Hey Dan – this is really cool! Thanks for putting this up.
I believe there is a typo in Session 9 where you have the transformers listed as “Output”, but they are in fact “Input” transformers.
I was going to ask if each transformer was properly terminated and/or using the correct Zobel network, but after blowing up the pic, looks like this is on each board. I didn’t notice anything on the Lundahl board, though. I know the spec sheet says “No termination necessary” but when using dynamic mics, mine sounds best with around 65K across the secondary. Also, is the Lundahl wired for 1:2.5 or 1:5 ratio? How about the other transformers?
Another question – what is the discrete op amp circuit without and cover (it has "2" wrote on it)? Did you do any test with it or the Burr Browns?
I will add that, from listening to the acoustic guitar samples, it is interesting how different op amps react differently with the same transformer. For example, the difference between the Cinemag and the Sowter are night and day using the ADK op amp, but not nearly as noticeable using the vintage 2520. I would be very interested to here the other op amps with the Jensen, Lundahl, and Sowter transformers.
Good eye. All the mic input tx boards have a termination and/or zobel network in place.
I recently changed the Lundahl daughter board PCB layout to add a terminating resistor. If I recall correctly I added one "underneath" the pcb to the board in the picture.
