Current Sensing Pickup Experiment Results

[bbsailor]

If anyone wants to do some quick and inexpensive current sensing experiments do a web search for the following low frequency current sensing transformer: CSE187L. It costs about $2.50 and less in quanties. It has a 1:500 turns ratio with the primary having a "U" shaped single turn through the core using very heavy wire with a 250 micro ohm resistance (AWG 12 about 2 inches long). The secondary has 21 ohms of resistance.

My quick experiments were done using a Tenma 72-6950 audio impedance meter which has a range of 1 to 2000 ohms in three ranges, X1, X10, X100.

The secondary impedance of the CSE187L has close to 500 ohm impedance when the low impedance input is properly loaded down. The reflected impedance is very sensitive to the primary loading and is reflected by the high turns ratio. Using 12 turns of AWG 18 on the 3/16" rod magnet resulted in about 1200 ohm reflected impedance.

I used 4 turns of AWG 18 around a 3/16" magnet and directly connected it to the transformer right next to the coil and got about 50mv output. The peak was closer to 100mv. When I connected it to a 500 ohm to 50K ohm Shure matching transformer, I got an output in the range of a typical guitar pickup. This adds another 1:10 output voltage boost. So, now with both transformers there is a 1:5000 voltage boost, less the losses in the transformers. This means that the output efficiency is very closly related to the current induced in the primary, the size of the wire and number of turns.

Here is where you could get some house hold copper wire, AWG 12 to 14 and strip off a six inch length, make one U-shaped loop around a magnet and get some measurable output on a scope and listening through a guitar/bass amp. You can add more turns to try to optimize the output but here the thickness of the wire and the connection to the transformer primary is critical. Look for some thin copper tubing the same ID as the transformer primary wire to make a quick coupling mechanism. This will allow you making quick changes while experimenting.

Measuring the impedance of four turns of AWG 18 reflects an output impeance on the transformer of about 600 ohms and increases by only 100 ohms when the manget slug in inserted in the center of the coil. This should give the experimenters some idea about how sensitive the low-Z side is to generating enough current to optimize the primary connection.

The specification sheet suggests a 60 ohm load resistor on the transformer secondary. I suspect that the optimum loading would occur when the primary reflects a 60 ohm secondary impedance. This would imply that the optimum impedance/resistance of the primary should be no more than 0.0154919 ohms (square root of 60 divided by 500) However, to obtain the maximum voltage output, a 10 X impedance loading of 600 ohms might be better. Anyone have any ideas on this?

Look at Lace Alimnitone pickups and see how thick the primary winding are. These are the actual aluminum loops around the magnet. The transformer is underneath coupled directly to the aluminum loops.

To minimize noise, run a short jumper from one side of the primary to the ground side of the secondary. I'm sure that someone could figure out how to make this a balanced output with the signal leads floating to feed a balanced input to the matching transformer like the Shure A95U pluged in directly to the amp input.

This current sensing pickup winding method would make a new breed of pickups that could be made very easy, have a wide bandwidth response, have minimal susceptibility to external noise, allow longer cable runs, and use a commonly available externally connected 500 ohm to 50K ohm matching transformer.

If anyone try this experiment, keep good notes about the wire size, number of turns around the magnet, transformer type, and total length of the primary winding around the magnets so you can post the results here so we can all benefit from the collective efforts of the pickup builder community.

Hopefully this post will get the ball rolling.

Joseph J. Rogowski

[David Schwab]

I've been meaning to try this kind of pickup out. Thanks for the step-by-step info!

[Possum]

Lace....

I know someone posted the patent for the alumitone pickups before, anyone still have it?
I found this sound sample for the strat alumitones, they sound good but you can hear the pick on the strings a little too much for my taste:
http://marksmitchell7.home.mindsprin...uckerBlues.mp3

This sounds like a fun idea to mess with. the alumitones are the only really new idea to come along in a long time, unfortunately I don't think they have much of a broad market appeal, they are very hi-fi. 2.7K

[bbsailor]

Lace Patents

For David and the other technically curious, here are a few patent numbers related to current sensing pickups.

Villen E. Khanagov has patents 5831196 and 5767431
Jeffrey Lace has patent 6897369

It should be noted that in order for the single turn primary to induce enough current to be transformed into a usable output, a good amount of current must be developed in the single primary turn. In the patent illustrations, see how large the primary winding is compared to smaller types of magnet wire typically used on guitar pickups, even low-Z pickups. In the new Lace Alumitone the primary winding is the outer metal structure of the pickup.

