This is a headphone amplifier with an integrated DAC (Khadas Tone Board). The amplifier circuit is designed by Wayne Colburn (Pass Labs) and was generously released for free to the DIY audio community via the DIYAudio forum. PCBs are also available via the DIYAudio Store. This design incorporates a dual op amp which is biased to run in class A.
I came across this project on the DIYAudio forum and though it would be interesting to build a solid state amp to compare with the OTL tube Headphone Amplifier I had built previously.
I ordered the PCB from DIYAudio and the rest of the parts from a combination of Mouser, Digi-Key and other local retailers. Once I had all the components in hand I populated the PCB and built temporary fixture to mount everything on. It’s always nice to be able to listen and test the electronic side of things before committing to the complete build.
On first power up I found that there was a loud hum/buzzing, the volume knob was particularly sensitive to touch. In the end, with the help of the forum I tracked it down to a grounding issue. An eyelet around the volume pot mounting boss connected to a circuit ground point was enough to ground the pot body and solve the issue.
My amp was built in the “LED reference” configuration.
As recommended by many builders on the forum thread, I used an 8-pin DIP socket to mount the dual op amp. The leads of the adjacent capacitors were also left slightly long so they could be leant to the side, allowing the use of larger op amps. In regards to op amps, I started with the standard recommendation which is a Texas Instruments RC4580. In the photo below you can see a “Burson V6 Vivid” installed, which is a discrete op amp and a significant step up from the RC4580. I am currently listening to the “Burson V6 Classic” version and think I prefer it to the Vivid, both of these op amps were provided to me by Burson for testing purposes. If I can convince myself to make the investment, I would like to try the OPA627 which is apparently one of the top choices for this amp. Part of the fun of this amp is trying out (rolling) different op amps, much like the joy of “tube rolling” in tube based amps.
After running it for a while I noticed that the regulators were running quite hot. I took some key voltage measurements which can be seen marked in red below. As an electronics amateur I relied on the helpful DIYAudio forum members for advice. The general consensus was that because I used a transformer with 22V secondary outputs, my voltages ere on the high side, but still ok. I could change to the “naked regulator” configuration to bring the voltage down to 15V but I decided to leave it as-is for now.
I did however take the precautionary measure to order the tallest heatsink which would fit in my planned enclosure. Seen below on the left is the heatsink specified in the standard BOM, on the right is the taller one I ended up using (31% taller).
Some preliminary listening, via the ODAC.
Once I was satisfied with the electronics side of things my attention turned to the enclosure. I carefully modelled all the key parts in CAD and began the construction.
I had this plank of walnut on hand, so I milled it down to the right dimensions and ensured that the mix of darker heart wood and lighter sap wood would be orientated in a pleasing way on the front face.
Using the table saw the plank was carefully cut into a mitred box whilst maintaining the grain orientation around all four sides.
As per my previous two amplifier builds, my CNC Router was employed to cut the detailed pockets in the front and rear panels.
The box was glued up and all the faces planed smooth and flat using my Veritas low-angle block plane.
Plywood was CNC routed and glued into the bottom of the case to mount the PCB.
Rear panel is cut from 2 mm aluminium sheet.
Top-plate is water-jet cut from 4mm thick aluminium plate.
A mounting plate which holds the Khadas Tone Board was cut from 1.5 mm stainless steel and then tumbled to break all the sharp edges which result from the water-jet cutting process.
Tapped and matching mounting holes drilled into the mounting rail. The mounting rails will be affixed near the top of the case and also supports the top plate.
This handy right angle drill attachment allowed me to drill the holes to attach the mounting rails.
Test fit of the mounting rails and DAC.
Rear panel was painted black and engraved using mu CNC router.
Rear panel was populated with input RCAs, pre-amp output RCAs, input selector switch, IEC socket etc. As recommended by the forum members, resistors were later installed in series with the pre-amp outputs.
The knob, dial ring insert and top plate components were prepared for anodising with a 220 grit bead blast.
The image below shows a top plate with brushed “scotch-brite” finish on the right and post bead blasting on the left.
Dial rings were laser engraved with radial graduations.
The flush-mounted, flat-faced power LED is one of my favourite details of this build. I took some leads from the forward most side of the unused R9 and R10 pads with a resistor in series to power the LED.
Finishing this project is bittersweet because on the day that my Father (Owen Young – DarkLantern) passed away we had arranged to meet and he was to borrow this amp. I know he would have loved to listen, evaluate and compare it to the OTL tube amp we built together last year.
I am very pleased with how this amp sounds, when paired with my HD6XX it is very enjoyable, detailed, spacious and with a lot of presence in the mid and low end when compared to my JDS Labs ODAC/O2.
Please enjoy the photos of the completed project below.