Ultra Low Impedance Pickups

[bbsailor]

Hello,

This is my first post on this forum.

I want to share an idea that I developed back in the 1970s when I was teaching Audio and TV Production in a NJ community college.

I hope this stimulates some experimentation as it can be tested very easily to better understand the full potential of this new approach to pickups.

When I taught my audio module about microphones, I covered the ribbon microphone and took one apart to show the students how the thin ribbon suspended between in a strong magnetic field actually transduces sound vibrations into electrical energy (which is the definition of a transducer...changing one for of energy into another). The ends of the ribbon are connected to the low impedance side of a step-up transformer. The higher impedance side of the transformer goes to the input of the amplifier circuit. After the class was over I got to thinking that a guitar/bass string could take the place of the ribbon and maybe this could be the basis of a new type of metal string pickup.

I took the ribbon microphone transformer home and using alligator clips, attached one end of the transformer to the string behind the nut and the other end to the string behind the bridge. Try this on an acoustic guitar first as the metal strings sitting on a metal bridge could short out or greatly reduce the result of this experiment.

When I attached the high impedance side of the transformer to my amp, I brought a large hand-held cermic magnet near the string after I plucked it and guess what... sound from that one string through the amp. Wow, I thought.. I am on to something.

Then, a little patent searching in the Philagelphia library (before the Internet) revealed that S. Vasilach filed patent number 2,293,373 in 1939 and was later isssued the patent in 1942. For those looking to educate themselves, go to patent number 7,105,731 issued in 2006 for a more modern refinement with a wealth of good references about low impedance pickups.

This discovery did not stop me from futher investigations and I ultimately converted an acoustic guitar to having each string independently produce its own fully isolated (from other strings) outputs. These outputs went through a 7-pin connector, one common ground and six hot outputs that fed a 6-channel mixer. Now I could adjust the level, EQ and L-R panning of each string.

The six strings conncted to the low impedance transformer acts like an antenna to pick up hum when ever I got near an AC transformer. To test this I used a Weller soldering gun. As I moved the gun near the strings while turned on I could hear the hum increasing. Then I thought about humbucking pickups and played with reversing the phase of each alternate string. Since each string vibrated independently and has a separate output there is no phase cancellation of the induced string pickup but there is a good reduction in picking up outside hum. Another soldering gun test confirmed this.

Here is how I modified the guitar to make this pickup work.

1. Removed the fretboard and plaved a .5" wide copper strip underthe fingerboard, jut over the truss rod, with the end going into the body of the guitar near the heel. This acts a the common ground return.

2. Removed the plastic nut and replaces it with a brass nut. I soldered the copper strip to the brass nut just beyond the fingerboard.

3. Inserted six copper rivits the the wood tail piece to thread the strings through. I soldered a wire to each copper rivit. This made the hot connection for the string.

4. Obtained six miniature output transformers 8 ohms to 10K ohms. The key is to have a good step up ratio. The turns ratio is the square root of the primary divided into the square root of the secondary or 2.82 dived into 100 or a step up ratio of about 34.

5. Mounted a ceramic magnet on a wood sound hole mount tht looks like an acoustig guitar pickup but it is just a magnet.

6. Mounted the six transformers in a copper case to the neel of the neck and ran the output of each transformer to a 7-pin connector.

You can try just about any audio step up transformer. Try a Radio Shack 70V line speaker matching transformer. These work very well with the 8 ohm side going across the string and the high impedance side going into the amp.

If you find an audio transformer with multiple low impedance windings, you can use each winding for a separate string. You can even use two strings for one winding; one transformer lead going up the string, make a short to the next string, and one tranformer lead to the adjacent string.

Moving the magnets between the bridge and the neck changes the tone. With an amp up loud, moving the magnets causes a static-like sound due by the displacement of electrons in the metal string.

This ultra low impedance pickup works well with all metal strings, guitar or bass. It should stimulate some interesting results among the pickup builders on this forum.

