Guitar pickups with just 1.15 Ohms DC resistance!

[Joe Gwinn]

Another way to look at it is to list the core size for each string and look at the low to high resistance ranking from that perspective. I think core size determines the overall conductivity.

I think a tensioned string is dominated by the core wire diameter, but a slack string will have added conductivity from the wire turns contacting one another. I'm pretty sure I was seeing this effect, which is also sensitive to bending the string into a circular arc.

[Joe Gwinn]

I got this on my 5 string bass:

G-0.045" = 0.5 Ω

D-0.065" = 0.5 Ω

A-0.080" = 0.6 Ω

E-0.100" = 0.7 Ω

B-0.130" = 0.4 Ω

The strings are D'Addario XL Nickel Round Wounds. I testing touching the probe right behind the bridge saddle and right behind the nut. The scale length is 34".

How were resistances measured?

By the way, the string length on my beater base guitar is 32". And, I measured resistance using the 4-wire configuration.

[David Schwab]

How were resistances measured?

I did the measurements touching one probe right behind the bridge saddle and one right behind the nut using my Radio Shack digital VOM.

[Mike Sulzer]

Well, if we are going to do this the easy way, a set of Di'addario XL110 (.010-.046) measured (1-6): 2.7, 1.6, .9, 1.3, 1.1. .7 on a 25.5" scale, behind nut to bridge. Using my RS DMM. One should subtract .1 from each based on the residual from touching the probes together, and figure a statistical uncertainly of at least .1, and who knows how big the systematic error is.

I did the measurements touching one probe right behind the bridge saddle and one right behind the nut using my Radio Shack digital VOM.

[Possum]

...

David don't you have the patent for that type of system somewhere? I think you've posted it before. Its an old idea using the strings themselves as the coil. One problem I could see would be making a good connection to the strings, I don't think just hooking a lead to a tuner would be all that great especially at such low impedances. Then each string would generate a different voltage probably as well....

[pneumoman]

Curious how you isolated the strings

How did you keep the other strings from acting as parallel load? Seems like you would have a voltage divider situation.

Joe and Steve

I do not have a 4-wire, low resistance bridge but I have remeasured the strings on my Fender Tele using 0.011" diameter high E string set. My Gibson is currently apart for some modifications.

I will list the Extech raw results in comma separated values below. Please note that the leads of the Extech LCR meter reach to just inside the nut and the bridge providing a reading of about 25" of string length. Anyone else is welcome to verify these results.

String Name, R@1KHz, R@120Hz, Inductance@1KHz, Inductance @120Hz
E wound, 0.3, 0.3, 4uH, 0.009mH
A wound, 0.32, 0.32 3.7uH, 0.009mH
D wound, 0.56, 0.56, 2.3uH, 0.008mH
G plain, 0.71, 0.71, 0.1uH, 0.005mH
B plain, 1.08, 1.08, 1.3uH, 0.004mH
E plain, 0.85, 0.81, 0.7uH, 0.003mH

The results from the plain E and B strings are counter to logic but that is what I measured several times while making sure I had a good connection between the test lead clip and the string.

Measurements can vary based on the following variables.

String temperature
String metal composition
String coatings
Dirt
Rust
String age and amount of stretch since new
Test lead resistance
Clip lead spring pressure on string connection
String core diameter for wound strings

The plain strings range from 0.71 to 1.08 ohms while the wound strings range from 0.3 to 0.56 ohms.

If you wanted the maximum voltage transfer to a transformer you would want an input impedance at least 10X the string source impedance. However, each string may not produce a musically balanced output with that 10X matching so some way to adjust the individual string output level should be provided. A commonly available transformer is the Mouser Electronics 42KM019 which is 20K ohm (CT) to 4 ohm (CT). The DC resistance is Primary:1000 ohms, and Secondary: 0.5 ohms. The key to maximum voltage transfer from the string to the transformer is to limit the resistance in series with the low impedance 4 ohms input side of the transformer (used backwards with the 4 ohm side being the input). A few mV generated or induced on the string will give you over 100mv at the output of the transformer. By using the full winding or the CT winding on the 4 ohm side can provide some matching alternatives between the wound strings with lower impedance and the plain strings with a higher impedance.

