There are sevaral areas where signal transformers have an traditional place as "first choice" component. One of those is the moving coil transformer.
The output signal of a MC pick up is only a few milivolts. This is really very low. e. The problem is not so much to amplify this signal, but rather how to do so without adding noise, and how to deal with milivolts signals without introducing hum. I mean, adding 200 microvolt noise to 200milivolt signal is 60dB, which is well in the audible range already. However most semiconductors already produce 200Microvolt noise by just looking at them. For hum, the situation is that the ears is less tolerant to hum, than to noise. Imagine all the 230 Volts mains cables surrounding your equipment, and you would not like 200 Microvolts hum added to a 200milivolts signal. Well, such solid state amplifiers excist, but they have a lot of heavy feedback, and the question is really if you want to treat your analog sound, all at the beginning at the chain, that is directly from the MC pick up, with a solid state amplifier with heavy feedback in it. For any zero feedback concept it gets spoiled this way, and once the signal is mistreated, it can not be made undone.
the Lundahls can give a factor of 32x amplification in a passive way, so adding no more noise than the so called barkhausem noise from the metals used. In real life that means no noise is added.
Then, and tone transformer by default breaks the parasitic ground loops, and any hum induced voltage will not be able to let current flow, and in that way the hum can not pass the transformer.
So isn't this an ideal situation, parasitic ground loops eliminated, up to 32x gain, and no noise added. If you ever have been struggling with noise problems and hum problems of electronic MC amplifiers, you will soon learn to estimate the advantages of a high quality transformer.
About Quality of MC transformers.
Since we talk about just milivolts of signal, maximum signal level is not the issue, but rather that we want to step up this signal without adding noise and hum.
The following statement may seem strange when you read it for the first time, but magnetism it is not a fully analog effect. At very very low level, the magnetism changes in tiny steps, due to iron crystal size. As soon as you are in the "volts" level of a transformer, no instrument can measure that effect of those magnetic steps, but when we talk about the 200 micro Volts, as produced by am MC cartridge, this volts, the quality of the magnetic core gets most important. The core must have a micro-fine magnetic structure. So some possible core materials are: Amorphous, mu-metall, or cobalt. Another issue with MC Transformers is signal loss. Since you want every milivolt to come out of the MC transformer, these are wound with relatively thick wire, to prevent copper loss. Now the thick wire will limit the number of windings, and therefore limit the maximum signal, but high signal is no issue. The thick wire will result also in less windings possible, giving a low impedance product. However also this is no issue, since MC pick ups have a very low impedance anyway. To increase the copper filling factor, the "thick" wire is made of several windings pararallel. As you see, the whole picture makes sense, and indeed making MC transformers is something that gives a very nice result. It is impossible to achieve the same result electronically for the same price. (if possible at all)
IMPORTANT -- > How to select a Lundahl transformer, fine tuning of Moving Coil transformers, and some more info
