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DIY 6DJ8 (ECC88) Tube Headphone Amplifier

Bruce Heran

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Low Power 6DJ8 (ECC88) Valve Headphone Amplifier

with No Coupling Capacitors and No Negative Feedback

This is a project that has roots to back about 2 years ago. I wanted a
quality headphone amplifier to be an addition to my Hi-Fi stereo system. It was
to be something to use when I didn't want to disturb everyone else in the house.
I tried several different tube types in the amplifier design process. I used
6J6, 12AU7, 6SN7, 6CG7/6FQ7 and 12AT7. Generally all worked, but none seemed to
have either the needed output level or didn't match well (likely to the choice
of transformers) and sounded on the dull side. The 6DJ8 (ECC88) dual triodes
worked like a charm. They had the combination of sufficient current flow,
modest voltages on the anodes (important for the matching transformers) and
enough gain to make the amplifier sing. The next best tube type was the
6CG7/6FQ7. Almost as good, but they are getting a bit scarce and the new
production JJ E88CC are now quite common and reasonably priced. I am sure other
triodes could be used and there is no reason why a high impedance ( on the
primary side) small push-pull output transformer would not work. I doubt that
you can find one much cheaper than the matching transformers I used. Certainly
not with similar response and distortion characteristics.

Figure 1: 6DJ8 (ECC88) Vacuum Tube Pinout Diagram

Building the 6DJ8 (ECC88) Tube Headphone Amp

This low power tube headphone amplifier is designed along the lines of the Oddwatt power
amplifiers, except that this headamp does not require a driver stage. All
the circuit gain is in the single output stage. Once again I used a constant
current source (CCS) IC in the cathodes but since only low power levels were
needed there is no provision for balancing the load between the two triode
sections. None was needed either as the output is quite symmetrical to levels
way above anything you could use with headphones. I wanted simplicity in the
circuit. It has neither coupling capacitors nor any feedback loop. In fact
there are a total of only 7 components including the tube in each channel. The
power supply, alas, has many more components. This is common in many of my
designs. I find that a very clean power supply is essential to achieve high
quality audio performance.

Figure 2: 6DJ8 (ECC88) Tube Headphone Amplifier
Schematic

Approximately 100 millivolts (mV) of signal input will result in about 50
milliwatts (mW) of output power. This is much more output than is needed for
really excessive sound levels with most headphones. The audio output
transformers used are Edcor
XSM10K/150. These transformers are designed for line level matching and are
rated at 2.5W with a center tapped primary impedance of 10k and a center tapped
secondary of 150 ohms. These impedance values lend themselves for use with many
Hi-Fi headphones. While there is some loss of efficiency in using headphones
below 32 ohms on the 75 ohm tap there was minimal effect on the quality of the
sound. For use with higher impedance headphones you can use the Edcor XSM10K/600. The primary impedance of the Edcor
XSM10K/600 is 10k with a 600 ohm center tapped secondary that is ideal for use
with 300 ohm and 600 ohm headphones.

The power supply provides direct current (DC) for the tube heaters and a well
filtered B+. I believe that using alternating current (AC) on the tube heaters
would also be just fine, but since I have the PCBs and components to get the DC,
that is what I used.

Figure 3: 6DJ8 (ECC88) Valve Headphone Amp Power Supply
Schematic

Aside from the Edcor transformers, most of the required parts for the tube
headphone amplifier project can be sourced from the likes of Parts Express and MCM Electronics, among
others. To help you along with the construction of the tube headamp here are
some suggestions for design and construction tips for vacuum tube
amplifiers and the tube amplifier wiring color code I use.

Warning: This tube headphone amplifier project uses high voltage in
the 250 to 270 Volt (V) range. Contact with voltages of this magnitude can
cause serious injury or possibly be fatal. If you do not know how to build
projects that have these potentials or you are not comfortable in with projects
that use these voltage levels, I strongly recommend you do not built this
headphone amplifier. As always, please follow your governing electrical code
for all wiring and connections.

