Many customers ask us for the difference in sound between our mu metal lamination transformers and our strip core amorphous transformers. We have forwarded the question to Kevin Carter at K&K Audio. Here are his impressions:

“When I first encountered the Lundahl LL1544A and LL1545A line level transformers, which use the same coils, but these are installed on cores made of different materials, I knew that it was a great opportunity to understand the role that these core materials play in “transformer sound” without being confused by other variables in transformer construction. A few years later, I was able to do the same sort of comparative listening in becoming familiar with the differences and similarities between identical coils assembled with different coil materials in the LL1931 and the LL1933 MC Input transformers. I found sonic consistency between the transformers using the same core material and differences between transformers of the same application type with different core materials. Here is a summary of my observations:
The amorphous core transformers (LL1544A and LL1931) provide a very open and detailed “picture” of the space that was recorded, providing a very high level of detail recovery. A particularly memorable example of this “see-through” quality was evident on a solo violin recording where the performer’s breathing was readily audible during playback of the performance with the amorphous core transformers, but was only evident with the mu-metal core transformers if the listener really focused on hearing the breathing. The breathing sounds were there with both transformers, but there is less of an emphasis on detail and more upper bass / lower midrange warmth with the mu-metal transformers (LL1545A and LL1933). The brush strokes are broader with the mu-metal core transformers and they convey more “body”, whereas the amorphous core transformers offer finer brush strokes, which results in hearing more of the details while listening to the whole.

The difference between the transformers made with the two different core materials is not large, but the detail and tonal shading differences are easy to hear with the appropriate recordings. If the equipment in which one of these transformers were to be installed with has plenty of sonic body already then I would choose the amorphous core version for its substantial transparency. On the other hand, if the sound is a bit light-weight, then a mu-metal version is probably the best way to go.”