For those seeking to experiment with this type of pickup, it is necessary to preserve very good and conductive connections in the very, very low-Z primary side. Otherwise, low output and noise will be present.

Here are some steps to try. Obtain a copper plate at the hobby shop. Cut it the size of a single coil pickup with the open side (after doing the next step) about .5" longer. Make a long slit across the center where the 6 magnets normally go forming an elongated "U" shape. Make this wide enough not to short out against the magnets. Black tape works well as in insulator. Where the end of the plate overhangs the normal pickup footprint, drill one hole on each side of the slitted copper 0.082" in diameter spaced 0.37" apart (on center). This size and spacing is so the primary windings of the Traid CSE187L transformer can be inserted and soldered into these holes for making a quick prototype.

If you try to experiment with this design, obtain two thicknesses of copper plate and make two pickup configurations to compare the output. The thicker plate will have a higher output as it generates more current in the primary low-Z side. The output should max out at a copper plate thickness between 3/32" and 1/8" based on my early, crude experiments. I'm sure there is room for improvement.

I would be interested to hear from anyone who tries this.

Joseph J. Rogowski

[David Schwab]

This sounds like a fun idea to mess with. the alumitones are the only really new idea to come along in a long time, unfortunately I don't think they have much of a broad market appeal, they are very hi-fi. 2.7K

I think the 2.7K reading is the transformer coil. I'd imagine that could be wound a bit more to change the tone.

They do seem to be overly bright.

It is a neat idea though. All the Lace pickups are very interesting from a design standpoint, and a bit less interesting as far as tone.

[David Schwab]

For David and the other technically curious, here are a few patent numbers related to current sensing pickups.

Villen E. Khanagov has patents 5831196 and 5767431
Jeffrey Lace has patent 6897369

Oh yeah, I had those patents.

Khanagov also makes Villex pickups. They have some interesting features, like an integrated midrange control, and he has a passive tone booster setup. I hear he just came out with a passive tone control system. The tone booster never sounded like it's boosting to me... more like some kind of LC resonant filter network.

The pickups sound interesting. I imagine they are based on his current sensing design.

[bbsailor]

Art/Music Versus Science/Engineering

I agree with Dave's assessment about the Alumitone being an interesting design. I also agree with their bright sound. This is due to the fact that there is very little wire in the 2.7K coil, which also has less capacitance, changing the perecived tone by shifting the resonant frequenct much higher.

Science deals with why things work, engineering deals with how things work and in what variations they can be made to work better or different...,i.e the Alumnitone. Art deals with the subjective value put on what is being presented, and in relation to music it encompasses the perceived value of the sound quality.

Once can always add capacitance in the form of a passive tone control to shunt highs to approximately match a high-Z pickup tonal response. However, once the tone control is at maximum resistance (setting 10), the coil self capacitance, cable length, volume pot and amp input loading will influence the quality of the sound. What the Alumitone does is allow a higher range of sounds to be made as if the tone control could go to 15, without needing active circuitry to isolate the pickups from external loading or boosting the treble, adding noise.

All pickups are sound generators as well as filters due to the interaction of their physical location(s) along the string, phasing, coil shape, inductance, capacitance, resistance and the resultant resonance and coil Q.

Given that fact that humbucking pickup kits can be purchased for about $25.00 from Sterart Mcdonald, it should be easy to wind low impedance pickup coils on traditional humbucking bobbins to match the 33 ohm input impedance of the Shure A95 series of external transformers and obtain a similar effect as the Alumitone pickups. This design does allow using a balanced line to the amp with longer cables less likely to pick up noise. Whether this is desireable is based on the real or perceived need for an expanded tonal range; the environment in which music is being played; and one's history with noise and other problems.

I approach the design of a guitar from the scientific and physics aspects to define why things are made the way they are and understand the interlationship of various factors. I approach the guitar from an engineering approach when I want to change what is currently being done and at least replicate current designs if not improve them. I approach the music from my own subjective impression of the net result of my efforts. When I take a very inexpensive guitar that has potential, (like my Ibanez GAX30 mod post) work it over from a setup perspective, reduce the noise, and expand the tonal capabilities and put it in the hands of some really good players and watch them, I see whether my science and engineering has any artist value.

The value in disecting new technology, like the Alumitone, is in better understanding the tradeoffs that each new technology brings, and be able to apply it in places and at times where it might offer value to musicians. Many things are done in science because they can be done. Whether they should be done is another issue.

Trees fall in forests all the time even though no one is there to listen.

