I did some unscientific listening tests and the 2520 and SC25 do sound a bit different to my ear. I looked as some frequencey curves while I listened back and they look remarkably close to identical, but while the 2 2520s will almost completely phase cancel with my matched MXL 603s they didn't come close to canceling with 1 2520 and 1 SC25.
The difference in the curves seems to be in the roll-off below 63 Hz. The 2520 seems to slope a little steeper for a higher overal reduction (not by much though). Funny, however, that the 2520 seems to have more pronounced low end above 63 hz, so it sounds fatter. The SC25 seems to have a more pronounced upper-mid range than the 2520 which makes it sound clearer and more present...but not in the highs like the N72, it is definitely in the upper mids. I want to say around 3.5-4.5k but I can't really tell by looking at the frequency graph (Waves PAZ). I need better frequency measurement equipment to get a better visual. I think the SC25 might help a solo instrument poke out of a busy mix a little better than the 2520. This is speculation though, since my tests were quick and not in the context of a mix.
So the 2520 (at least the old model) and the SC25 certainly offer different flavors. I can't imgaine why they sound so different though, and look so close to identical on the frequency plot. I guess my ears are getting more sensitive to these types of differences. That, or the frequency plot isn't worth a damn. I am thinking of keeping the 2520s loaded and building another A12 pair for the SC25s.
This says a lot about the flexibility of the a12 as far as color goes. You could probably have a full rack of A12s and have 4 completely different sounding pairs! I imagine I will find the same true of the J99 when I get it.
Shane
Update - I did some tests with Right Mark analyzer, although I'm not sure how my setup affected the accuracy. I simply ran the balanced out of my RME into the mic in of the A12s. I had to turn the output fader on the soundcard to near it's minimum position to get the levels low enough. i am not sure if the impedance difference will skew the test considerably. The noise figures are almost completely useless since I was certainly amplifying the soundcards noise a far amount.
the curves do look REMARKABLY close, but not identical. The 2520 is about .05 dB hotter up to 1kHz, and then the SC25 takes over. The SC25 is about .05 - .08 dB hotter up to about 12 khZ where it starts to become about .1 dB hotter. Both taper off considerably after that and go about 3dB down at 35kHz. There is a noticable bit more harmonic distortion in the 2520 than the SC25 (about 25-30% on average) but the peaks are all at the same points. I imagine the SC25 is probably built around the later model 2520, and is probably pretty close to an exact clone if there is such a thing. I certianly feel they both have value, but after listening to an acoustic guitar track for a while I began to prefer the SC25. It seemed clearer and more focused. It lacked a bit of low end that the 2520 had, but it only seemed to add a bit of mud so it was not missed. The SC25 certainly had enough low end of it's own though.
Both Op Amps produced the most pronounced peak at about 15hz. There is a huge bump there, probably from the transformers and not the op-amps though. I wonder if this affects things in the audible range? Could it help add punch somehow?
Funny, in headphones there seemed to be some annoying upper mid frequencies in the SC25 track that were missing from the 2520. In my monitors this was not the case (Audix Studio 1a).
For kicks I compared the A12 with the 2520 to the N72 1272 clone. I was surprised to see that the 1272 drops about a decibel at 20k, with the slope starting around 16k. The 2520 held flat all the way up to 35k where it was opnly down a few dB. I always thought the N72 sounded like it had more high end than the A12, but I'll chalk it up to harmonics. Anyone care to help explain?
Shane