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Class-A
12AU7 Tube Headphone Amplifier

Class-A 12AU7 Tube Headphone Amplifier

This is
simple to build audiophile class-A tube headphone
amplifier. It is based around 12AU7 / ECC82 audiophile
vacuum tube that provides warm, rich and smooth sound
expected from audiophile amplifiers. The 12AU7 (ECC82)
is a Twin Triode vacuum tube, it is very popular in the
audio world because it is rather rugged and can be
operated at lower voltages. Headphone amplifier and
12AU7 tube is powered by just 12V DC voltage. This is
great news for those new to vacuum tubes that want
experience and learn more about them. Typically vacuum
tubes operate at high and dangerous voltages so you must
have some experience and know what you are doing. On the
other hand this headphone amplifier operates at low 12V
voltage so it is safe to build and experiment.

This
simple headphone amplifier allows an entry level builder
to experience assembling and listening to their own
creation. I use the term builder as electronic
experience combined with innovation which allows the
creation of a device, rather than simple board stuffing.
This amplifier can take on many shapes and sizes; I
especially like when builders reuse older devices as
cases and even recycle some components from various
discarded power supplies. I try to keep price at a
minimum because this amplifier is very basic and allows
the builder to seek out theory, and actually "listen" to
their music, and grow to build more complex projects in
the future.

There are plenty of wonderful
websites out there that can explain tube theory, I
learned from the US Navy NEETS module 6. I will not go
to in depth into the theory, but will introduce you to
the 12AU7 (known in Europe as ECC82).

The 12AU7
(ECC82) is a Twin Triode vacuum tube, it is very popular
in the audio world because it is rather rugged and can
be operated at lower voltages. You will find these tubes
in vintage amplifiers and organs. They are even used in
older vacuum tube volt meters and their life span can
reach into decades. The 12AU7 has an amplification
factor (µ) of about 17, this is moderate as compared to
its cousin the 12AX7 that comes in at 100. For purpose
of the headphone amplifier the 12AU7 tube will be used
in a common cathode configuration, and the incoming
signal will be amplified by approximately 10dB. The
12AU7 is usually operated at plate voltages of over 120
volts, but fortunately it can be operated at lower
voltages with decent results.

12AU7 (ECC82)
Vacuum Tubes / Valves

As you see in the load
lone graph below, we will operate in the 6 volt region.
6 volts allows enough "swing" for the signal to reach 12
volts and down to the 0 volt line. Since this amplifier
is single-ended we will not be applying an external
negative voltage, but due to grid leak bias the voltage
does sit above common.

12AU7 Load Line Graph

The tube has 9 pins and follows the EIA 9A pin
out configuration. Remember that pin out diagrams are
always read from the bottom of the tube, this little
detail may save you much rework in the future.

12AU7 (ECC82) Tube Pinout Diagram

The
great thing about this tube - it is basically two tubes
in one. The other plus is that the heaters (filaments)
can be powered by 6.3 or 12.6 volts AC or DC. In lower
voltage amps, using DC for the heaters is preferred to
reduce hum. For the this headphone amplifier we will be
using a 12 volt sealed lead acid (SLA) battery as the
power supply (hence v12). The two filaments are pinned 4
to 9 and 9 to 5, with pin 9 as a center tap. One can
apply 12.6 volts across pins 4 and 5, or 6.3 volts to
pins 4 and 5 with pin 9 used for common (negative), or
6.3 volts can be applied to pin 9 with pins 4 and 5 used
as common.

Power Supply: 12AU7 (ECC82) / IRF510
Headphone Amp

The power supply is rather simple
and straightforward, a 12v SLA battery. The battery is
rated at 1.3 ampere-hour (Ah) and the total amplifier
draw is about 400 mA, so we can expect some hours of
playback between charges. A battery is the perfect
source for voltage as there is no ripple or noise that
will be injected into the signal, this is very important
with headphone applications.

An alternate is a
regulated switch mode power supply. I use a Cannon
K30120 13v 1.8A portable printer power supply. The
voltage is kept constant, great filtering, and most of
all there is a built in over current protect. These
power supplies can easily be found in thrift stores, the
cost is miniscule to building a similar supply. It is
important that the power supply be regulated to keep the
unwanted noise levels low.

12AU7 (ECC82) /
IRF510 Headphone Amp Circuit

The circuit
consists of two stages: 1. a common cathode tube volt
amp stage (gain), 2. a MOSFET source follower for
current gain (with a LM317 regulator used as a constant
current source). Since most headphones are less than 50
ohms a little current is necessary to run them
efficiently.