Construction of the headphone amplifier is straight forward and
uncomplicated. The layout of the circuit is not critical as the amplifier gain
is relatively low. The PCB and chassis you see in the photographs are left over
parts from Forewatt
12AU7 Tube line stage preamplifier prototypes. Being somewhat frugal, I
tend to use parts that are available. The 1 uF and 0.1 uF capacitors in the
circuit should be "poly" types. The 0.1 uF capacitors across the tube heaters
can be low voltage ceramic types. For best performance the 1 uF capacitor shown
on the amplifier section near the audio output transformer should be physically
located close to the actual B+ connection on the audio output transformer. You
can use the standard size of LM317
voltage regulator in place of the LM317L which comes in a TO-92 transistor
case if you wish. The power actual dissipation is rather small at about 75
mW. The 7806 voltage regulator used for the heater circuit should have a small
heat sink as it will be dissipating a little over one Watt if you use the Edcor
transformer shown in the schematic. I recommend against using standard type
rectifiers in the HT (B+) power supply. The ultra-fast UF400X series rectifiers
work far better and generate considerably less noise than the 1N400X types do
and are nearly as cheap. The bridge rectifier in the heater circuit can however
be a generic one. As always, be careful not to create ground loops. For more
informations about grounding, see my article about grounding
and shielding for your DIY audio projects.

Photograph 1: 6DJ8 (ECC88) Valve Headphone Amplifier -
Inside

Performance - 6DJ8 (ECC88) Tube Headamp

The amplifier will deliver over 100 mW of output into pretty much anything
from about 32 ohms to 300 ohms. With the Edcor XSM10K/150 audio output
transformers the optimum match is at 75 and 150 ohms. Using headphones with an
impedance below 32 ohms will result in decreased performance as the load on the
tubes will be below the best range. Above 300 ohms it will also be less than
the best match, but many phones in that range seem to be more sensitive than
those with lower impedance and it may not matter. The frequency response at the
50 mW level is from 20 Hz to 28 kHz within 0.5 dB and the wide band
signal-to-noise is at -84 dB. I didn't run a full distortion series on the
amplifier as the output levels are rather low (voltage wise) and the ambient EMI
in my shop would tend to confuse the results. On a spectrum analyzer I was able
to get as much as 2 volts output before there were obvious harmonics appearing
on the display. At more typical listening levels (about 0.1 to 0.2 volts into
100 ohms) there was no sign of any harmonics above the noise floor. I would
conclude from this that the actual distortion levels at such levels are well
below 1%. Power consumption of the headphone amplifier is approximately 20
Watts (W).

The testing equipment setup used to measure the amplifier performance
consisted of:

HP 331A Distortion Analyzer

Tenma 25MHz Dual Channel Digital Storage
Oscilloscope

Velleman Dual Channel USB PC
Oscilloscope

Tenma Low Distortion Signal Generator (0.05%
residual THD)

APC H10 Power Conditioner (120VAC output)

Several digital voltmeters

Photograph 2: 6DJ8 (ECC88) Tube Headphone Amplifier

Sound - 6DJ8 (ECC88) Tube Headphone Amp

So to the important question - How does it sound? The sound in short, is
excellent. With my heaphones - Koss Pro4AAT (250 ohms) and Sennheiser HD-280
PRO (64 ohms) there was no audible noise or hum and the response was rich and
clean. I won't give up my electrostatic speakers and other gear to swap for
headphones and the headphone amp but it sure is nice for those occasions when
private listening is needed. The quality of the sound will depend a lot on your
choice of headphones. The amplifier can not make cruddy headphones sound great
but will sound great on quality phones. I found the sound particularity nice on
my Sennheiser HD-280 PRO headphones. They have a rather neutral overall sound
and a lot of detail. I have used them on other sources, most notably with my
OPPO BDP83SE Blu-ray player. For those of you unfamiliar with the OPPO player,
it earned a class A rating in Stereophile for audio performance. When I feed
the output of the OPPO player into the headphone amp I can not really tell that
the sound has changed in any obvious way. My conclusion is that the amp is
pretty linear and clean. There is no hum and noise with the Sennheiser
headphones which are rather sensitive. Solid bass, clean mids and highs, nice
detail and obviously wide sound stage. I actually considered adding a blend
control as I prefer some channel mixing similar to what you would have with
speaker use. There are plenty of tube options as the tube headphone amplifier
should work well with most the 6DJ8 / ECC88 and 6922 / E88CC tubes.

I hope you enjoy this headphone amp project as much as I have. Comments and
questions about this project are welcome in the 6DJ8
(ECC88) Tube Headphone Amp Project thread.

Good listening, Bruce

About the Author

Bruce Heran is the VP for Design and Support of Oddwatt Audio. He has been involved in the field of
electronics in one form or another for nearly 50 years. Bruce is a strong
supporter of DIY audio projects and generally specializes in vacuum tube designs
for Hi-Fi. One of his main goals is to educate and encourage new comers in the
building of affordable high performance audio equipment.

Last update 9 September 2014. Created 24 November 2012. © Bruce
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