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Hybrid
Tube Headphone Amplifier

Hybrid Tube Headphone Amplifier

Here's
a simple to build hybrid tube headphone amplifier built
around 12AU7 / ECC82 vacuum tube. I have always been
intrigued by tube amplifiers, but most DIY kits are very
expensive and use very high voltage. So I decided to
build an amplifier that would be inexpensive and had the
least amount of parts necessary to drive a pair of 32
ohm Grado headphones. Having built several YAHA amps
based on the fa-schmidt design, and a Szekeres Mosfet
follower I wondered how the two would sound together. So
I built the schematic into TINA-TI, a free spice based
program to test circuits before the build, and the
results were remarkable. Nearly 20dB of gain across
20Hz-100kHz from a 13VDC power supply. As you see in the
schematic and parts list, there are less than 30
discrete components and most DIY'ers will have them as
spares from other builds. I chose the 12AU7 / ECC82
vacuum tube because it can be driven with low voltage
and the filament voltage is 12.6 volts, so there is no
need to regulate the voltage any further. I used 1/4W
resistors in the first stage and 2W in the second. The
2W resistors may be overkill but I did not want to
change them later. The 20ohm resistor must be a minimum
of 5W and do not use wire wound, as the inductive
characteristics will distort the response curve.

Figure 2: 12AU7 Tube / IRF612 Mosfet Headphone
Amp Schematic
Table 1: Parts List - 12AU7 / IRF612
Headphone Amp

100k
100nF
100nF
1M

1uF
20ohm 5W
200 ohm 2W
220k 2W

470uF 50V
47k
5k 2W
12AU7 / ECC82

IRF612
LED Holder
LED
9-pin Tube
Socket

Additional Notes - Parts List:

Heatsink must dissipate around 3W so 2.5-3 square
inches, or less if you use a fan. Use heatsink paste and
mica insulators as well as insulated washer. The tab of
the Mosfet is at (12-13vdc)
Chassis of your choice,
plan for ventilation.
13VDC switching power supply,
can be between 12 and 13 volts to heat the filaments.
Power jack is a generic DC jack from Radio Shack.

The MOSFET (T1) can be replaced by an IRF510, IRF610
or IRF611.
Construction - Tube / MOSFET Hybrid
Headphone Amp
For the enclosure I used a Lansing
MicroPak "C"; which I found on eBay for approx $8. This
item was an overstock of some surround decoders and came
with RCA connectors and pre-punched faceplates. The tube
socket should be a 9 pin, this one was for older pixie
tubes so there are more holes, but the 9-pin match
perfectly.

Photograph 1: Lansing MicroPak
Enclosure and PCB
I used RCA connectors for input
and a 1/4" jack for an output, this was to accommodate
my Grado SR125's and the holes in the plates were
punched for a 1/4" jack.
The PCB had pre-existing
solder runs and a ground bus so this dictated the
location of the components If you build this on proto
board make sure to leave room for the heatsink and the
20 ohm resistors to dissipate heat. Make sure to
insulate the Mosfet from the heatsink with mica, and
also use heatsink compound for better heat transfer. The
tab of the Mosfet is at 12-13 volts and will destroy the
power supply and component if it shorts to ground.

Photograph 2: Construction of Hybrid Headphone
Amplifier
After all of the components were soldered
I smoke tested the unit and made any necessary
corrections.

Photograph 3: Tube / Mosfet Hybrid
Amplifier Smoke Test
The tube heater draws 150mA at
12.6 and the whole amp draws 580 mA at startup and
stabilizes at 550mA after the tube warms up; which takes
a few seconds. I used a Canon AD-360U switching power
supply from their small bubble jet printer. Since the
drain of the Mosfet is connected directly to the
positive rail any noise will be amplified. This power
supply had virtually no noise.

Photograph 4:
Hybrid Headphone Amplifier - Front View

Photograph 5: Hybrid Headphone Amplifier - Rear
View
I used a 12V muffin fan dropped to 9V to cool
the amp, two holes in the top allow air to rush in and
the existing holes in the front and rear plate allow the
air to flow well.

Photograph 6: Tube / Mosfet
Hybrid Headphone Amplifier
Measurements - Tube /
MOSFET Hybrid Headphone Amp
Here are a few scope
shots of the 10 Hz sine wave response, as well as the
square wave response at 100 Hz. The performance was very
consistent and the voltage stayed the same over the most
of the sweep. The TINA curve shows approx 19dB of gain
over the spectrum of the audible range. The sound from
my Grado SR125 headphones is crisp with a tight lower
end. This amp is very nice considering the low voltage
and low component count. It may not be true audiophile
quality, but the average cost is only $40 USD or less.

Photograph 7: 10Hz Sine Wave

Photograph
8: 100 Hz Square Wave Response
This amp is perfect
for the novice builder and the components are available
at mouser, digikey. You can substitute other Mosfets as
long as the resistance is similar and the internal
capacitance does not effect the response curve. It is
best to use TINA-TI to build the schematic and make any
changes, this way you can check the ac output before
building. This will save you much time.
Lastly, make
sure that your headphones are not plugged into the jack
while powering up or down, this amp as well as other DIY
builds have a large rush of current at power up and down
and damage can occur to you headphones if you are not
careful.
I am sure you will be pleased with the
sound of this little amp and better yet the price.

UPDATE 28 August 2008: I have built another version
of this amplifier which is smaller, has less parts and
runs off of a 12V battery. For full details, see the
12AU7 Tube / IRF612 MOSFET Hybrid Headphone Amplifier
update on the DIY Audio Projects Forum.

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