voltage no matter how many turns the pickup has, that is, for a given fixed cross section of copper in the coil.)

[bea]

BTW: what about replacing the transformer by an active circuit like a transimpedance amplifier?
(You would obviously need to solve the possibly nontrivial problem to solder the two ends of a 10mm^2 wire to the tiny legs of an opamp...)

[bea]

BTW: what about replacing the transformer by an active circuit like a transimpedance amplifier?
(You would obviously need to solve the possibly nontrivial problem to solder the two ends of a 10mm^2 wire to the tiny legs of an opamp...)

[bbsailor]

BTW: what about replacing the transformer by an active circuit like a transimpedance amplifier?
(You would obviously need to solve the possibly nontrivial problem to solder the two ends of a 10mm^2 wire to the tiny legs of an opamp...)

Bea, and other forum members having the same thoughts,

The ability to extract a usable voltage from a high current source (the string loop of heavy wire) requires having a method to convert a very, very, very low voltage to a higher, usable voltage by an active element with an excessive amount of gain but with the consequence of picking up a lot of internal active electronics noise as well as external noise. The most efficient way to overcome this limitation is to use a passive transformer to convert a string loop induced amperage in the .01A to .05A range with a micro-ohm range of the string loop being between about 800 micro ohms to 400 micro ohms and still keep the output impedance in the range of your chosen common input, which for an XLR mic input is about 240 ohms. Just multiply the string loop resistance by the current transformer turns ratio squared and you can calculate approximate output impedance. Typically, the current transformer leakage inductance will add between 5 and 10 percent to that number.

Ribbon microphones have been around for many, many years but all of them still use a transformer directly connected to the corrugated aluminum ribbon mounted between two very strong magnets. Unlike a ferrous guitar string inducing a voltage/current in the pickup coil or the string loop, the voltage/current is being taken directly from the ribbon (to the transformer) as it moves from responding to the sound levels in the air and causing the ribbon to vibrate in the magnetic field. Typically the turns ratio of a ribbon microphone is 1:35 up to about 1:50 to keep the output impedance in the 240 ohm range to ensure a full high frequency response. Some new ribbon microphones use higher turns ratio transformers but use a sold state buffer to transition a higher output impedance (due to the higher turns ratio) to the typical XLR mic input impedance.

The reason why this is unlike a guitar pickup is that the current/voltage is being induced in the vibrating element which is aluminum and not attracted to the nearby strong magnetic field of the ribbon mic. This is unlike the ferrous guitar string moving the magnetic field to induce the voltage into a nearby coil or string loop. Guitar strings could be damped if the magnets are too strong or too close to the strings.

See this link: http://criticalrecordingstudio.com/b...06/ribbon1.gif

Also, see this link:http://www.nyu.edu/classes/bello/FMT...icrophones.pdf

Also, do a web search on "ribbon microphone theory" to relate to how this well established microphone transducer technology, using transformers, relates to my answer and your question.

I posted a topic called "Moving coil pickups for the technically curious" (based on ribbon mic theory) on this forum to show members how to use the voltage induced directly in the strings as a new type of pickup.

For those tinkering with this new idea, just alligator clip the 8 ohm side of a transformer across one guitar string and connect the high impedance side of that transformer to the amp input or mic input. Pluck the string and hand hold a magnet near that string and listen!!!

Joseph J. Rogowski

[bea]

Josef,

as soon as You have active electronics in Your instrument anyway, the idea of the transimpedance amplifier will become interesting, especially if we find a cheap traditional (low noise, low power consumption) opamp that will do the job. Which actually means that i'd have a lot t learn until i could make some prototype. (i must have a few TL071 in my box... ) The active design will be more compact than the passive design and allow for traditional shapes or pickups that could be fit to an archtop

But anyway: at least for the new future i'll concentrate on the passive version. There is a lot to do, even in prototypes (reduce skin effect, maybe also proximity effect).

Do You think, something like nail polish or some other NC laquer would suffice as an insulation for a stranded wire? I would like to use the wire of electrical installation material, e.g. 1.5 mm^2, diameter 0.67 mm.