These transformers go into several groups:
Audio Input transformers are to my opinion the best accessory, to add to HiFi equipment. These are seldom used by high end companies, because those transformers can not be bought somewhere cheap, and you need two of them for stereo. So given, they cost 150 Euro for a pair, there are few so called 'high end' companies willing to invest that sum. With commonly used profit factors, it would make the final product 400 Euro more expensive. Which becomes difficult for the manufacturer, as he is not forced to use them. hat simple explains it.
Here are some of the advantages I know, and perhaps I forgot some.
- Ground loop breakage. A ground loop is an circle of ground wire, which can by the mains cables, and signal cables, which all have ground. Such a loop will pick up a hum signal when magnetic flux passes through it. Just like with any air coupled transformer. It can become very tricky, when the mains cables itself are part of the loop. Before you know it, there is some hum introduced, and the source of this is hard to find. Many designers have difficulties with this, and I have to say, it is not easy indeed. Ground loops are everywhere. The common lecturing about star grounding, or as a minimum a bar grounding, is simply ignored. Though when connecting HiFi equipment with grounded chassis, some forms of ground loops always exist. You just need to hope it doesn't hum. Open up a random amplifier and look for the "star" grounding. Likely you will not see it. It is considered "too complicated". A ground bar, works also very good, as long as you don't make it a ground loop! Here is a nice example of a ground bar, in a Yamamoto amplifier. In case, your amplifier has no ground bar, and no star wiring, and there is a hum problem, this might have to do with ground loops inside the amplifier. Specially with hand wired concepts there is always that risk, a sensitive wire becomes part of a ground loop, unknowingly, and it is magnetically coupled with some AC magnetic field. These fields are not visible, and hard to find. Such ground loops have no tag on it, saying "hello, I am a ground loop" and many times it's just a needle in a hay stack you need to find. The best way to break a ground loop, is use signal inputs which need no ground! Such are called symmetrical inputs. These are those 3-points XLR connectors. However, and this is what the writing is about, it is possible to change am unsymmetrical input into a symmetrical input with a 1:1 tone transformer. In case of unexplainable hum via the inputs, this solves the problem in many cases.
- Grid step up. The lower the signal, the easier it picks up hum. Note, a step up transformer will give totally hum free and noise free voltage amplification. This is additional to the ground loop breaking, which it will also do. It is really possible to fully eliminate the first tube from an amplifier, and replace it by a grid step up transformer. A very nice one is the LL1951.It gives a gain of 7x or 14x, but there are many others from Lundahl too. At the same time, a grid step up transformer functions as a grid choke for the next tube, because the secondary of a step up transformer is very high inductance. What else do you want :)
Here is a possible application for a grid step up / input transformer. By using an isolated ground type of RCA connector, it it still possible to use this convenient connector, and yet the ground loop is broken. So this input responds the same like a XLS input, but using an RCA connector. For the small price of a toen transformer, the first gain stage is now the transformer instead of a tube, and there is not better way to construct a hum free and noise free amplifier.