A Ribbon Microphone- Design 001, Rev A

Since I have to give away the Austin Ribbon Mic that I built, I decided that I wanted one of my own. I purchased raw materials for four more ribbon mics and have been working diligently on another mic. It isn’t completed yet, but I’ve done enough (and it looks good enough) that I can show off some preliminary photos.

My progress so far on ribbon mic no. 2, which incorporates a large number of design changes from the previous mic.

This microphone is similar to the Austin ribbon, but it’s a completely new design inside. The body style is similar, and I’m using a similar motor frame of clear acrylic plastic with drilled vent holes as a frame for the motor, but that’s where the similarity ends.

The Austin mic sounds great, but my outboard preamps are… well… let’s just say they aren’t top-of-the-line models. (Except for my lovely pair of VP26 pres from Classic Audio Products of Illinois, but I don’t have a Lunchbox yet to mount them in. I’ll get them racked one of these days, but the studio needs to generate some income first.)

So as a result of my gear, I’m having to crank the preamps up a good bit on the Austin mic. It sounds great on a source like an acoustic guitar, but for voice it requires a super clean preamp. (The Crest preamp that I recently purchased is a significant improvement.) This new design is an attempt to increase the output a little so that I can get a better signal with more common equipment.

My redesigned ribbon motor frame with magnets in place. A central groove is milled in the plastic for positioning the ribbon. The magnet spacer is just a piece of 4mm aluminum.

I’ve redesigned the motor frame using magnets that are slightly larger at 5mm x 10mm, and a ribbon that is slightly narrower, about 3.5mm. It’s counterintuitive, but narrower ribbons have a higher output. I’m using a 2.5 micron aluminum ribbon foil. The plastic frame is machined with a slot down the middle to hold the ribbon, and a pair a milled aluminum clamps to hold the ribbon in place. I didn’t want to use copper here because of galvanic reaction concerns. Everything is held in place by 4-40 capscrews, which are magnetic & will probably be a pain in the ass to install, but we’ll see. I’m waiting until most of the work is completed before I install the magnets and the ribbon, since the magnets will attract bits of ferrous crud in handling & the ribbon is so very fragile. That’s why they aren’t in place in the photo above.

The frame installed on the motor mounting tube. The ribbon clamps have been machined and are in place. I’ll probably secure the wiring with a dab of glue once the ribbon is installed.

Another difference is in the motor mount. I’ve built a secondary can that slides inside the mic body from a small piece of galvanized fence tube with a fender washer silver soldered to the top. This will shield the transformer almost as well as a Mu-metal can, and it makes a sort-of “mic-within-a-mic.” This same type of construction is used with great results on the Electro-Voice RE50 reporter’s mic, one of my favorites. This sub-assembly is isolated from the outer shell of the mic with a wrap of foam. This will be of limited benefit, since a ribbon mic is never handheld except when used as a prop in music videos. But I did want some way to take the mic apart and put it back together as a single unit… the Austin design has the motor pressed against the mic body with strips of neoprene foam. It works well, but isn’t ideally suited to taking the mic apart. I expect to do that a bit more with this mic, as I experiment with things like silks around the ribbon or waffle-plate resonators.

I’m calling this one Revision A, since I redesigned & rebuilt the mount to correct a grounding problem. I’m already working on Revision B, which will be largely similar except for the ribbon clamps and  more space at the bottom of the frame for mounting the motor with screws. (Revision A’s mount is secured with glue.) I’ve got some other ideas to try with magnet sizes and frame designs, but I need to give these an extensive listening test first.

I’m still waiting on a few parts… the transformer and the XLR, mainly… but I hope to have this project finished soon, and I’m anxious to hear the results. In the meantime, I’ve started milling parts for another one.

The semi-completed shell. I still need to machine the brass cap at the bottom and mount the XLR connector. Overall length is just about six inches.

The cost for this project is fairly high, mainly due to the amount of time that is spent in construction. If you add in the cost of the machine tools, the cost goes from fairly high to astronomical. I’m using a benchtop drill press, a 7×10 Chinese lathe and Taig milling machine to make these. All three have been essential for various parts of the work. While it would be theoretically possible to build a mic with hand tools, it is a lot easier to get good-looking results in less time with some heavy machinery. (If you lack the tools, Rick Wilkinson’s Austin ribbon mic kit is highly recommended, see my previous post.) And actually, my setup is pretty minimal… I could use a larger lathe & a CNC mill for faster and better results… but now we’re talking about an investment that would require going into business as a mic manufacturer, which isn’t my intent. These are experiments. I’ll probably make a few available for sale at some point, just to recoup some of the cost of the parts. And I expect the price on them to be rather high, just because they take so very long to make. But these are primarily built for my own satisfaction. It won’t hurt the resume, either.

I owe a great debt to Rick Wilkinson and Les Watts (former mic designer with Shure and EV) for teaching me a large majority of what I know about building ribbon mics. Their help is very much appreciated.

UPDATE: I’ve very nearly completed this mic… I recut an existing XLR connector on my lathe, so now it fits the microphone. The magnets were fitted to the frame, and I mounted a 2.5 micron ribbon yesterday. I’m guessing it’s about 3.6 mm wide. It’s a nice, tight fit. The ferrous capscrews were a pain in the ass, but not impossible to deal with.

The ribbon motor completed, with magnets and ribbon installed. Using 2.5 micron ribbon material is much easier than signwriters leaf, though it’s still not a simple process to install.

Now I’m just waiting on the transformers to arrive. These will be a special custom-wound ribbon transformer that should meet or exceed specifications from the usual suppliers. Unfortunately, it probably won’t arrive in time for the DIY seminar… I was hoping to have this working before then.

This mic comes with a custom walnut mic box that I had built by a local cabinetmaker. Nothing on this mic is sourced from Chinese suppliers… that sort of thing is easy enough to get from just about anywhere these days. If nothing else, this mic will be different from the common stuff that is so prevalent these days.

UPDATED UPDATE: When I went to use this mic recently, I discovered that the glue had failed on one of the magnets and it had separated from the frame, bringing the two together and turning the carefully-placed ribbon into aluminum dust. So while this is an easy-to-build example of how ribbon mics work, it isn’t the best design in terms of longevity.

My new design incorporates a steel frame rather than plastic. This is slightly better from a magnetic perspective, and it’s naturally stronger for threads, etc. I’m thinking of trying a frame made from small pieces of 1/4″ square steel. Machining from solid would be a possibility, or I could even get crazy and go to the blacksmith shop and forge something… I was formerly a full time blacksmith and still have access to some large and heavy tooling. And I’m pretty sure that nobody else is hot forging their ribbon mic parts.

But the key design element will be a small notch to hold the magnets apart. I should have done this before, as it could be incorporated into my earlier frame design. It’s yet another thing to put on my long list of projects.

The finished mic in its custom-built, handmade walnut box.