Originally Posted by lucifix
Thanks for posting your observations!
Anyway, since changing the AD797 opamps the noise floor is almost non-existent.
Do you experience hissing with dual AD797 opamps with your IEMs?
To confirm the noise issue once and for all and to find out why iBsso has choosen AD797 as default, I reinstalled two AD797 opamps in L/R sockets with two transistor buffers in buffer sockets and one default LM6172 in 3/4 socket. Note I need current output of transistor buffers to drive AKG K601 instead of using default dummy adapters. The gain remained as default 10dB high setting. iPod nano was connected to P3 through LOD all the time. The test was performed with my only IEM V-Modal Vibe (::: v-moda :::
) with sensitivity of 92dB.
With 9V DC power adapter as power source (all batteries removed) and iPod turned off, humming noise (rather than hissing) was prominent even more so when volume was increased from zero to full scale. With 6 AAA rechargable batteries as power source under same condition (iPod off and volume adjusted from minimum to maximum), no noise at all until iPod was turned on (in pause state) and volume was increased to full scale (hissing rather than humming).
I suppose the humming noise was caused by ground loop and hissing was caused by noise from iPod. If you touch metal casing of P3 with DC power adapter connected, you should notice increase of humming noise. When batteries are used as power source, no ground loop can be formed thus there was no humming noise at all. Hissing noise originated from iPod (turned on) was amplified by P3 when volume had increased pass 90% of full scale. But with sensitive IEM, no one can listen at such high volume. Further more hissing noise was not noticable when music was playing.
Thus conclusion is AD797 works fine with sensitive IEM when batteries are used as power source. The gain setting can be lowered to 3dB to minimize hissing noise originated from portable player. Gound loop (humming noise) is inevitable whenever 9V DC power adapter is used regardless of what opamps are installed. You may want to hunt for 9V DC power adapters designed to break the ground loop at outputs. I would rather have iBasso not bundle any power adapter of questionable quality and lower the price of P3.