Here is the reply from Mr Lundahl for the LL2785B design.
Comments on the LL2785B design
The big challenge, when designing audio transformers with reasonably good HF
response, is to minimize leakage inductance AND minimize capacitive coupling.
Unfortunately these two ambitions are normally in opposition. In order to
reduce leakage inductance we increase the sectioning of the transformer, which
means that we interleave parts (sections) of the primary winding with parts of
the secondary winding. But one can easily see that the more we increase the
sectioning (in order to reduce the leakage inductance) the more capacitive
coupling there will be between the primary and secondary windings. However, if
the voltage swing across the section interface could be reduced, less capacitive
energy will be stored in the interface and thus the HF response will be
improved.
In our cooperation with Elekit and VK Music we have developed a new winding
structure for improved HF response and in particular for less phase shift in
feedback taps. This new winding structure is used in the new transformer for
the TU-8600S transformer upgrade kit. The LL2785B is built up of six sections,
two primary sections and four secondary sections. It is built with our own
silicon iron C-cores and is wound with our normal stick-winding technique.
What is unique with LL2785B is that its’ sections have been arranged in such a
way that all primary winding surfaces that face a secondary winding have no or
very little swinging signal. For this reason the capacitive energy stored in the
interface between the primary and secondary windings are kept to a minimum.
Per Lundahl
Comments on the LL2785B design
The big challenge, when designing audio transformers with reasonably good HF
response, is to minimize leakage inductance AND minimize capacitive coupling.
Unfortunately these two ambitions are normally in opposition. In order to
reduce leakage inductance we increase the sectioning of the transformer, which
means that we interleave parts (sections) of the primary winding with parts of
the secondary winding. But one can easily see that the more we increase the
sectioning (in order to reduce the leakage inductance) the more capacitive
coupling there will be between the primary and secondary windings. However, if
the voltage swing across the section interface could be reduced, less capacitive
energy will be stored in the interface and thus the HF response will be
improved.
In our cooperation with Elekit and VK Music we have developed a new winding
structure for improved HF response and in particular for less phase shift in
feedback taps. This new winding structure is used in the new transformer for
the TU-8600S transformer upgrade kit. The LL2785B is built up of six sections,
two primary sections and four secondary sections. It is built with our own
silicon iron C-cores and is wound with our normal stick-winding technique.
What is unique with LL2785B is that its’ sections have been arranged in such a
way that all primary winding surfaces that face a secondary winding have no or
very little swinging signal. For this reason the capacitive energy stored in the
interface between the primary and secondary windings are kept to a minimum.
Per Lundahl
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