Building a ribbon microphone

[bbsailor]

I stumbled upon this old thread. The microphone is finished and sounds very good. It's still subject to my tampering/modding though. :-) At the moment I'm toying the idea of adding a jfet stage to boost the out signal, which is somewhat low. I'll try to see to that I can upload some sound files.

Nice Job! The physics of ribbon microphones is dependent on the total resistance of the ribbon plus the wire lead and transformer primary resistance times the turns ratio squared. If you have a 0.1 ohm ribbon plus a 0.1 wire resistance (wire lead plus transformer primary) the total resistance is .2 ohms times the turns ratio (TR). Assume it is a TR of 40 then the output impedance would be 1600 X .2 or 320 ohms or near the upper limit for the modern mic bridging impedance of modern mixer inputs, typically about 2400 ohms.

If you use a higher turns ratio transformer to obtain a higher passive output you will quickly produce a higher output impedance that exceeds the bridging load recommendations. Then, a FET buffer/amplifier can be put after the higher TR transformer to minimize the load on the transformer as this load gets reflected back into the ribbon and affects the perceived openness of the sound. If you want to get the extra gain from a higher TR then you also affect the tonal balance more than adding a little more gain in the FET amplifier stage with a lower TR transformer where there is less loading reflected back into the ribbon. Let you ear be the final judge. Just knowing how all the pieces of the ribbon microphone signal chain connect together makes evaluating the sound easier.

A good test to demonstrate this concept is to put a 5K ohm variable resistor (pot) across the ribbon mic output transformer and vary the load and listen to how reducing the load affects the sound quality. High frequency response, transients and openness are affected first.

I hope this helps.

Joseph J. Rogowski

[überfuzz]

Thanks for the post! There actually quite a bit more to it than balancing the impedance. Not really electronic related but here are some examples: The wave equation can be used to predict behaviours of the ribbon set up. Maxwell's equations predicts how motion is transformed into electricity (current). Newton's 2:d law can be used to predict how vibrations are imposed on the ribbon. If you're interested I can send a paper I wrote on the subject.

Cheers!

[bbsailor]

Thanks for the post! There actually quite a bit more to it than balancing the impedance. Not really electronic related but here are some examples: The wave equation can be used to predict behaviours of the ribbon set up. Maxwell's equations predicts how motion is transformed into electricity (current). Newton's 2:d law can be used to predict how vibrations are imposed on the ribbon. If you're interested I can send a paper I wrote on the subject.

Cheers!

Yes, I would enjoy reading it. Send me a private MEF message with an attachment or post it here.

Thanks

Joseph J. Rogowski

[überfuzz]

Okay! I had a brief overlook and saw that some parts in the text don't match the current set up of the microphone. E.g. the cathode is not bypassed, and some minor alterations I've done. I put the paper in my google drive: Active ribbon microphone.

[bbsailor]

Okay! I had a brief overlook and saw that some parts in the text don't match the current set up of the microphone. E.g. the cathode is not bypassed, and some minor alterations I've done. I put the paper in my google drive: Active ribbon microphone.

Per, Good reading! Thanks

You may be interested in how guitars strings can be used like ribbons in a unique type of guitar pickup. See this old thread that I started in 2010. http://music-electronics-forum.com/t14952-2/ "Moving Coil Pickups for the Technically Curious". Just alligator clip the low side (4 or 8 ohms side) of a matching transformer across one guitar string. Attach the high impedance transformer side (10K to 100K side) to an amp input; hand hold a magnet near the string; plunk the string and now listen! From building the ribbon microphone, you should be able to quickly grasp the theory involved. Have Fun!

Joseph J. Rogowski

[überfuzz]

Thanks Joseph!

I added a jFet stage to both hot and cold line out. The microphone still sounds bitching good. The out signal is now more like what you'd get out of a standard studio mic. I can't hear any alteration to the tone. Further, I can't see (recorded and glanced the file in Cubase) or hear any added noise.

[überfuzz]

Enters next generation of ribbon microphone:




I designed the pcb board in a software called kicad, the hardware i designed in a software called freeCAD.

[bbsailor]

Enters next generation of ribbon microphone:

[ATTACH=CONFIG]46455[/ATTACH]
[ATTACH=CONFIG]46456[/ATTACH]

I designed the pcb board in a software called kicad, the hardware i designed in a software called freeCAD.

The jFet acts like a high impedance load that isolates the transformer from reflecting back any XLR microphone input circuit loading, typically 2400 ohms, from influencing the high frequency range of the ribbon microphone response.

Unlike guitar strings which are very loosely coupled to the pickup, the ribbon is tightly coupled to the very strong magnetic field very close to it and will be affected by any passive load on the thansformer but your use of a jFet prevents this from happening.

Nice job.

Joseph J. Rogowski

[überfuzz]

bsailor - quite rightand thanks!

I just put together common ground stages which borrows the anode side from the phantom power set up. i seem to remember its 48V supplied over 6.8 kohms.