The Sennheiser HD 650 is a great headphone with a little bit of sonic ‘magic’ appeal. The attractively priced MassDrop HD 6XX version is a special bargain.
A popular and nice match for the Sennheiser HD 650 / HD 6XX is a vacuum tube headphone amplifier.
The project here was built by Jeremy Young. It was inspired by the Bottlehead OTL headphone amp, but circuit ‘operating points’ were determined to suit a locally made power transformer.
The Output Transformer-less (OTL) topology is attractive cost-wise, however a big DC-blocking capacitor is required at the output. In other words, there is no ‘free lunch’ !
Similar tubes are used, a 12AU7 twin-triode and a 6080 triode, direct-coupled.
The tubes that we had to hand were a GE 12AU7 and a GEC CV2984 (6080).
This is the (Sennheiser) Massdrop HD 6XX ‘open back’ headphone:
Jeremy’s concept model for the headphone amp chassis:
Jeremy decided to make a Walnut wood case with ‘dove-tail’ corner-jointing.
Japanese ‘pull-saws’ were used to cut the dove-tails.
A DIY CNC router was used to cut holes and pockets for the volume pot, headphone socket and rear panel.
Finally, glued together (with Aliphatic Resin adhesive), hand-planed smooth and finished with mineral oil and wax.
CNC router cutting and glue-fixing a plywood inner-frame to support the aluminium double top plates:
The lower top plate to support the power transformer is 6mm thick aluminium. The rear plate (and a spare) is only 1.5mm thick. Both were water-jet cut at Jeremy’s work-place:
Assembly at last….
Wiring it up (the fun part)….
0.7-0.8mm untinned solid core wiring was used throughout, except stranded wire for mains power AC. Thick 1.6mm diameter solid copper for the Ground bus.
UF diodes, Panasonic TSUP filter caps are used. Resistors are mostly Mills non-inductive wirewounds. The Output capacitor initially will be just a generic 100uF electrolytic bypassed with a 1uF metallised polypropylene cap. Volume pot is the popular ALPS RK27.
Initial heater and HT voltage checks:
Listening test passed !
2 more cosmetic items to complete – (i) top cover plate (ii) power transformer cover.
The top cover plate being cut on the CNC router:
Transformer cover was milled from solid 6061-T6 aluminium. (Done on a 5-axis milling machine at Jeremy’s workplace engineering shop.)
Spray-paint finished with Rustoleum ‘Flat Soft Iron’ colour:
A Khadas ‘Tone Board’ (ES9038Q2M) DAC was chosen to process USB digital signal input :
Testing the Khadas DAC board:
A tight fit inside the chassis, the DAC board is mounted on an aluminium side-bracket (which will also assist as heat-shielding) :
The finished headphone amp:
The sound:
The Sennheiser / Massdrop HD 6XX has high input impedance, around 300 ohms, which allows a tube (typically highish output impedance) headphone amp to be used.
The HD 650 / 6XX have a nice, open, warm and ‘relaxing’ sound. Initially, they may sound too ‘laid back’, lacking some ‘bite’ and ‘punch’. But they are rewarding for long-term listening. Subjectively ‘natural’ (low distortion), ‘revealing’ without being ‘analytical’.
Low bass & upper treble is a little rolled off and a tube amp may not help too much here. However, the tube amp has an attractive treble tone, which suits the HD 650 / 6XX.
With the ‘see-through’ transparency and ‘spaciousness’ of the tube amp, this is a nice combination.
Initial impression is that the Khadas DAC is tonally ‘neutral’ and high resolution. Bass seems strong and treble is extended, without harshness. These strengths help to make a suitable match with the HD 650 / 6XX.
After more hours ‘running in’ we will try some different tubes and fine-tune other components, such as the sonically important output capacitor/s.
PS. This circuit inverts phase. So it would be good to have a balanced output connection, so that phase could be corrected again.
One of the disadvantages of OTL connection is that absolute phase cannot be easily reverted at the output of the amp.
You can see more detail about Jeremy’s project here at Jeremy Young Design.
D a r k L a n t e r n 
Absolutely fascinating and fantastic pictures. Thanks so much for posting.
Thanks John – glad you enjoyed.
A pic is worth a thousand words 😉
Cheers, Owen