Input Transformer
We do not see many DIYers messing around with input
transformers. Why? No one really knows the real reason. For us, we think that
it is super useful!
Input transformer is one of the crucial components to
achieve a two stage tube amplifier, or even a single stage amplifier with
sufficient gain. Input transformer, being more efficient than just a load resistor
or volume pot, can fully utilize the drive strength present in the source,
where most of the CD players or DAC now possess very low output impedance. Don’t
let that go to waste!
Excellent driver tubes with gain of >40 are very rare
nowadays. EC8020 or WE437 were two great driver tubes but it is as rare as
unobtanium now. Only such tubes, without the need of input transformer, can
drive the power stage to full power. For other driver tubes, they would need
another pre-amp stage to provide the gain for them to swing for full load at
the power stage, like the 6SN7, 5687, 6DJ8, 6922 and similar tubes.
Why not let the input transformer do the job instead of adding
another pre-amp stage? A 1:2 or 1:4 input transformer will be sufficient to
provide enough gain to the driver tube to reasonably drive the power tube! Of
course, in the 1st place, the input transformer would need to be of
excellent quality before we can leave the job to them.
Due to the size, quality and performance requirement of
input transformer, it has made it a daunting task to design and manufacture
them. What makes so difficult, one may ask. Let’s look into it.
Small signal input transformer is very sensitive to external
noise or interference as it has a lot of tiny thin wires / windings for good
sensitivity. It needs very good shielding. Good shielding requires special
material to provide the magnetic shielding needed to protect the input
transformer from surrounding noises. Some may even have several layers of
shielding to do a fool proof job, like having mu-metal shield + copper shield.
- 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.
Without having rich and real experience in design and manufacturing
audio transformers, not to mention have the expensive measurement equipment to
do a full sweep, and the technical know-how to resolve the frequency response /
impedance irregularities, it is impossible to design a wind bandwidth input
transformer (other audio transformers too!).
Now the more you understand, the more you feel that audio
transformer design is a very in depth subject.
J&K Audio Design
21/12/2013
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