Joe Rogowski

[Mark Hammer]

As first posts go, it's a honey of a post! Thanks!!

Much to sink one's teeth into here. Personally, I'll need a number of read-throughs, and will need to look up the patent dopcuments noted, before having anything more to say.

Just out of curiosity, what's it sound like?

[bbsailor]

Ultra Low Impedance Pickup Sound Quality

Mark,

The sound quality of the Ultra Low Impedance (ULI) pickup sounds more like an acoustic sound rather than the classic electric guitar sound with the characteristic resonant hump in the 2kHz to 5kHz range. Bass guitar designers may also find this ULI pickup approach interesting and worth a little experimentation.

Traditionally, guitar pickups have been high impedance to match the input characteristics of the available amplifiers. When you sweep a signal generator through a stimulation coil mounted near a guitar pickup and look at the output on an oscilloscope or high input impedance voltmeter you will see the classic resonant hump at the point where the impedance of the pickup is the highest. This is the result of the pickup inductance and the capacitance between the coil windings forming a resonant circuit. Optimum loading dictates that the load impedance should be 10 times the higest impedance of the source device (the pickup). That is why volume pots used in single coils are 250K and humbuckers are 500k to follow this design practice. The length of the guitar cable adds about 20 to 30pf per foot and tends to have the greatest audio effect by lowering the self resonant point on higher impedance pickups, which have more windings. All of these factors contribute to the classic guitar pickup sound that we have become accoustomed to hear.

Add one more factor, the Fletcher-Munson Curve, which shows that the ear is most sensitive to frequencies in the range between 1kHz and 5kHz and it is easy to see why guitarists have beome very attuned to their pickups and their sound. The pickup resonant hump is in the same range as the ear's most sensitive tonal range.

The ULI pickup uses the string as a single turn loop, functionally equivalent to the ribbon in a ribbon microphone. It feeds the transformer which converts the ultra low impedance of the string in about the 1 to 3 ohm range to a more usable higher impedance with a higher output than just the induced voltage on the string. Since each guitar string only represent a potential two-octave range, the impedance of any string can be optimized to match a transformer connection with the resultant output being rather flat into the higher end of the audio spectrum without the classic electric guitar pickup resonant hump.

Another trick you can do using this technique is to feed the output of a signal generator into a 70V speaker transformer (high impedance side) and connect the low impedance side across the string. Place a magnet near the string and sweep the signal generator to the primary string frequency and it will start vibrating. You can also listen to the various harmonics of the string and plot the harmonic signature of each string and string type (various metal compositions). This works best on an acoustic guitar where you can amplify the under-bridge non-magnetic pickup and plot the output more accurately using a meter or oscilloscope rather than just listening.

Any college students reading this might want to try this as an interesting research project and post the results on this forum.

[David Schwab]

Nice post, thanks for bring this subject up. I've always wanted to try this idea out, but never got around to it.

Martin Clevinger has two patents on this very idea, 4408513 granted in 1983 and 4499809 granted in 1985. Clevinger makes electric upright basses, but I don't think he's ever used this pickup system on any of them.

Roland's original guitar synth, the one that looked like a Les Paul, had a sustainer that worked this way, but in reverse. They had a large ceramic magnet under the strings where the neck pickup would be. They had nylon saddles on the bridge, and each string was wired up at the tailpiece and nut. They treated each string as a speaker coil, and ran the guitar's signal into that string. I read at the time that a string was about 8 Ohms. Guitar players inadvertently disabled the system by replacing the nylon saddles with metal ones, thinking they were improving the tone of the guitar.

The Lace Alumatone and Transsensor pickups work in a similar way to a ribbon mic. They used the induced current in the aluminum or copper pickup structure, which forms one loop, and then feeds into a step up transformer, which is also part of the pickup.

