Choosing the OPT core material always depends on use.
Usually -for amplifier OPT- I prefer "classical" FE (grain oriented, HiB etc.) cores, but use amorphous (for example Metglas) cores too. Tried nanocrystalline too, but not of my cup of tea, it "fills up" (saturating at LF) too quickly.
The high -primary- impedance line output transformer generally use high coercivity core (less number of turns is needed), it's commonly amorphous or nanocrystalline (as Finemet). There is -usually- no large current, so induction manageable even at low frequencies (where the music signal large) too.
I have nickel core ITs from Dave Slagle link, I don't know if they also make high impedance ones.
Try to search another (not nano) core 10-24k line OPT.
In the past (Brian) Sowter made such good transformers samples.
If your hearing is not perfect, old UTC transformers are also possible.
Usually -for amplifier OPT- I prefer "classical" FE (grain oriented, HiB etc.) cores, but use amorphous (for example Metglas) cores too. Tried nanocrystalline too, but not of my cup of tea, it "fills up" (saturating at LF) too quickly.
The high -primary- impedance line output transformer generally use high coercivity core (less number of turns is needed), it's commonly amorphous or nanocrystalline (as Finemet). There is -usually- no large current, so induction manageable even at low frequencies (where the music signal large) too.
I have nickel core ITs from Dave Slagle link, I don't know if they also make high impedance ones.
Try to search another (not nano) core 10-24k line OPT.
In the past (Brian) Sowter made such good transformers samples.
If your hearing is not perfect, old UTC transformers are also possible.
I use AZ1 valvo big mesh for preamp B406 with line stage output 10k/600 from Hashimoto. AZ1 rectifire FW which is good for low HV with not large current. The problem of the miniwatt B4xx DHT series is microphonic sensitivity. PS HV filter with C input from Oil type 10uf with L1 (5h) C2 L3 (5H)C4, blader resistor 100K 2 watt on C2.
Using a line stage transformer is good with the impression of a wide and deep stage output and linear details without coloring. But the compensation from that there is a decrease in amplication, B405 with mu 5 and OTL 10K/600 becomes small, like a buffer and this needs to select the appropriate high impedance input of Amp.
The selection of preamp input with AVC and TVC (usually 10K) looks good (Sakuma San design), initial schematic design with AVC parafeed is difficult to do, but with TVC it becomes an interesting thing to try!
Using a line stage transformer is good with the impression of a wide and deep stage output and linear details without coloring. But the compensation from that there is a decrease in amplication, B405 with mu 5 and OTL 10K/600 becomes small, like a buffer and this needs to select the appropriate high impedance input of Amp.
The selection of preamp input with AVC and TVC (usually 10K) looks good (Sakuma San design), initial schematic design with AVC parafeed is difficult to do, but with TVC it becomes an interesting thing to try!