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The DIY Ribbon Velocity Microphone, a home project

The Transformer's Role

New updates:

 
An update:  Good quality transformers are available on the Noisyhaus website for $21.95 plus shipping.  In fact all the parts need to build a great mic are there.  Check it out.
 

This is an incredibly important part of the design, and I recommend finding a transformer before you get started, unless you decide to use a record player preamp, or you want to screw around and make one that is intended to show the principles only.
 
I've used the Ludahls, the Cinemags,  Jensens, and a whole host of other transformers (even winding a few myself here and there).    All the commercially available ones are all great in certain types of microphones.  Almost all of them are very affordable now.  If you want quality vs. price, I suggest the Lundahl.  But I have to tell you, there has always been something about the Jensen that just sounded so sweet.
 
Just do a search on google for the names above followed by "ribbon microphone transformer," and you'll get there.

     Transformers.  A basic rundown of what they are.
 
     Transformers are (usually) cores of metal (mostly a soft iron) that have two sides:  one is called the primary and one is called the secondary.  Both of these sides are wrapped in wire to achieve a desired effect for a circuit. The primary serves to change whatever is put into it into a magnet field, then the secondary takes the magnetic field and changes it into an electrical source again.  The number of windings on each side expressed in a ration like 1:1, 2:1, 10:1, 1:10, etc.  In the case of 2:1, there are twice as many windings on the primary than on the secondary.  Here is an example of the ratio and how it relates:
 
     One effect of a transformer is that it blocks Direct Current. So one can get a transformer that has the same number of wire windings on each side (1:1), also known as an isolation transformer.  This allows an A.C. signal to pass through (but there is no physical connection between the two sides) , and D.C is blocked. Another effect is the ability of transformers to step-up or step-down Voltage.  Yet another is the changing or matching of impedences from one piece of gear to another.  Lots of uses, and lots of importance.
 
     The voltage induced by the Ribbon mic is, of course, Alternating Current.  But the amount of voltage is so little, how do we get great sounds out of something so weak?  Transformers, baby.  A step-up transformer, to be exact.
The voltage induced by the ribbon in the magnet field has to be transmitted through cables, wires, etc. for it to be any use to us.  More importantly, the A.C. needs to be in an impedance that is compatible to most mixing consoles.  What good is a mic that you can't use with your favorite Preamp?
 
     The ribbon material itself has very little resistance.  Mine has a resistance of .6 Ohms.  POINT 6.  almost none.  Basically about as close to a straight wire as possible.  But by manipulating the current and resistance of any circuit, you can change the voltage.  If Ohm's law popped in your head, then you are on the right track.
 
     The tranformer I used (and most ribbons use) is a step up of 1:12 ratio.  It steps the voltage up 12 time for every 1 that you put in. 
 
     Step up transformers are not rare, but ones for use in AUDIO applications are.  The quality of the transformer is extremely important for the quality of the audio.  Noise rejection, output, and frequency response all rely on the transformer.  So you can see, I cannot stress enough how important it is for quality in a ribbon mic.
 
     Other concerns for selecting a transformer...
     Input and output impedence, distortion, noise, and frequency response.
    
     There are quite a few manufacturers for audio quality step-up transformers.  The only problem is that they are expensive, and usually not just a little.  If you are familiar with phonographs, then you should know that there is a subculture of audiophiles that spend years creating the best possible phonographs and amplifiers possible.  As it turns out, phonograph transformers for moving coil needles (pick-ups, stylus, etc.) are very similar to what we need for ribbon mics.  Since there are many high quality moving coil tranducers that have very low resistance, there are many choices for a transformer.
 
It would take some math to explain, but the input impedance doesn't have to be .6 ohms.  In fact, the transformer I use has an input impedance of 46 Ohms, and looks for a source impedence of up to 5 Ohms, and it still works great.  The effect, I believe, is on the frequency response of the transformer (correct me if I'm wrong).  Since my transformer has a frequency response of -3 dB from .3 Hz to 200 kHz with an source of 5 Ohms, the response is not that affected (at least not in the audible range).  I've got a lot of room to play. 
 
There are a lot of other factors that I'm not going to go into here.  If you have a question, let me know.  I'm NOT a transformer guru, and I am just now getting into transformer design and physics (after years of working on tube amps.  Yeah, they kinda scared me, but not anymore).  I'll try to let you know an answer, or at least put you in touch with someone who can help.
 
 
 
 
 

Again, here is the schematic I used to wire up the transformer.  It is really for a phonograph, but it works for ribbon microphones just as well.
 
The address is (again):
 

transformer2.jpg

Some of the manufacturers are floating around the internet.  Look around.  see if you can find a used one for a decent price.  But BE CAREFUL.  There is no telling what people have done to them. 
 
My suggestion is to buy one new.  I got mine as a gracious donation from JENSEN (THE name in transformers) for this project.  Its the JT-346-AX.  Please do NOT contact them asking for a transformer.  You will succeed in only pissing them off and making them mad at me.  Other wise, it would cost over $200.00.  I cannot thank them enough, and will be giving them all my business from now on, because this thing sounds incredible, and the specs are fantastic for every area that counts. Use my link to check them out.  Also, check out the PDF in the schematic section for the adaptation of the transformer to ribbon mic use.
 
There are other companys, such as Sowter and whatnot, but I have not tried their transformers.  You will probably have to get in touch with them to custom make you a transformer for this purpose.  It'll cost you, but if everything is done right, you will be well rewarded with a great sounding microphone that you built completely with your own two hands.
 
Another suggestion if you want to make a mic just for the hell of it.  If you are doing it purely out of curiosity , use a phonograph preamp.  The RIAA curve is a little annoying, but it still works.  It won't sound good by any means and will be noisy as HELL, but it will let you hear the sound of your own voice.  This was a suggestion of Larry Killip (see the links) from early on in the project, and it worked like a champ.
 
     Since then, Erik (from Buellton) has let me know of a transformer for 44 American dollars (if you Guys knew how many over-seas people I had emailing me, the you would understand the "American Dollars" thing).  Try a Google search of Cinemag XFMR and see what you come up with.  Thanks Erik!

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