I make low impedance bass pickups, but with standard magnet/coil structures. You are right that guitar players have become accustomed to the sound of high impedance pickups and passive volume tone circuits. Bass players are more open to the more hi-fi tone of low Z pickups.

[bbsailor]

David,

Try this on a 4 string bass. It requires no modifications just a few alligator clips and one dual primary low-Z transformer or two transformers.

Just clip one lead of each transformer primary low-Z coil to two adjacent strings. Get a small piece of copper foil to lay in the nut to short the two adjacent strings together. You could also place a jumper on the adjacent string machine head mounting screws to short two strings together. If the transformer has two primaties, all four strings will be mixed in the transformer.
If you use two transformers, just wire them in series. This works best with transformers with turns ratios 10 or higher; the higher the better for more output. Look in your junk box for some transformers.

I showed this to Stewart Spector when he had his shop in a loft in Brooklyn NY many years ago.

I suggested that his neck-through design could use a molded hollow lower piece made of a resin material that cold be screwed on to the center neck block to gain access to the electronics. I showed him how to use the ground rod as a common ground return that is fed into to the main body and the electronics compartment in the resin shell. Then, each string could have a small trim pot to adjust the level of each string independently.

The magnets do not need to be mounted in a fixed position like pickups but could be put on a slider assembly to adjust the tone by moving one, two or more activation magnets along the string path between the bridge and the neck. As the magnets are placed closer to the bridge, they need to be mounted closer to the string as the srring moves less and the magnets need to be closer to the string to hear the effect of that rear magnet on the higher overtones.

You can try to use ring magnets with the bass string actually going through the magnet to have a very good effect. The ring magnets can be built right on to the bass bridge assembly.

I found the link below for making a guitar bridge like I described above.
http://www.kjmagnetics.com/proddetail.asp?prod=R834DIA You will need to look for a magnet that is .5" OD or larger with a larger center opening to accomodate a bass string. It will work! Check out this web site for magnets that will work on a bass.

You can try this experiment with a transforme, some alligator clips, and a hand held magnet in a few mintutes to see the effect for yourself. It is an eye opener...or should I say, ear opener.

Joe Rogowski

[David Schwab]

Try this on a 4 string bass. It requires no modifications just a few alligator clips and one dual primary low-Z transformer or two transformers.

Is that an 8 Ohm to 10K transformer?

I found the link below for making a guitar bridge like I described above.
http://www.kjmagnetics.com/proddetail.asp?prod=R834DIA You will need to look for a magnet that is .5" OD or larger with a larger center opening to accomodate a bass string. It will work! Check out this web site for magnets that will work on a bass.

That's where I get my neo magnets. Cool! I have to give this a try when I get a chance.

Thanks for the info.

[bbsailor]

David,

You can try an 8 ohm to 10K and it will have a 1:35 turns ratio. This is calculated by dividing the square root of the primary into the square root of the secondary. You can also parallel two transformer primaries and put the secondaries in series. This will in effect double the turns ratio, in this case a 1:70 turns ratio. Three transformers will give you over 1:100 turns ratio. The Clevenger patent 4408513 shows this technique using two transformers to provide a more optimum match.

What I did to optimize the match to the impedance of the strings is to keep adding a parallel transformers and linking the secondaries in series until adding the next transformer caused the output to become lower. This method requires the least amount of test equipment and complex calculations. I found that using between 3 and 4 transformers on the different guitar strings tended to produce optimum results with my transformers.

You will get the best results by keeping the leads to the transformers short and thicker than the transformer primary wire size. I have a special lab transformer that has both a 1:100 and a 1:1000 turns ratios. The 1:100 ratio set works better. http://www.signalrecovery.com/1900page.htm

If you use one primary winding to attach to two strings in series (up one string, jump across to the next string and back to the other transformer lead) you can only adjust the level of a pair of strings. To have individual string control, you need a common ground return from the nut, like the truss rod. Then you need 4 transformers, one for each string. Once you get the signal to a good working level with a 1:10 or 1:12 truns ratio transformer, you can use a quad op amp to make a mixer with a gain of about 10 each and individual string volume controls to provide a good operating level for the amp and isolate the transformers from coax loading to preserve the high frequency response.

