Single Loop / Low Z pickup

[RobinDavey]

Hello Everyone,

As a research project in my senior year, i have chosen to design a Single Loop Pickup. I've seen some great concepts and designs and i am ready to dig in. the only downside is my electronical knowledge and experience with building pickups. I've made a design how I would like to build it and it is just simply a magnet with one loop of probably copper and let the loop go through a tranformer (1/500). The transformer goes to a xlr-output.

I will add my drawing down below (don't mind the text, it's dutch). I would like to know if it's a viable design and what sort of magnet and material/length of loop i need.

Robin de Bruin

[R.G.]

The pickup design is trivial compared to the design of the transformer. Transformer design gets harder the higher the transformation ratio. 1:500 is tough.

Magnetic pickups involve the lower-reluctance strings dragging flux loops around as they loop from the top of the magnet around to the bottom. The pickup voltage is created when the flux loops are dragged across a conductor loop. In the classical pickup, this is thousands of turns of fine wire. In your pickup, it's only one wire, and so the issue of where the wire is being "cut" by the changing magnetic field is a much bigger issue, as a single wire is only in one physical place.

Hmmm. I think I just invented a new kind of single-loop pickup. I've never seen this before. I believe that distributing a single loop over a volume of space around the magnet would produce more and better response than one single physical loop. For your senior project, it would be much more interesting to not only do the single loop from a single conductor, but to do a "single loop" composed of ten, twenty, or a hundred loops distributed outside the magnet in a volume and then paralleled at the transformer to "catch" more of the flux loop movement.

It would also be interesting to compare and contrast multiple parallel loops with the same number of loops in series, and compare that to a real single loop.

That ought to get you some extra credit!

[rjb]

Much, much, much discussion & information here:
http://music-electronics-forum.com/t5447-6/
Have fun.

-rb

[RobinDavey]

Thank You R.G. i will try to put this into work, making multiple pickups with different amounts of loops and compare them to each other!

[bbsailor]

Hello Everyone,

As a research project in my senior year, i have chosen to design a Single Loop Pickup. I've seen some great concepts and designs and i am ready to dig in. the only downside is my electronical knowledge and experience with building pickups. I've made a design how I would like to build it and it is just simply a magnet with one loop of probably copper and let the loop go through a tranformer (1/500). The transformer goes to a xlr-output.

I will add my drawing down below (don't mind the text, it's dutch). I would like to know if it's a viable design and what sort of magnet and material/length of loop i need.

Robin de Bruin

[ATTACH=CONFIG]31541[/ATTACH]

Robin,

Try this experiment. Obtain some identical toroid type current transformers (CT) with 500 turns and 1000 turns. Place the thickest piece of copper wire as will fit in the center opening. As an aternative, use multiple smaller wires. Use thin wall copper tubing to make thick-wire low resistance joiners when soldered. Remember, these are current transformers and only work well when the resistance of the string loop is very low or near its published resistance in the micro ohms per inch range. Put the string loop through 2 identical current transformers, one on each end of the loop. Make it so that this assembly will suspend in the 4 inch open hole of an acoustic guitar as your test platform. Put a single (0.25" wide, 2" long , 0.125" thick) Neodymium magnet or 6 individual magnets in the center of the string loop. Attach one CT to the XLR microphone connector pins 2 and 3 with the shield of this wire going to the string loop. Plug the newly wired XLR plug into your audio amp. Strum some notes or chords and listen to the balance of high and low sound tones. Now short out the other CT and the sound volume as well as the tone will change. You are changing or lowering the impedance of the string loop by shorting out the other CT. This is the same effect as using a thicker wire or adding another string loop of wire in parallel around the magnet and through the CT primary.