12AU7 (ECC82) / IRF510 Headphone
Amplifier Schematic

A Bipolar Junction
Transistor (BJT) can be used in place of the MOSFET, but
MOSFETs tend to be more stable with temperature and
current shifts. Use caution when handling MOSFETs as
they are very static sensitive.

The signal
enters the grid via a coupling capacitor, it leaves the
plate (anode) amplified and 180 degrees out of phase, it
is then directly coupled to the gate of the MOSFET and
leaves the source. The signal is then coupled to the
headphones via a electrolytic capacitor, which blocks
the DC from your headphones. Since the sleeve of your
headphones is common it completes the circuit.

The MOSFET is biased into class-A operation and
will be constantly conducting at approximately 125 mA.
The LM317 regulator is configured as a constant current
source and regulates at 125 mA in the given
configuration. You can use the online LM317 regulator
calculator to determine the current through the
regulator by adjusting the program resistor. It is
suggested that a 10 ohm 1/2W program resistors is used
for R4 (you can use two 1/4W 20 ohm resistors in
parallel). Note that the regulator and MOSFET devices
will heat up and radiate heat. There are some real in
depth calculations for heat, but know that the MOSFET
can dissipate at least 1.6 Watts and the LM317 2 Watts
to air, at room temp. I tested a prototype over a
continuous 24 hours period in a 150 cubic-centimeter
(about 9 cubic-inch) enclosure and there were no thermal
stability issues. You can add heatsinks to the devices,
just ensure that if you gang the FETs together to use
mica and silicone washers to prevent the 12V supply from
transferring to the heat sinks.

The schematic
above shows only one channel, you will have to wire the
the tube for both channels using different pins. The
only wiring that is common for the 12AU7 tube between
the two channels is the heater.

Construction
Notes: 12AU7 (ECC82) / IRF510 Headphone Amp

With
any project there are specialized tools that are
required; in this project you will need some basic
electronic test equipment and tools.

Ohm's Law
deals with three basic measurements, Voltage, Resistance
and Current. Knowing at least two of the variables, we
can extrapolate a multitude of others. To make good
readings, one must use a digital multimeter (DMM or
DVOM). There are hundreds of DMMs on the market and you
do not need to spend $200 on a DMM if you are only doing
hobby work, but try to stay away from the very cheap $10
knock offs. One should use a meter that reads all three
parameters and have probes that are safe up to 1000
volts.

The second most important tool is the
soldering iron. You can use a soldering pencil, but for
best results, a soldering station equivalent to the
Weller WLC100 will save you headaches down the line.
Nothing like leaving your soldering pencil on the table
and finding it with your arm, or burning your laptop,
etc.. Rosin core 60/40 solder is the best for new
builders, there are others who swear by silver solder
and other fancy solder, but for this application, the
60/40 flows well and will last years. Lastly purchase
some liquid flux in a needle type applicator. Flux helps
to transfer the heat to the joint, and will make the job
so much easier. The solder has a flux core, but again
the job will go much better with flux. Cleaning of the
flux can be done with an acid brush and 99% isopropyl
alcohol; you do not have to go crazy with cleaning, as
the low voltage will not be an issue. Stay away from
plumbing flux, it is conductive and can destroy your
project.

Purchase some cheap headphones for
testing. Similar to the ones you use on the airplane,
nothing like frying your $300 Sennheiser headphones to
make a project go upside down.

12AU7 (ECC82) /
IRF510 Headphone Amp Parts

The bill of materials
(BOM) is pretty straightforward, I selected readily
available parts, 90% of them can be purchased at Radio
Shack. If you have access to similar components, or you
want to buy higher quality components, you can use
Nichicon FG, KZ or Elna electrolytic capacitors and
maybe Solen polypropylene film for C1. The only issue is
shipping charges can make the project expensive, if you
use a local Guitar Center and Radio Shack you can cut
out shipping costs completely. The BOM for this 12AU7
(ECC82) / IRF510 Headphone Amp is provided

R1

R2
R3
R4
R5
U1
C1
C2
C3

Q1
Socket
V1
P1
Case
Mica

Heatsink
Jack
Battery
100k, 1/4W

150, 1/4W
4.7k, 1/4W
10, 1/4W
1k, 1/4W

LM317
2.2uF, 16V
100uF, 16V
470uF, 16V

IRF510
9pin PCB
12AU7
50k
12V 1.3Ah

N/A - Not Avalailable
* - 50-Piece
1/4-Watt Metal-Film Resistor Assortment, Radio Shack
Part # 271-309
** - Misc Resistors at TubeDepot, it
is suggested you use a 1/2W 10 ohm resistor for R4

*** - Misc 1/2W Resistors and Jacks available from
Parts Express

R3 - A fixed resistor (R3) may be
used in place of the 50k potentiometer (P1). 4k7 worked
for many 12AU7 tube types, but you should pick a value
to get half the supply voltage (see setting the bias
Photograph 14).

