Hi All!
Earlier today I lurked through the last 40 pages of this thread because I'm really interested in the A-100. After that, I pulled the trigger and picked one of these up on Amazon.
Like many other posters I was struck by Mr. Zeos's enthusiasm and it seemed to do him well with the Argons, which I've ordered. I think I need to stop watching his channel. Aside from Z-reviews, one of the things that pushed me over the edge on buying this was seeing the designer chip in on this thread. In my limited experience it's pretty rare to see THE hardware designer come in and offer up info on the design and insights on how to use it. So cool!
Anyway, I figured I'd share some of my thoughts on the amp. Mainly about the noise, as to me this is a defining factor on how useful this is as a headphone amp. It sort of makes sense to me why there could be noise with jumpers placed. AND, since I've had two fingers of Arberlour, I'll type out how I see things... please excuse egregious grammatical or mathematical errors or if I'm being too obvious or pedantic...
The SNR on this thing is pretty darn good at 110dB (at full scale output... sqrt(50Wrms*8Ohms)=20Vrms). However, if you're just driving some 16 ohm IEMs your true "full scale" may very well be less than 1Vrms (50-ish mW). Since you're making use of only 5% of the actual ~20V full scale (or at most 20*log10(1/20) = -26dB) your relative SNR will be closer to 110dB - 26dB = 84dB. The T50RPs, being beasts at 3W / 50ohm, would use up more like 12V. Assuming you REALLY enjoy listening to them at MAX volume, you'd be making use of 60% of the amplifier headroom (only ~4.5dB down from full scale) and your relative SNR would be up to ~106dB. I think a lot of this is dependent on how YOU hear, but, loosely equating lower effective SNR to higher amounts of audible noise, you should hear more noise with more sensitive and lower resistance headphones... that is, if you hear any noise at all. This also explains why adding series resistance decreases noise as you'd be using more output voltage.
Luckily for me, the A-100 is going to be pulling double duty and its day job is going to be driving some Klipsch RP-15ms... if the hiss is audible w/my headphones, I'm still getting at least 50% of the intended use out of this thing. I will post again in a few months with my actual experiences with the A-100 when I've finally got those lovely Argons in-hand.
Ramble over. Glad to be on the forum!