Most XLR microphones are reated at 150 ohms (called nominal impedance) but in practice they can range from 150 to 300 ohms and still be considered as low impedance. In the old days of audio they used matching input and output impedances and 600 ohms was a common input and output value then. Today, however we use what is called a bridging impedance to keep the voltage level developed in the microphone or in his case, the guitar pickup, high enough to provide and good signal to noise level. This means that the actual input impedance of todays low impedance XLR mic connections is between 1500 ohms to 2400 ohms. Why is this important? The resistance of the string loop will dictate the output impedance of the current transformer and to keep higher turn ratio CT in the 300 ohms output range, requires a thicker single string loop wire or the ability to add miltiple thinner parallel strands. Just multiply the string loop resistance by the CT secondary turns squared. AWG 11 wire is 105 micro ohms per inch. An 8" string loop is 840 micro ohms or .000840 times 250,000 or 210 ohms. Add another approximately 20 percent of about 40 ohms for leakage inductance and you will be very close to the measured output using the Extech LCR meter. If you used a 1000 turn CT you would multiply the same string loop resistance of 840 microohms by 1,000,000 and now have and output of 840 ohms. When the CT output impedance goes too high because of using too thin of a string loop wire or using a very high turns ratio CT like with 1,000 or 2,000 turns, the microphone circuit input impedance load will begin to cut off the upper frequencies and harmonics of this current based pickup. The way you wind the string loop can have a frequency shifting effect on how the CT based pickup responds to vibrating strings, where the flat, peaked or cut off response is in this pickup's audio spectrum. Obtain some thin wall copper tubing from a hobby or craft store. This will allow you to make very good, low resistance joints in string loops when soldered properly.


This is a good show, listen and tell exeriment that will also give you a reason to learn something new. Try to use wood craft stickes (same a popsicle sticks) to support your pickup across the 4" hole of a flat top acoustic guitar. Obtain some good clear packing tape to secure this pickup and wire to the guitar. To aid experimentation, put 3 insulated alligator clips on the end of the XLR microphone cable to quickly attach your wire to the current transformer and the string loop. If you use multiple parallel string loop wires, make sure they are all connected to a common ground point to minimize noise pickup.

Joseph Rogowski

[RobinDavey]

Thank you very much Joseph. i've seen your other work with low z pickups and i've gained alot of information from it. Your information was just the right thing I needed. Many thanks,

Robin

[Mick Bailey]

The low Z pickups I'm familiar with consist of maybe a few hundred turns in a conventional construction. I wasn't even aware of low Z going down to a single turn and being practical. I remember an interview with Les Paul where he was experimenting with the string itself being the 'turn' and generating the signal - the pickup being nothing more than a magnet.

Is there a preference to using a transformer rather than op-amps - what's the reasoning?

[bbsailor]

The low Z pickups I'm familiar with consist of maybe a few hundred turns in a conventional construction. I wasn't even aware of low Z going down to a single turn and being practical. I remember an interview with Les Paul where he was experimenting with the string itself being the 'turn' and generating the signal - the pickup being nothing more than a magnet.

Is there a preference to using a transformer rather than op-amps - what's the reasoning?

Mike,

See this thread for more details. http://music-electronics-forum.com/t14952-2/

If you measure the non loaded voltage induced in a vibrating metal string you will find it to be about 1 to 2 mV. If you load the string down by its fractional ohm impedance, you will drop the voltage to about half or between 0.5mV to 1mV. I have found that using an 8 ohm to about 20K ohm miniature transformer works well in producing over near 100 mV output. If you use a 3.2 ohm primary transformer you will get a little more output. Use this transformer with the low impedance side connected behind the nut and bridge on the same string. Transformers are preferred to op amps because the transformer offers a lower noise solution as the low voltage and higher current is transformed by the turns ratio and the impedance is changed by the square of the turns ratio. Read up on the history of ribbon microphones for some interesting and related background information. Once you understand the electronic principles involved, you can use a wide variety of commercial off the shelf toroid transformers and metal frame E-I type current transformers like the Triad CSE-187L to make an alternative for 5,000 to 10,000 turns pickups with very fine wire and the associated resonant peak which produces the characteristic electric guitar sound. I'll guess that Les Paul understood the concept of a moving wire (string) in a magnetic field being a generator.

Joseph Rogowski

[Mick Bailey]

That lifts the lid on a whole new area for me; I was looking for a new direction for experiment and kept coming back to optical pickups, but the low-Z idea looks a more interesting departure. I can see the parallels to the ribbon mic now you mention it.

Years ago I worked with a guy in his early 20s who'd never eaten a peach. Didn't even know what one was. I feel like that guy.