Recently I reviewed the Nord Three SE 1ET400A Dual Mono Stereo Amplifier . That unit came with the Sonic Imagery's 990ENH discrete operational amplifier ("opamp") in the buffer stage (pre-amplifier). The owner then ordered a set of Sparkos Pro SS2590 discrete opamps. This gave me a chance to compare the two in the same platform.Alas, this analysis while of some value, won't be conclusive as power amplifiers can have temperature and run to run variations. Still, the Purifi 1ET400A modules are very low distortion so the data is more likely to be valid than not.The opamps are socketed in the buffer board provided by Nord making swapping out easy. I especially appreciated the beefy gold coated pins which made it easy to pull them out and insert. Here is the original configuration with Sonic Imagery 990ENH:I like that the Sonic Imagery Labs parts come fully enclosed. The Sparkos in contrast, are bare PC boards:The Sparkos Pro opamps cost US $59 from the company. Sonic Imagery doesn't show the prices but I found some on ebay for US $69. Seeing how four of them are used in this amplifier, the premium is fair bit for both.Let's start with our dashboard. First, the Sonic Imagery 990 ENH from last review:Notice the lack of harmonic distortion to the right of our 1 kHz tall tone. Now with the Sparkos SS2590:Our overall SINAD remains roughly similar because it is noise dominated. On distortion front though, the SparkOS has fair bit higher distortion (> 10 dB). Whether this is run to run variation, I can't say for sure but seems to be too big of a difference to be so.Noise performance is the same for both:We had fair bit of channel inconsistency with respect to THD+N with the Sonic Imagery 990enh. Let's see if that has changed with Sparkos:It has indeed. The two channels (brown and red) track each other much better now. But the best channel (red) is worse than the best channel with Sonic Imagery (light blue).Here is the same into 8 ohms:Similar picture emerges here. More importantly, both sets of measurements underperform the Purifi's own reference design (in dashed green). Hard to say if that is just due to OpAmp differences. Directionally it indicates so.This is not the most precise study of the differences between these two sets of discrete opamps. If taken at face value, the Sonic Imagery 990 enh Ticha seems to have lower distortion, but higher part to part variation than Sparkos SS2590. Both appear to lose to stock Purifi implementation so not sure either is worth any kind of premium.The differences are very small regardless so I would not sleep over this choice one way or the other. From what I see, Nord only offers these two choices and the Sparkos is the cheaper one.--------As always, questions, comments, corrections, etc. are welcome.This is a killer Christmas season. I have spent more money this year than any other and by far. So I appreciate you allusing : https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/