- Precision engineering – input transformer requires very accurate and balanced winding. Due to the size of the wires (0.003 – 0.005mm) and complexity of winding a 1:1, 1:2 or 1:4 ratio, input transformers requires a lot of windings to be done in such a small place. That requires very accurate winding machine, as accurate as to 0.001mm or lower. Due to the size of the wire, windings will be difficult as wires tend to overlap and cause impedance irregularities.
- Such precision machines are very rare and not every manufacturer can afford them. Also, the bobbin for input transformer will need to be precision made. Again, it is very rare too. Most of the bobbins (probably injection molded) are made for power transformers instead of signal transformers. Using such huge tolerance method will not make a very good input transformer.
- Core material – input transformers need very high Gauss core material, specially made for audio use. Such cores are expensive, and it’s not easily available, not to mention in small quantity. The electrical characteristics are very good though compared to the size.