There are many common transformers that are 8 ohms CT to 10K or even 15K. If you get one of these try using the CT or the 4 ohm setting for a better turns ratio. The one thing that is critical at bass frequencies is the inductive coupling at 40Hz. Make sure that your bass transformers have a good size core. A good source of these is to use 70V line matching transformers used in commercial speaker systems. Use the 4 ohm tap and the higest Z secondary tap. Check on line for the transformer specifications. These can be had for a few dollars each. Radio Shack even stocks them. These will work in a pinch to quickly get you started. Just get two of them and parallel the primaries and series the secondaries.

A good test is to feed a signal generator into the low Z side through a 100 ohm isolation resistor and then load the hi Z side with a resistor 10X the hi-Z impedance. Sweep the signal generator while keeping the input level to the transformer at a constant level in the 0.1V range. Then see what happens when you sweep down to 40Hz while monitoring the hi Z side on a scope. This will ensure that the transformers are not cutting off due to poor low frequency response.

I hope this helps?

Joe Rogowski

[Ds1]

Hey did anyone end up trying this on a bass? I'd love to do this on my 6er I'm building. Also, where and how exactly are the transformers wired in?

Thanks

[bbsailor]

DS1

Before you finish the neck do this.

Make a truss rod collar to electrically connect a brass nut to the truss rod behind or under the nut.

Make the end of the trus rod have a connection in the heel of the neck to route a heavy ground return wire to the bridge area or control compartment area. This can be abandoned if you later want to use traditional pickups.

Use a bridge that electrically isolates the bass strings from each other. An easy way to do this is to use an open copper rivet that each string passes through with a wire attached soldered to it. The transformer is placed across the string using the truss rod as a covenient ground return to connect inside the bass body.

Connect one end of the copper rivet wire and the truss rod ground retrn to the low impedance side of the step up audio transformer. You can play with some different transformers by simply clipping one low impedance transformer connection behind the nut and the other low impedance connection behind the bridge. Connect the high impedance side of the transformer to your amp directly. Bring a hand held magnet near the string and plun it. You will then will hear the sound of that string amplified.

You will need one transformer per string or find a transformer with multiple low impedance windings and mix all the strings in the transformer.

With separate transformers for each string you can adjust the level of each string electronically rather than mechanically.

You will need to place the magnets under the strings to allow current to be induced in the strings when vibrating in the magnetic field. Here you can play with various ceramic magnets, alnico or other higher power magnets to get a usable output and pleasing tone. You can use a single or multiple set of magnets to emphasize various harmonics. Magnets placed nearer the bridge can be closer to the strings without causing too much string damping.

If you use transformers with a 1 to 20 or 1 to 40 turns ratio you can use a small quad op amp to make a mixer with a volume control for each string boosting the gain of the transformers by about 20X to 25X to drive your amp.

Just playing with this by alligator clipping various transformers across a bass string should be very educational.

Give it a try!!

Joseph Rogowski

[Ds1]

thanks Joe, I will definitely give it a try and let you know who it works out!

[tbryanh]

Awsome

[tbryanh]

I would like to try this on an electric guitar, but the bridge and saddle are metal.

Any ideas were I can buy or how I can make a non-conductive bridge and saddle?

[bbsailor]

I would like to try this on an electric guitar, but the bridge and saddle are metal.

Any ideas were I can buy or how I can make a non-conductive bridge and saddle?

If you have the Gibson-like Tune-O-matic bridge, you can get nylon slider inserts.

If you have a Fender type bridge you can get Graphtech bridge pieces.

The real trick is to keep the balls of the strings electrically isolated from touching metal behind the bridge.

