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.
