DIY Ribbon Microphone

January, 2018.

I have modified a few gold diaphagm condenser microphones using Royer modifications as described in old Tape Op magazines. The large diaphragm tube mics are the best. See here for the most detailed description. I have heard many good things about ribbon microphones, decided to see and hear what the fuss is all about.

From a high school physics perspective, a basic ribbon mic is simply a thin, corrugated strip of aluminum foil suspended between the north and south poles of strong magents, with the ends of the foil connected to a transformer to boost the varying voltage produced when the ribbon vibrates in the magnetic field.

The transformer. One of the most expensive parts of DIY microphones is the audio transformer. I decided to try to wind my own, especially after seeing some posts of people trying with stuff they had lying around. A toroidal transformer seems to be the easiest to wind because of the availability of a simple torus and easy access to the core. Here is a toroidal transformer I pulled off a toasted computer power supply board:

I cut the copper wires with a cutting disk on my Dremel:

Pulled off the wires and plastic insert:

All ribbon mic transformers I researched on line have primary-to-secondary ratios from about 1:25 to 1:40. One well-reviewed transformer for which I found specs has a 35:1 turn ratio and static resistances of 0.2 ohms (primary) and 68 ohms (secondary). I pulled some magnet wire from scrap transformers...

...found it to be about 26 gauge by measuring wire diameter with a digital caliper. Tables abound on the net that provide resistance data for copper wire. 26 gauge wire has a resistance of 41 ohms per 1000 feet of wire. I have a spool of 42 gauge magnet wire left over from winding guitar pickups :-)

This is very, very thin wire with a resistance of 1660 ohms per 1000 feet. To get a resistance of 68 ohms, I will need to wind 41 feet of this wire around the torus. After failing badly trying to pull the loose thin wire through the tiny torus, I figured I will need to wind the 41 feet of wire onto a bobbin first. Here is the torus with many turns already wound:

I threaded the bobbin through the hole in the torus, unwinding the wire from the bobbin as I went. I got about 35 feet of wire through before a tangle forced me to abort. I measured the resistance with an ohm metter, found it to be 46 ohms. I decided that was good for a first try. I estimated the number of turns to be about 240 turns based on the length of wire to make one turn. I decided on a 1:30 ratio and made about 8 turns with the 26 gauge magnet wire I scavenged from a scrap transformer, and I soldered bigger wires to the transformer wires:

Transformer done!

Ribbon mic motor. The ribbon mic "motor" is actually a generator, isn't it? It converts mechanical energy of a vibrating aluminum ribbon (from pressure sound waves in the air) into electrical energy of moving electrons. I looked at several ribbon motors on line. Here is an especially helpful site because it gives pictures with measurements!

http://recordinghacks.com/2008/11/01/chinese-ribbon-microphone-designs/

I ordered a set of 4 neodymium bar magnets from Amazon, 60 x 10 x 5 mm, for about $8. I designed the motor around the magnet dimensions. I drew the design on paper and tried to make the motor base from a piece of hard, thick, clear plastic, but the Dremel did not play nicely with it, melting and grabbing the plastic. So I went to the garage and found a scrap piece of oak flooring about 3/8" thick. I rough cut the shape with a band saw and radial arm saw, cut and glued the end piece:

I cut copper contacts out of an old piece of copper sheet and cleaned them up with the Dremel:

I drilled holes in the copper pieces and through the wood base simultaneously so the holes would line up. I hot glued the transformer with its thin fragile wires to a "breadboard" and soldered wires to the copper contacts. I had an XLR plug left over from my recent MXL mic mod, which I soldered to the secondary wires:

Motor base done!

Aluminum ribbon. I measured household aluminum foil with a digital caliper. My best measurements showed a thickness of 15 microns. This by all accounts is much too thick for a microphone ribbon. (Is it?) I tried to scrape household aluminum foil with a razor and with 0000 steel wool. It seemed to have no appreciable effect on the foil thickness, or it destroyed the foil. I scoured the web for 1.8 to 2.5 micron foil, the recommended thicknesses for a "good" ribbon mic. The best I could find was about $20 for a small sheet of foil. Trying to be frugal, I decided to try aluminum leaf, found "25 Sheets of Genuine Aluminum Leaf in a pack. 5-1/2-inch x 5-1/2-inch. Japanese Raw Material" for $7.50 on Amazon. Worth a gamble since some forum posts suggest that aluminum leaf can work in a ribbon microphone. Here I try cutting a strip:

The foil must be crimped into a corrugated ribbon. I found this toothpaste tube crimper on Amazon for about $8:

This thin foil is difficult to crimp. Below you can see the ribbon is almost flat:

I gently placed the foil on the contacts in the base, screwed on the top contacts and plugged the mic in. At first I got absolutely no sound. Then I realized I had not put the magnets in yet. Oops. Then I bumped the ribbon putting in the magnets. Prodding the set up, turning up the gain on the mixing board, I got a couple little bumping sounds and maybe a ghost of my voice. Maybe. Back to the work table. I decided to try household foil because the aluminum leaf was so delicate and hard to work with. Household foil is easy to crimp:

With household foil, I got sound! It was faint, it was muffled, but it was real sound! Click the text below to hear it.

Household foil mic

I went back again and crimped a new piece of aluminum leaf. Even though it had some pin holes and a little tear, I tried it anyway. Chalk it up to practice working with the very delicate foil. Here is the mic in the studio ready to test:

This time, I got respectable sound. Very exciting. Not as crisp as my $100 condenser mic, but good sound from $4 in magnets and scrap parts. There was also noticible 60 Hz hum and radio noise. Here it at the end of this sample:

First aluminum leaf sample

I tried cutting and crimping another piece of leaf, getting more experience handling the delicate foil. This time I got better sound, louder, with wider frequency range and less noise:

Second aluminum leaf sample

January 15. I tried sandwiching the cut ribbon between two pieces of 2-ply facial tissue:

Best crimped ribbon yet! Holding it up to the light shows a faint cracks but not as bad as the earlier tries. Here is the ribbon on edge:

Not only is this ribbon more tightly crimped, but it can be tensioned more tightly than previous ribbons. Here is the ribbon in the motor base:

Through this mic, the acoustic guitar sounds like this:

Best crimped aluminum leaf sample

The mic must go into a case to shield it from mecanical damage, 60 Hz. hum and stray electromagnetic radiation. I found a brass drain pipe in my plumbing bin, some aluminum screen, a metal cap from a cardboard mailing tube. First I had to join the motor and transformer together so they would slip into the drain pipe. This what I rigged up. A PVC coupler screwed to a metal bar to which I screwed the transformer board and motor. I soldered a ground wire to the ground pin of the XLR plug:

This slipped with some coaxing into the pipe. Then I secured the rig to the pipe with two screws. I secured the ground wire under one of the screws:

The mic sounds great for a $4 mic, with no 60 Hz hum, but there is still radio noise. I will reflect on additional shielding, especially for the transformer. I also have to close up the bottom of the mic.

Mu metal, an alloy made mostly of nickel and iron, is supposed to be very good at shielding, but I don't have any. I'll use soft iron instead:

I cut a piece of iron from the can and rolled it into a sleeve:

I painted some liquid tape on the fine transformer wires. Here is the sleeve before I slipped it over the transformer:

I crimped the ends of the sleeve somewhat, hot glued the wires going in and out of the can. Before final assembly, I hotglued the sleeve to the board (not shown). I fashioned a better basket for the motor by hot gluing a piece of screen to the top and gluing the screen to a copper strip which I screwed to the mic body.

I closed the bottom with a hot glued washer with a 5/8" hole. I hot glued the XLR plug in the hole:

The mic as of January 20, 2018:

. This is what is sounds like right now:

Sample 5

There is no 60 Hz hum. The mic picks up the sound of the computer but I don't think it is picking up radio noise. The sensitive mic is picking up the sound of the power supply fan.

To get the very best sound from a ribbon mic, I want to modify and test the motor and transformer.

I tried winding another transformer, trying to get more turns before tangle. I also wanted to distribute the turns of the secondary around the torus, like this:

I also wanted to make another motor base. This time I cut strips of oak to try to make a motor with better defined dimensions:

I cut, sanded and glued the sticks together and clamped them:

I cut more copper sheet, cleaned it up with a little wire brush in the Dremel. I also cut a support piece for the transformer:

Here is the new motor after more sanding and filing:

Wouldn't it be good to have a 3d printer to easily make several motors of various dimensions? I'm working on that. Here is a rendering of a motor I made in Blender:

Now if I only had a 3d printer...

Here is the new motor in an upright position to correctly evaluate:

It's hard to compare it to the first mic:

I must set them up together, play the same samples into each. Working on that. I need connectors, cables and a simple test station.