LUNDAHL "XL" transformers
First of all, the standard transformers are very good already. The XL types have a larger dimensions, which boosts their specifications a little bit. Like in so many cases, the last little bit of improvement costs the most.
Let me explain the difference, and the design considerations. The cores are approximately 45% larger.
Transformers have some electrical properties: Inductance, Windings Capacitance, Copper resistance. There are a few more, but mainly the XL differs with those three.
Using a 45% larger core would significantly increase the induction of the transformer accordingly, if the windings number is unchanged. Looking (for instance) at the LL1678, it has already a low frequency of 10Hz at 1dBm which is excellent. This needs no improvement. So the number of windings can be reduced by 45% and the low range stays at 10Hz still. The lower number of windings will already reduce the copper resistance. But... the windings are longer. So to overcome this, the wire can be made thicker, because the larger core has more room for copper.
The longer wire loops will have more windings capacitance (from each wire to all of the others) but we have less windings now.
By careful tuning of the design, a transformer will result with the same frequency range, but now wound with thicker wire, having less copper resistance. For the output this is not really important, but some improvement can be achieved. Most of the improvement is at the input side, where we have now less than half the copper loss. This will improve the efficiency of the transformer, and by this give higher output signal at the same input signal, which means a better signal to noise ratio at output of the Phono amplifier. This was what we have been doing all of this for.. :)
The user is not bothered with inductance and capacitance values, but instead the frequency range of a tone transformer is given, allowing a quick compare. In case you compare with other manufacturers,
Note, that Lundahl mostly specifies the loss at 1dB point, which is close to a flat range. Whereas 3dB, as seen in some competition data sheets, means 50% signal loss. This is too much! So try to compare at 1dB, or 1.5dB, if these numbers are available.
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We have a new addition to our moving coil step-up transformer family: The LL1678XL. This upgraded version of the LL1678 fill the gap between LL1678 and LL1933/LL1941. For this XL variant we have used the 45% increased amorphous core cross section to reduce the number of turns in the windings and to increase wire diameter. Benefits of this is increased bandwidth and reduced winding resistance while maintaining the inductive characteristics of LL1678. The result is a better match to the low impedance, low output level MC cartridges that will normally be used with the LL1678XL.