Joseph J. Rogowski

[David Schwab]

I agree with Dave's assessment about the Alumitone being an interesting design. I also agree with their bright sound. This is due to the fact that there is very little wire in the 2.7K coil, which also has less capacitance, changing the perecived tone by shifting the resonant frequenct much higher.

I wind pickups to about that resistance, but they have fairly high inductance as compared to the Alumitones. So they have the highs, and they have good lows too.

I think one issue faced as a pickup maker is the resistance to new ideas and tonalities offered by a different way of doing things. I find in general that guitar players want the same sounds they grew up hearing, or that are popular, and those sounds are mostly "vintage" in nature. It's based on old technology, and of one or two makers. Then we have the whole form factor thing... you mostly see various Gibson styles or Fender styles, as if no other pickup ever existed. Try doing something radical shaped like a Jazz Bass pickup! Not much room in there.

Bass players are generally a little more open to new technology. I think this is because bass was always in the background, and we didn't have a lot of options, no magazines about bass, and dinky under powered amps. That has all changed of course, and basses tend to be more high tech... active pickups, graphite necks, and so on.

But then the ugly head of marketing and the whole cattle mentality has reared and lately all I see are Jazz basses played through Ampeg amps. My two least favorite pieces of gear.

I started making pickups to satisfy my own desire to hear the actual tone of the basses I built, and not the sound of the pickups. So that's the direction I move in.

Trees fall in forests all the time even though no one is there to listen.

I like to answer that question by saying if no living thing is there to perceive the sound made by the falling tree, than it does not in fact make any sound.

Meanwhile....

Has observing the universe hastened its end?

[bbsailor]

Low-Z Coil Matching Technical References

Here is some reading for the technically curious. Guitar pickups are similar in operation to the "sound powered telephone". To see how the Navy used and repaired these devices, go to the following web site. http://www.crystalradio.net/soundpowered/rcamanual/

One of the best references on impedance matching was written by Ben Tongue. http://www.bentongue.com/xtalset/5hpXform/5hpXform.html

The technical issues can be best summarized by looking at the power being generated by a string in a specified magnet field as having a certain power output. Power is a function of voltage and current (meaning the load on the coil). When you make a low impedance coil you have a lower voltage, higher current pickup output that you are trying to match to a high-Z amplifier input. It is all about selecting the optimum number of pickup coil turns to transfer the available potential power in the varying magnetic field and transform the impedance across the desired frequency spectrum with minimal losses.

High output (hi-Z) pickups evolved for two primary reasons.

1. To create input overload to produce controlled harmonically rich and pleasing distortion (even harmonics).
2. Improve the signal-to-noise ratio in traditionally noisy high impedance circuits with high-Z pickups which act as noise antennas.

If you send a lower noise signal to an amplifier, you can get away with using a little more gain without adding too much noise. 600 Ohm microphones do this all the time.

Where low impedance pickups have a potential edge over high impedance pickups is in two areas.
1. Low-Z pickup coils are less affected by the capacitance of the connecting cable and loading of on-board volume control.
2. Coils windings can stay closer to the stronger magnetic field of the pole piece.

Point 2 bears some explaination. All pickups produce more output when more turns are added. As a consequence of these additional turns, more capacitance is added and the coils tends to expand farther away from the stronger flux of the magnet. Thus, the outer turns produce less output per turn than the inner turns.

Keeping the turns in a low-Z pickup coil closer to the stronger magnetic field and using the turns ratio of the transformer to effectively transform the pickup coil turns into an equivalent hi-Z pickup output, requires selecting an optimum matching of the coil to the transformer. All transformers have some losses, but these losses can be offset by the stronger concentration of the magnetic field close in to the low-Z pickup coil. Ben Tongue's article deals with effective impedance transformation in crystal radios which converts the antenna received radio signals into audio using only passive components. The discussions and issues described in this article apply also to passive guitar pickups seeking to optimize the transformation of energy.

There is a limit to how powerful the magnetic field near a string can be, as that same magnetic field tends to damp the string vibrations when placed too close to the string. One approach to minimize this effect is to make a pickup where the string passes through a ring magnet that has a coil wound on each side of the magnet like a humbucking pickup but with the string passing through the coil, the magnet and the other coil. Mount these magnets on the bridge assembly or near the bridge and you could build a pickup that operates like the sound powered telephone and minimally damps the string vibrations. The string would become a vibrating armature in a very strong, concentrated magnetic field. Get ring magnets that are 3/8" OD , 3/16" ID and build a matching bobbins that have 10 to 25 ohms of wire on each bobbin, and you will have something to talk about.

Joseph J. Rogowski