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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.

http://make_a_ribbon_mic.tripod.com/noisyhaus/

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):

http://www.comtran.com/as/as067.pdf

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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