One way to make a quick Gibson-like tailpiece is to get a piece of .5" thick plexiglass or Lexan and make a tailpiece with copper rivets that the strings can pass through. Cut it so it can rest on the tailpiece studs. Then, solder a wire to each rivet before setting them into the passthrough holes. If you have a Gibson-like tailpiece this would be a very easy way to try out this idea without making any permanent changes.

Keep the wire wire leads short and rather thick so as not to get too much resistance in these leads going to the transformer(s) mounted onboard the guitar.

You can try this out on any guitar with individual tuners, not electrically connected, by just alligator clipping a low impedance transformer with a high turns ratio across one string (behind the nut and bridge) and attach the high Z transformer side to an amp input or another matching transformer like a Shure A95 series to get an additional boost.

This experiment works espicially well on an acoustic guitar since there is no metal bridge to contend with.

What you will hear is the upper harmonics of a guitar string that are normally beyond the pickup resonant peak that typically falls off at 12db per octave or more.

By moving a hand-held magnet between the bridge and neck, you will hear different harmonics empahasized.

Have fun. This will be very educational.

Joseph Rogowski

[tbryanh]

If you have the Gibson-like Tune-O-matic bridge, you can get nylon slider inserts.

I have a Gibson-like Tune-O-matic bridge.

OK, so I can make the bridge out of plexiglass, good idea.

So the issue now is the saddle. It is an adjustable saddle for intonating the guitar.

Are the nylon slider inserts for the saddle? If so, who sells these?

Thanks

[bbsailor]

I have a Gibson-like Tune-O-matic bridge.

OK, so I can make the bridge out of plexiglass, good idea.

So the issue now is the saddle. It is an adjustable saddle for intonating the guitar.

Are the nylon slider inserts for the saddle? If so, who sells these?

Thanks

Make the tailpiece out of plexiglass, not the bridge. You could make the sliders out of plexiglass if you have access to some machine shop tools. Make one strip about 2.5" long in the shape of the slider and the slice them up into individual slider pieces with a threaded hole to match the screw threads.

At one time the nylon Tune-o-matic saddle inserts were common. But now the most common alternative is the polymer String Saver saddles available from www.stewmac.com. As long as they are not metal, anything will work. Look for a small audio output transformer that is 4 ohms to about 10K impedance which is about a 1:50 turns ratio using the 4 ohms side as the input.

As an alternative, you can purchase one Triad CST-1005 current transformer which has on open core. You just pass the common ground return at the nut side of the strings through this transformer and you can hear the individual string sounds in the amp when the output of the CST-1005 is connected to an amp or another transformer. You just use a short alligator clip on the ground return wire to attach to the ball end of each string to complete the current loop with the string being part of a single turn loop. With a 1000 turns on the CST-1005 you should get a signal in the neighborhood of 20mv to 50mv directly out of the transformer with a good size magnet under the strings.

I just did some experiments with one turn (total wire length of 7 inches forming an oblong loop) of AWG 6 wire through a CST-1005 with a neo magnet (.125" thick X .5" wide X 2" long) in the center, like a one-turn pickup coil. The impedance of the CST-1005 is 261 ohms at 120Hz and 278 ohms at 1KHz.

When this wire loop is thinner or has more turns, the difference in reflected impedance results in a greater variance of impedance readings at the different measurement frequencies of the Extech LCR meter. Thus you can balance the output to favor either the higher or lower frequencies by the impedance matching of the string loop around the magnet placed under the strings.

This experiment moves away from the traditional voltage-based pickup design with thousands of turns of wire and the resultant impedance consequences to a current-based approach where any resonances are well above the audio spectrum. Matching the low impedance loop of either the string loop itself or the low Z loop using a thick string loop coil wire creates a new set of matching design issues that can be effectively balanced with a good LCR meter.

Once you start playing with this you will be entering a new and fun design territory.

Let us know how things work out.

Joseph Rogowski