One key point to the maximizing the output level is the length of string that is in the magnetic field; longer string length in a magnetic field produces more output, with wide variation in output between short (1 inch) and long (6 inch) field length. The long magnetic field should taper closer to the string as it gets closer to the bridge as the string vibration amplitude is reduced near the bridge.

I hope this stimulates some good technical discussions.

Joseph Rogowski

[bbsailor]

How did you keep the other strings from acting as parallel load? Seems like you would have a voltage divider situation.

pneumoman,

You use two strings, in pairs to make a simple test pickup with three sets of strings on a guitar or two sets on a 4-string bass. You need to make the bridge have non-metallic bridge inserts and keep the strings electrically isolated on a tailpiece. Then you make a copper foil jumper on the nut to short two strings together. The two ends of the strings behind the bridge go to a low impedance transformer 3.2 ohms up to about 8 ohms on the primary side and 50K to 200K on the secondary side.

If you want individual string control then you need to make the truss rod be the ground return and use a brass nut connected to the truss rod with a metal collar. Then the tail end of the truss rod needs to have an electrical connection that runs into the body of the guitar and becomes the common ground return point for each string going to it's own transformer primary behind the bridge and the other connection of the transformer primary going to the the common ground on the truss rod ground return. The secondary of each transformer can feed its own volume control and stay separate for MIDI or mixed for a balanced sound from each string.


Playing with magnet placement on this design allows you to hear more of the upper harmonics that normally get lost in a traditional high impedance pickup.

Try it out on an acoustic guitar to quickly see and hear how this works. A few alligator clips , miniature audio transformers and flat magnets (Radio Shack .75" X 1" with a 3/16 hole in the center available in a 5-pack) are all you need to do some quick and fun experiments.

Strings will not act as a parallel load if the bridge and tailpiece is non-conductive and each string stays electrically isolated and goes to it's own transformer.


I hope this helps

Joseph Rogowski

[StarryNight]

I'm probably missing some key knowledge here so please bare with, but why do the strings need to be paired? I gather it is because there needs to be a parallel load but I'm not sure what that means. Could each string not have it's own transformer?

Would the transformers need to be different values to compesate for the differences in string mass?

are the paired strings joined electrically at the low end of the transformer and the step up would be the output? At what point does the ground to the truss rod happen?

Do not most acoustic guitar strings use non-ferrous material for the wound strings, and would that not affect output?

[bbsailor]

I'm probably missing some key knowledge here so please bare with, but why do the strings need to be paired? I gather it is because there needs to be a parallel load but I'm not sure what that means. Could each string not have it's own transformer?

Would the transformers need to be different values to compesate for the differences in string mass?

are the paired strings joined electrically at the low end of the transformer and the step up would be the output? At what point does the ground to the truss rod happen?

Do not most acoustic guitar strings use non-ferrous material for the wound strings, and would that not affect output?

String pairs allow an easy way to quickly try it out on an acousitc guitar without any modification. Just treat the length of the two strings as a single string loop but with the strings shorted together behind the nut or on the nut. The transformer(s) can then be mounted behind the bridge to try it out on 2, 4 or 6 strings. The raw voltage generated on the guitar string is about 2mv to 5mv depending on the string gauge, and strength of the magnetic field. If you put a transformer with a 1:50 turns ratio then you will have an output in the 100mv to about 250mv output. Try different transformers with a 3.2 ohm to about 8 ohms primary with as high of a secondary impedance as you can find (20K to 200K). Try single strings and string pairs to see, hear and measure the optimum output. This is the fun part where the real learning occurs.

