The device works very well by default, lot of power, class-A output stage, very good looking, but some mods might improve it a bit, at least for my taste.
With sensitive cans there might be some background noise (I was using board revision v1.6, but with board v2.1 and higher the noise settled down a lot!), especially when using 16 Ohms IEMs, so a linear power supply to replace the built-in SMPS/DC-DC converters could be an answer to resolve this. Basically, this combo it's getting power from a 70W 12V SMPS external PSU, then 2 x DC-DC boost converters are creating +/-15V; also the initial 12V is lowered by linear regulators to 5V (several RC filter are in there as well).
1. LPSU on the bench (PCBs are from ANT-X from diyforum, just a bit modified to adjust the Japanese NJR 7812/7815/7915 regulators):
Now there's no more noise on the 6.3mm headphones jack, even if 16-Ohms IEM's are used, so getting rid of DC-DC boost converters and upgrading to linear PSU did helped here.
For board revision v2.1 or higher you might not be needing this LPSU-upgrade mod, as the new board is having 20 more ELNA caps and ground plane has been increased to about 100% of the board, so noise is minimised.
2. RCA output seems to be connected directly to output stage transistors. To further minimise the noise on the RCA outputs I was able to cut the PCB traces that go to the RCA plugs and add a couple of shielded wires directly from the Low-Pass Filter op-amp (the one from the middle), via additional resistors (anything between 10 -50 kOhm should do).
My cleanest and darkest headamp I have is the Objective 2; basically, with volume to the max, gain of 3.7X and 16 Ohms IEM's used I can't really hear any noise. Connecting the PLAY to the O2 didn't changed anything...same darkness. Of course, no music was playing, but PLAY was connected to USB of the computer.
3. Op-amp testing, besides the SS V5 and V6, I was able to test many more from the below pics. However, I choose AD8599 (or MUSES8920) for I/V and LPF and AD797 (or MUSES8920 with DIY dual-to-single adapter) for pre-amplification stage
The dual-to-single adapter is DIY, because I was unable to find such thing on eBay...not sure it even exists somewhere honestly.
4. Gain adjust pins/jumpers could be easily soldered on the green ovals. It used to be 220pF caps in there on a former board revision, but manufacturer decided to remove them, so paralleling the existing 2.1 kOhms resistors with something around 5 kOhms will lower the output volume enough to make the device much friendly with high sensitive headphones.
With sensitive cans there might be some background noise (I was using board revision v1.6, but with board v2.1 and higher the noise settled down a lot!), especially when using 16 Ohms IEMs, so a linear power supply to replace the built-in SMPS/DC-DC converters could be an answer to resolve this. Basically, this combo it's getting power from a 70W 12V SMPS external PSU, then 2 x DC-DC boost converters are creating +/-15V; also the initial 12V is lowered by linear regulators to 5V (several RC filter are in there as well).
1. LPSU on the bench (PCBs are from ANT-X from diyforum, just a bit modified to adjust the Japanese NJR 7812/7815/7915 regulators):
Now there's no more noise on the 6.3mm headphones jack, even if 16-Ohms IEM's are used, so getting rid of DC-DC boost converters and upgrading to linear PSU did helped here.
For board revision v2.1 or higher you might not be needing this LPSU-upgrade mod, as the new board is having 20 more ELNA caps and ground plane has been increased to about 100% of the board, so noise is minimised.
2. RCA output seems to be connected directly to output stage transistors. To further minimise the noise on the RCA outputs I was able to cut the PCB traces that go to the RCA plugs and add a couple of shielded wires directly from the Low-Pass Filter op-amp (the one from the middle), via additional resistors (anything between 10 -50 kOhm should do).
My cleanest and darkest headamp I have is the Objective 2; basically, with volume to the max, gain of 3.7X and 16 Ohms IEM's used I can't really hear any noise. Connecting the PLAY to the O2 didn't changed anything...same darkness. Of course, no music was playing, but PLAY was connected to USB of the computer.
3. Op-amp testing, besides the SS V5 and V6, I was able to test many more from the below pics. However, I choose AD8599 (or MUSES8920) for I/V and LPF and AD797 (or MUSES8920 with DIY dual-to-single adapter) for pre-amplification stage
The dual-to-single adapter is DIY, because I was unable to find such thing on eBay...not sure it even exists somewhere honestly.
4. Gain adjust pins/jumpers could be easily soldered on the green ovals. It used to be 220pF caps in there on a former board revision, but manufacturer decided to remove them, so paralleling the existing 2.1 kOhms resistors with something around 5 kOhms will lower the output volume enough to make the device much friendly with high sensitive headphones.