Originally posted by bea
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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
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