Hello Uncola,
Thanks for doing that testing
l have also been doing some sort of testing and researching. l have researched on HPs amplifiers, noise, hum, and all that's involved. Unsurprisingly, l found that when it comes to noise/hum/hiss, there is one variable that is always involved: sensitivity. So, l went back to look at my HPs, and checked the hum coming from them. This would be the order, from more hum to none (please, pay attention to the sensitivity levels):
1- AKG K550 (
114db / 32 Ohms)
2- Sennheiser HD558 (
112db / 50 Ohms)
3- Audio-Tehnica PRO700MK2 (
106db / 38 Ohms)
4- Philips Fidelio X1 (
100db / 30 Ohms)
5- Beyerdynamic DT770 (
96db / 250 Ohms)
6- Hifiman HE400 (
92.5db / 35 Ohms)
As you can see, there is a negative correlation: as the sensitivity diminishes so does the hum. Now, more interestingly, l happened to find out just by chance (l felt like a scientist making a casual discovery) the following:
l was testing the Audio-Technicas with the normal cable on them (not the coiled one these HPs also provide). Normally, l use the 1/4 adapter that came in the box with the HE400s. Today, given the chaos l had on my desk (all the HPs + cables + adapters around) l reached a different adapter (the one that came with the Audio-Technicas). l couldn't believe my ears (l asked my wife to check it out, too!): the hum was gone. Zero (¡Nada!). Unbelievable! Just because l changed one 1/4 adapter for another!
Thus, following the list above, the Audio-Technicas (using the 1/4 adapter they provide) and the Hifimans do make
no noise/hum, at all.
No need to say that after the Audio-Technicas 'discovery', l went crazy trying all HPs again with this (supposed to be) 'magic' adapter. l also tried different cables with, unfortunately, same results. The Audio-Technica's 1/4 adapter only makes the difference with the Audio-Technica HPs. Don't ask me why
Changing topics and answering to your question:
1- l am not sure whether l am using the right RCA outputs or not. Usually, l connect one of the D2's analog out (listed on the device: 'Normal' - 'TubePreOut') to the only A1 analog in. lt appears that by connecting the D2's TubePreOut to the A1's analog in, l would be 'double-tubing' as you mentioned. However, the sound is practically the same (if not absolutely the same) whether connecting the A1 to the D2's 'normal' or 'TubePreOut' analog out.
Moreover, l know that the hum comes from the A1 only. l switched off the D2 (and disconnected the RCA cables from the back of the A1, too). Then, l connected the A1 via 'line in' to my phone/Fiio X3: same results/hum. Besides, l tried different plugs in the house. Same again.
Finally, l tried something very simple: l simply switched off the D2, while the A1 was on. The hum continued to be there.
So no, the D2 is not the problem, it is out of the equation
lt's seems like the A1 is a 'humy' one (if you allow me to make up a new word for your English dictionary
)
Maybe we get lucky and one of these super knowledgeable Head-fiers happens to read my ignorant posts
, and decide to take my ignorance to an end, once and for all. Maybe even Ryan from Maverick Audio reads these posts, and decide to give us a much appreciated hand... Who knows?
Anyway, enjoy the weekend!