Making a Ribbon Microphone Using a Commercial Element
I purchased a ribbon microphone element branded "Alcatron". I made a simple wooden body from some reclaimed English Oak and mounted the ribbon assembly inside a rectangular routed pocket.
The transformer is home made and looks it. The laminations are taken from a small valve output transformer, I sure they will be plain iron, though I don't know. They measure 2 inch * 1.5 inch and I used a stack height of 0.3 inches. This a large core for a ribbon transformer but it was to hand. The voltages generated by the ribbon are tiny so there is no possibility of the core saturating. To give some perspective, I calculated at what voltage saturation would begin. For a 0.25 inch square core with 20 turns, 64 millivolts would give a peak flux density of 1 Tesla at 20 Hz. The ribbon output will be one thousandth of that.
The windings are in five sections, (the more sections, the less leakage inductance and the better frequency response). The winding scheme is shown in the table below. The secondary windings are in series and the primaries in parallel, giving a ratio of 40:1. The winding was done, very carefully, by hand. The primary resistance is negligible, being wound in 1 mm dia copper wire. The secondary measures 150 ohm, not negligible. Referred to the primary it is equivalent to 150/ratio2 or about 0.1 ohm which is comparable to the 2 inch ribbon of the microphone element. So the secondary resistance will increase the self noise of the microphone by 3 dB. The transformer can be seen here in the lower routed cavity.
Thinking about the sources of noise in a ribbon microphone, resistances which produce Johnson noise will be the only self-noise source, although further noise will be added by the amplification equipment. The effective resistance of the noise source will be the sum of the all the resistances, referred to one side or the other of the transformer. Referring to the primary, these will be: ribbon resistance; wiring resistance; primary resistance; secondary resistance/ratio2 and connection resistances. It is evident that the connections to the ribbon are all important because they are in such a low-resistance circuit. I wonder how good they are in practice. If the ribbon clamps are made of copper, then there is a risk of corrosion at the interface between the dissimilar metals which would likely increase the resistance.
Here is the microphone undergoing a test. The mounting yolk can be seen here. It is made from laminated ash formed on the guitar bending iron and glued together. The metalwork is taken from an old (and useless) shock-mount.
So how does it perform? Well, the first thing to say is that there is insufficient gain available from my mixer (Soundcraft Spirit Folio F1), a complaint I have heard before relating to ribbon mics. However, it is sufficiently sensitive to record classical guitar.Secondly, it sounds very smooth in comparison to my cheap condenser mics. I like the sound on classical guitar and it records guitar amplifiers very nicely.
I had noticed for some time that the performance of this mic had deteriorated, so much so that I had stopped using it. Some time later, I wanted to use it in a M-S (Mid-Side) pair so I tried it again. It did not work at all. The pictures show why. They show the ribbon displaced from its proper position and the plating flaking off one of the Neo magnets.
I happened to have collected the materials to make a my own ribbon motor so I fitted two new magnets and made a new ribbon. Here is repaired unit. I don't know why it appears gold but here you are. It works at lease as well as the original