Ground isolation, DC rejection, RF shielding, circuit balancing, impedance matching and line splitting or combining are all very good reasons for knowing about audio transformers. Transformers have grown to become a very integral part of the sound of high-end studio equipment. A magnetized core made up of varying parts of different metals, such as nickel, steel, etc. This "bobbin" is created by laminating thin sheets of metal and stacking them, insulated from one another. This core is wound with a type of insulated copper wire. All in all a very simple design which, fundementally, has remained the same for way over half a century. For more on the fundemental principles of all transformers, see Faraday's law of induction.
Bill Whitlock of Jensen transformers has a detailed explanation of the concept, parts and construction that is specific to audio transformers HERE
RE: 48v and vintage transformers via Oliver of Tab-Funkenwerk:
"Phantom Powering
One of the biggest concerns with these historic microphone pre-amp input and output transformers, is the phantom powering. Developed in the beginning of the 60s, and initially a sub-standard of powering microphones, this has now moved on to become the industry standard, applying 48V via two resistors to each leg of the modulation (pin 2/3) so there is no potential difference and using the shield as return path/0V....
It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insulated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.
If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitive property of the transformer can charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years."....great stuff, Oliver...
Below are a few of the biggest transformer companies.
Jensen
Sowter
Lundahl
Cinemag
Edcor
Carnhill
Among the vintage brands there are: UTC, Triad, Haufe, Stancor, Marinair, St.Ives, Peerless, Thordarson, Altran, Chicago, Western Electric, Reichenbach, API, Hammond, and more.
Below are some handy wiring diagrams for UTC's if anyone happens to need them:
UTC documents
If you are building a classic studio piece, this link is a great reference for what transformers will work:
Transformer by project list
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