The inner core of most bronze strings are magnetic, that is why bronze strings still work on magnetic pickups but the B-string tends to dominate the balance because it is thicker and higher in frequency. If you choose to use a common ground return, you will be able to use six independent transformers possibly with different turns ratios to better match string outputs. You could also obtain a toroid current transformer (CT) with 1000 to 2000 turns, then loop a few turns from the string end and the common ground return through the CT, varing the number of turns to balance the output. Prem Magnetics sells the SPCT-251 current transformer (E-I Frame) with 2000 turns and a square primary opening of 0.162" allowing six primary wires to attach to each string plus common ground for mixing all strings in one CT.

Having ferrous or magnetic strings is not necessary as the string is acting as a moving conductor in a magnetic field. Ribbon microphones have very light-weight alumium ribbons suspended between a strong magnetic field and still generate voltage. Ferrous strings are only necessary on magnetic pickups with magnets and coils. Moving strings can be any metal that is conductive and moving near a magnetic field. The impedance of a string is so low that the optimum matching to the low impedance primary loop requires experimentation to optimize the match to the individual string frequency, and harmonic range. Start out with a primary winding having the same resistance as the string for an optimum power transfer. However, most modern input devices have between 5 and 10 times higher impedance than the source impedance to make a "bridging impedance" to preserve the optimum voltage level. With this moving coil pickup design you are working in an area between current and voltage transformers. Try out my tips and post your results.

Joseph Rogowski

[ken]

How much 'skin effect' would the outer wrap of a wound string actually have compared to a plain string? In other words, if you used a plain string the same thickness of wire as a wound string even for the low strings, what would be the actual perfomance differences compared to using a 'normal' wound string set here? For example, you have a .100 plain string and a .100 wound string. What would be the performance differences be between the two when used in this circuit?

I thought only the center core of the string would be used here.

ken

[bbsailor]

How much 'skin effect' would the outer wrap of a wound string actually have compared to a plain string? In other words, if you used a plain string the same thickness of wire as a wound string even for the low strings, what would be the actual perfomance differences compared to using a 'normal' wound string set here? For example, you have a .100 plain string and a .100 wound string. What would be the performance differences be between the two when used in this circuit?

I thought only the center core of the string would be used here.

ken

ken,

There are two issues here.

The first issue is acoustic. Once the string diameter gets much thicker than .018" in diameter they are wound to allow the strings to vibrate better and maintain enough mass with enough flexibility to play in tune with thinner strings on instruments where the frets are at right angles to the strings. Even on pianos, thicker strings become wound with one or more wraped layers to cover the full frequency spectrum of the piano.

The second electrical issue is related to two sub-issues (1) frequency and (2) string ferrous mass.

Typically pickups are more efficient (generate more voltage) at higher frequencies. Strings with a ferrous outer wind appear to have more ferrous mass than a bronze wound string with only a ferrous core. Therefore, the higher ferrous mass of the wound strings helps produce a more balanced output with the plain non-wound strings. Try using a magnetic pickup on an acoustic guitar with bronge wound strings. The magnetic field under the B and E strings is noticably weaker. Run a small screwdriver over the top of the pickup and you can feel the stronger and weaker areas of the magnetic field in an attempt to produce a more balanced output.

Eddy currents tend to rise with frequency but would only tend to increase the resistance of the thinner plain strings very slightly (at these higher frequencies) which are already more efficient than the thinner ferrous cores of the lower strings. I do not think it is practical to use a solid or plain string .100" thick as it would be very stiff and not very musical sounding. Some bass guitars use low E-strings .104" diameter but they are wound ferrous strings.

Eddy currents do come into play with Alumitone-like pickups with a thick, low impedance string loop where the upper frequencies and harmonics would tend to present a slightly higher impedance to the transformer action and thus produce less higher harmonics in the final output of the transformer secondary. I see the evolution of the Lace Alumitone pickups or similar designs to be made with a laminated frame of thin copper/aluminum sheets that are insulated from each other to minimize eddy currents. A large bundle of Litz wire could be used as the low impedance string loop to achieve the same effect using current transformers.

Joseph Rogowski