Making a Ribbon Microphone Using a Commercial Element

 

 

Mic Body with Homemade Transformer

 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.

 Winding Section Secondary  Primary 
1 266 turns  
2   20 turns
3  266 turns  
4   20 turns
5  266 turns  
 
   

 Self Noise

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.

 

 
 
  
Mic Showng GrillePlates were made for the front and back, about 3/8 inch thick, with cut-outs for the grilles. They are rebated from the back to form a flange so as to secure the grille mesh. There are two layers of stainless steel mesh 2 mm pitch on the outside and 1 mm pitch on the inside with a very fine nylon mesh between. I expect the grilles will affect the sound to a fair degree but they are necessary to protect the delicate ribbon from finger damage or from metal particle ingress. This aspect of the design is the least understood by me and is very much "finger in the air". The picture shows a plate screwed in place with tiny wood screws (guitar scratch-plate screws). Not shown is the XLR connector. It is a standard cable socket glued in a hole at the bottom. 
 

Mic in actionHere 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.

 

 

 Mic Alongside Commercial Transformer I was a little sensitive about the quality of my home made transformer, so I bought one to give it a try. This one has a nickel-iron core and a ratio of 36:1. I don't know what the winding pattern is but the frequency response is quoted as 20 Hz to 20 kHz +- 1dB. More than adequate and probably much better than mine. Whether it makes any difference to the sound, I am unsure. I did not detect any difference after the change.
On the subect of Nickel-iron cores, I am at a loss as to why they may be superior in this application. There are numerous claims to suggest that they are. However, the flux density levels are very low so distortion will be simlarly very low. It is quite different to an output transformer application where flux densities will be much closer to saturation.
 
  

Mic withe New Transformer

 
 

 

 Microphone Failure

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

 



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