Following up on Part 2, it’s time to talk about the output stage.
This output stage is the brainchild of my friend Kostas, all I did was lay out the PCB.
It is a fully discreet single-ended class-A output stage, outputting ~2.4V RMS.
This is its schematic:
This is the PCB:
And this is the BoM: AK4490 dual mono DAC – Discreet Analog stage BOM (509 downloads)
The BoM includes part numbers for most parts from Mouser. The only parts that are not in production and must be found elsewhere are the UPA68H. Ebay is a good bet. Chances of getting fake parts are pretty small, but just in case do this to double check the ones that you bought: http://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post5330311.html (Thank you Salas for the info and the idea to use them in the first place!)
The only parts that need matching are T8A with T9A and T8B with T9B. There’s no need to go crazy with the matching – within 5% should be enough.
Power should be ideally +/-16VDC. A bit less is OK (I did my initial testing with +/-12VDC) but more will most likely damage the board. The board is running in class-A so current draw is constant. A power supply with 100mA current capacity should be enough.
Bias current is adjusted by the multi-turn trimmers R26A and R26B. They should be adjusted to their mid value before soldering to the board (~1K). To adjust bias just measure current consumption at one of the rails while turning the pot. Adjust for ~25mA total current draw per rail and per channel. Current draw on the negative rail should be about 1mA higher than on the positive rail. Bias should be re-adjusted if the power supply voltage needs to change.
After bias adjustment and with no input signal you should check for DC at the outputs. If everything went well you should be seeing anywhere between 0 to a few mV of DC voltage.