Saturday, May 23, 2009

The Homebrew 6DJ8 Single Tube Headphone Project.

New beginnings, I am embarking on the construction of a 6DJ8 Single tube headphone amplifier and will during the course of construction add photos and description on the work and components. This is a tube hybrid amplifier. The first set of photos are of the cabinet and the pcb board being measured and prepared for mounting to the cabinet. the cabinet is lovely DIY cabinet called "Oceania" which is designed for headphone amplifier projects. The power for the pcb is 24V DC (low voltage) and on board regulation is accomplished by the LM317 (in my case I have upgraded to Linear Technology LT-1085). The volume use in this instance is a Blue Alps 2 x 100K A pot.

The power supply used will be a 24 V switching power supply and will be outboard and not housed in the same cabinet.

Stage 1 ( Cabinet & Hardware):

Above: The PCB for the amp layed in the bottom cabinet being measured for placement.

Above: The 6DJ8 Headphone Amp. front panel facia.

Above: The top half of the slide on cabinet.

Above: The hardware, knobs, feet and on/off button.

Stage 2 (PCB population):

Stage 2 is the population of components on the PCB, the following photos show the speed of which the components have already become populated.

Sorry for the quality of some of the photos, the digital camera will need to be replaced at some stage!

Above: The front panel beginning to take shape, the phone socket fitted, the Alps volume pot fitted and a nice custom made Goncalo Alves Wooden 1/4 gloss finish knob fitted.


Tube Socket: Gold Plated / Ceramic Tube Socket for 6DJ8

Tube: NIB NOS Tungsram 6DJ8

Regulators: LM-317 - Replaced with Linear Technology LT-1085

Semicon.: K117 x 2, K310 x 2

2SK117 x 2 :
Field Effect Transistor Silicon N Channel Junction Type Low Noise Audio Amplifier Applications.

Toshiba 2SK117 Data Sheet PDF.

3SK310 x 2 : 2SK310 Silicon N-Channel MOS FET (High Speed Power Switching) - Hitachi Semiconductor.

Hitachi 2SK310 Data Sheet.

Resistors: Dale Vishay & Tokyo Ko-on

10 ohms x 2
13 ohms x 2

15 ohms x 2
35 ohms x 2
100 ohms x 2
220 ohms x 2
2K ohm x 1

2.2K ohms x 2
1M Ohms x 2 (1watt)

VR - Blue Alps 2 x 100K A

On board VR: 2 x 1K ohms Bourns

Fuse (pcb mount): Silver plated contacts


Musicap "Hoviland" 0.1uF PP x 2

220uF x 2 Black Gate 16V
1000uF Elna "For Audio" 50V
1000uF Nichicon 35V


1 x Headphone Chassis

1 x gold plated panel mount headphone socket
2 x gold plated panel mount RCA sockets

1 x 24 DC panel mount DC connector
1 x LED

1 x push button on/off for panel mount
4 x isolation feet
1 x 24 DC switching power supply housed seperately
4 x heatsinks for semicons

The Finished Project:

I finally finished the project on 26th June, 2009 and here are the photos of the completed single tube 6DJ8 headphone amplifier which is up and running and using a NOS Tungsram tube:

The heatsinks were replaced on the FET's and regulators with larger ones, the amp does not run too warm and is quite cool. One drawback is the LED configuration (refer to the circuit diagram), this is a bad design to be taking voltage for the LED from this location as it does cause oscillation when powering up the amplifier and you "Must" not plug in the headphones as you will hear a high pitched whinning sound. Only when the LED is fully lit and the circuit becomes stabilized should you plug in the headphones. The designer should rethink about the LED issue! I am using an off the shelf 24V DC (2amp) switching mode power supply which is available everywhere (ebay) and it is very quiet, the supply is not housed in the same cabinet and is external to the unit to minimize on any further noise. Of course you may choose to use a linear supply if you wish.

Dynamics and transients are extremely good, bass impact is extremely good and mids-highs are well balance with high resolution. The NIB NOS 6DJ8 from Tungsram is an extremely high resolve tube and well suited to the amplifier. The sound stage is extremely deep and location of musical instruments is very precise. I expect things to further improve as the Hoviland "Musicaps" begin to burn in, since I have currently about 50 hours of time on the amp. One can hear the changes each time, the bass appears to be becoming tighter and better controlled.

Linear Technology LT-1085 regulators will be a night and day difference over the original LM-317 regulators and my advice is not to skimp on these regulators and change them to the Linear Technology regulators! You will have a much faster amp and the difference is stunning to say the least.

I have used
Musicap "Hoviland" 0.1uF PP x 2 caps. as I have found that these give the warm SQ that I want from this tube. Other caps used ar Black Gates and Nichicon, you must experiment with caps as these will alter the SQ signature of the sound. Overall, it is a very quiet amp and audio qulaity is very good once completely warmed up. The circuit board can be found on ebay and sells for about US$8.00.

I have received enquires about the cabinet and volume knob, the cabinet is off the shelf made for headphone amplifiers and can be found on ebay, whilst the
Goncalo Alves Wooden 1/4 gloss finish knob is a custom turned knob and is a once off item.

No comments: