Seeking Advice on Selecting Material for Ribbon Mic Motor

[tomerttb]

Hi everyone,

I hope you're all doing well. I have a question regarding the selection of the correct material for a ribbon mic motor, and I'd greatly appreciate your insights.

I'm currently trying to understand whether the motor should be magnetic or not. Specifically, I'm curious if the magnetic field should come solely from the magnets or if a combination of magnets with a magnetic material motor is preferable.

Depending on the answer to my first question, I'm also looking for recommendations on materials that would be a good choice for the ribbon mic motor.

Thank you in advance for any guidance or recommendations you can provide.
Tom

 

[doqmemory]

This seems like one of those "AI" posts to me.

Sorry if you are a real person. It sounds like fun to work on ribbon microphones. Good luck.

 

[ccaudle]

This seems like one of those "AI" posts to me.

Not really, it is not obvious at first glance whether it makes a difference to have a "return path" for the magnetic fields. I put that in quotes because that would be the closest term I can think of for an electrical circuit, but I'm not sure if that same term is used for magnetic circuits or not.

But to the original question: a ribbon mic will work with non-magnetic materials holding the magnet in place (plastic, aluminum, brass, etc.). The somewhat well known Austin Mic DIY ribbon microphone plans use plastic for example, and there are plenty of reports of that working fine.

What I found in some ribbon microphone reference documents is that having a ferromagnetic material connecting the backs of the magnets together (assuming you are using the typical modern style of two small bar magnets on either side of the ribbon) produced higher sensitivity and less problem with interference from outside magnetic fields by focusing the magnetic field more closely between the magnet poles.

So ferromagnetic material recommended, but not strictly necessary. I don't think there is much choice, the ferromagnetic materials are basically iron or mild steel, nickel, and cobalt. Nickel and cobalt are not really practical to use, so that leaves iron or mild steel. High carbon and stainless steels are harder to machine and depending on the alloy may not be magnetic, so you have the fortuitous situation that the best material to use is also the cheapest.
The downside is that mild steel will rust easily. You need close tolerances for a ribbon mic motor, which doesn't really lend itself to priming and painting. Most commercial motors probably nickel plate the steel after machining to prevent rust. You might be able to use a bluing type stain (like traditionally used on guns and some tools), or an oil finish (basically heat the material up to red hot and dunk it in oil, which gives it a nice black finish).

 

[tomerttb]

Hi ccauld,
Thank you for your answer.
If I've grasped it correctly, commercially available ribbon mics typically utilize a magnetic material for the motor, often treated with an anti-rust coating.
If yes, I think it's a good place to start experimenting

Tom (Im not AI)

 

[KLDVOX]

i thought the ribbons were alumunium foil. cutting stainless is not a problem A2 works well, just never let it get hot while cutting as that will have the effect of hardening the steel. if i was build a ribbon motor i would be tempted to use a hard plastic or even a hard wood such as african black wood (clarinet wood).

 

[KLDVOX]

a bluing type stain (like traditionally used on guns and some tools), or an oil finish (basically heat the material up to red hot and dunk it in oil, which gives it a nice black finish).

the normal bluing is clean the steel, coat it with a thin film of oil THEN heat till the oil is burnt on to the steel. i think you are getting mixed up with oil hardening of alloy steels. ( the steel is heat treated and quenched in oil some of the carbon in the oil is absorbed into the surface of the steel ) there are propriety bluing solutions avaiable but again these are painted on prior to heating, the steel does not need to be red hot as that is done to anneal/soften the steel

 

[dmp]

I'm currently trying to understand whether the motor should be magnetic or not. Specifically, I'm curious if the magnetic field should come solely from the magnets or if a combination of magnets with a magnetic material motor is preferable.

Yes, the magnets provide the magnetic field across the ribbon. The 'motor' basically holds the magnets in the right place and usually has a iron 'return path' for the magnetic field to follow. Some ribbon mics use the steel microphone body for the return path. If you buy magnets make sure they are oriented N-S correctly to the ribbon.

See this thread which has some discussion and pictures of RCAs. Studying the construction of the old RCAs ribbon mics is a great place to start.

 

[ccaudle]

i think you are getting mixed up

You are correct, I went back and looked at some of the source material I found previously, and it is definitely not red hot, much lower than that. I had seen descriptions of "dunking" but also brushing on oil. I had not previously seen an example of your description where the oil is applied first then the work piece heated, but it makes sense as long as you are using oil with a high enough smoke point. I think the point of immersing in oil was that the entire surface could be coated while still at a relatively even temperature, and you did not have to worry about making sure you had a natural bristle brush which would no melt like synthetic bristles if brushing oil onto a hot surface. Of course dipping into a large volume of oil cools the piece pretty quickly, so I think brushing on either before or after heating would probably be better.

A metal worker explained that the process described should correctly be called oil blackening, as bluing is a specific process that is chemically different.

Traditional rust blueing might work as well, use a slightly caustic solution to form a thin rust layer on the part, then boil in water to convert the red oxide to black oxide.

A couple of months ago KingKorg tried out home nickel plating and had some success:
https://groupdiy.com/threads/nickel-plating-headgrille.85109/

 

[KLDVOX]

thanks for the info on using caustic stuff its not a process i have come cross. ill have to have a look at this

 

[cyrano]

the normal bluing is clean the steel, coat it with a thin film of oil THEN heat till the oil is burnt on to the steel. i think you are getting mixed up with oil hardening of alloy steels. ( the steel is heat treated and quenched in oil some of the carbon in the oil is absorbed into the surface of the steel ) there are propriety bluing solutions avaiable but again these are painted on prior to heating, the steel does not need to be red hot as that is done to anneal/soften the steel

The same way you prepare a wok, or a cast iron skillet for cooking...

 

[tomerttb]

Thanks everyone for helping with this subject.

 

[Marik]

The main purpose of the ferromagnetic steel for the frame is not even a return path. With modern Neodymium magnets the return path has much less effect, as it easily gets saturated. The main advantage of the steel is magnetic flux from the back of the magnets leaks through the air into the magnetic gap causing severe cancellation of the field. The steel effectively increases that distance, which greatly helps to avoid losses/field cancellation with square ratio relationship and thus, increases the efficiency.

In general, the non magneic frames will work, but have quite a bit lower efficiency. In a such a low signal system as ribbon microphone we want to squeeze out every last dB of signal. Another consideration, it is much easier to assemble the frame when the magnets can get stuck to it.

Best, M

 

[s wave]

I hope your DIY microphone building has been successful. By now you probably have the hang of it. Many factors affect the mics sensitivity. 3D printing is probably the best and fastest route to design and engineer a ribbon microphone. FreeCad is a great program. Learning curve takes a day or a week to get the mindset of how to make 3D prototypes. But the motor chassis is relatively simple. Once you have a design you like. Export it to an .STL file on a flash drive or SD card. Bring it down to your local library and print it for free. Most libraries give you 20 grams (a day) of PLA for free - enough to make to chassis/motors. PLA is a plastic that is pretty good for what you are doing. I just printed one up and am more than happy with the motor chassis. I glued the magnets in place using JBWeld epoxy. (many people use CA glue aka (Super Glue). Keep us updated on how your project is going. tc