AnentHi Anent,
Thanks for the well-researched and thoughtful question. :)
As the product manager for this amplifier, I thought of the following criteria:
-Vanishingly low distortion that AAA tech is known for
-Usability from IEMs (low gain option) to more demanding headphones (2 more gain options with headroom for current-hungry planars & voltage-dependent dynamic drivers)
-Very competitive pricing
There are many who believe in the most transparent upstream gear possible...purists in the amplification sense. The objective measurements are the key to proving the purity of the amplified signal.
After that- power, input/output flexibility, single-ended plus balanced output, and low cost are the most important factors for our headphone audiophile community. Providing the target specification (amount of power plus features above) to THX, our recipe called for 788 performance (world class low noise, low distortion, low cross-talk) and 788 cost, but 888 power. Took some design tweaks to the circuit and a helpful partner to bring this new amp to the community. So what do we call it then? 789.
The AAA topology is still seemingly vague (though I'm not in the audio EE biz)....
How similar is the 789 to the following? (Yes, kind of apples to oranges, but....) Obviously swap out the Li-battery for the wall-wart. The 88 shows an earthed ⏚ analog ground -OR- should the circuit actually show a (floating) common signal ▽ ground point? https://www.triadsemi.com/product/tsthx88/ https://www.triadsemi.com/wp-content/uploads/dlm_uploads/2016/12/triadsemi_tsthx88_productbrief.2016-12-19.pdf
AND :) the THX patents have some interesting tidbits..... :) https://patents.google.com/patent/US8004355B2 https://patents.google.com/patent/US8421531B2