Q1 - The MOSFET (Q1) can be
replaced by an IRF610, IRF611 or IRF612, all of which
will work as well. Stay away from IRF530 or IRF540 types
(commonly found in power supplies) as there will be
terrible roll-off of the highs.

C1 -
Non-polarized electrolytic or film type capacitors may
be used for C1 (2.2uF or greater). Most will find that a
film type capacitor will sound best. Generic Polyester
or Polypropylene types work well.

HEATSINKS - As
previously noted, it is completely fine to run the
MOSFETS and LM317 regulators to air, I did the math from
the datasheets and the current is low enough to keep
them stable. However, please do note that they do get
hot. The metal tabs will sizzle water from your finger
tip, like checking a clothes iron, but have not had a
failure yet. There is not too much radiant heat though.
If you are worried about heat you can use a small heat
sink on the devices.

PCB: 12AU7 (ECC82) / IRF510
Headphone Amp

After a few years the circuit has
been modified from my original 12AU7 Tube / IRF612
MOSFET Hybrid Headphone Amplifier build to fit on a
single layer printed circuit board (PCB). Although it
can easily be constructed point-to-point on a 750 hole
proto board, a PCB is a cost effective upgrade. There
are several sites that give in depth instructions on
etching a PCB at home, and the whole process can be very
rewarding. Just know that etching utilizes some very
caustic and nasty chemicals, so proper ventilation and
use of personal protective equipment (PPE) will make the
process easy, fun and safe. Muriatic acid and hydrogen
peroxide yielded the best results and also is much
cheaper than ferric chloride.

The board layouts
are included in the attachments; you can print them out
and etch them using the toner transfer method.

I
used the free version of the CADSOFT Eagle program to
construct the board on a single plane. The process is
done with a standard Toner Transfer method. Basically
print the trace design onto cheap glossy photo paper.
You must use a laser printer, or a Xerox copier on the
darkest setting possible. The dark setting will allow
the most toner to build up on the paper. Follow one of
the many instruction set found online, I found the
glossy magazine transfer method the best and easiest.
You will have a transfer, just touch up and then finish
the etching process. Another key note is to have a
dremel press, or use two hands when drilling holes. Use
the smallest bit possible then move up in size. If you
remove too much copper, it will make the process of
soldering cumbersome.

Copper Clad Board

After many modifications the PCB was reduced to
a 2-1/4" x 4-3/4" and then down to 2" x 4" (50 x 100 mm)
with thicker traces. The build is demonstrated on the
first version (slightly larger PCB).

Assembly:
12AU7 (ECC82) / IRF510 Headphone Amp

The
components are fed through the top of the PCB and
soldered onto the bottom (copper side). There are
several types of 9 pin sockets, some will fit better
than others, but with a little diligence either socket
will work fine.

B9A 9-pin ceramic vacuum tube /
valve socket
Photograph 3: B9A 9-pin ceramic vacuum
tube / valve socket

Solder in the tube socket,
the pins will fit in the hole pattern, just use caution
not to overstress the pins or they will break.

9-pin vacuum tube / valve socket on PCB

Photograph 4: 9-pin vacuum tube / valve socket on
PCB

Solder the resistors next, the PCB spacing
is for 1/4 watt metal film types, if you only have 1/2
watt or higher resistors, you will have to bend the
leads to fit the PCB.

Bend Resistor Leads to Fit
on PCB
Photograph 5: Bend Resistor Leads to Fit on
PCB

After populating resistors, solder in the
capacitors. The C1 capacitors are polyester film types
(non-polar) and can be placed in either direction. The
remaining capacitors are electrolytic, and must follow
proper polarity. C2 has the ground pin facing the tube,
and C3 have the ground pin facing back.

Populate
12AU7 / IRF510 Tube Headphone Amp
Photograph 6:
Populating the PCB

Solder the MOSFETS,
LM317voltage regulators, and the remaining components.

Populate 12AU7 Tube / IRF510 Mosfet Headphone
Amp
Photograph 7: Populating the PCB

Solder
in the power jack, which in this case is a terminal
block. If you prefer, you can run wires to a jack on the
enclosure.

Populate 12AU7 Valve / IRF510 Mosfet
Headphone Amp
Photograph 8: Populating the PCB

Test fit the PCB into the enclosure and trim the
sides as necessary.

Dry fit the PCB into the
enclosure
Photograph 9: Dry fit the PCB into the
enclosure

With the board complete, prepare the
case for the sockets and volume potentiometer. A stepped
drill bit works great for drilling the holes into the
enclosure.

Chassis for 12AU7 Tube / IRF510
Mosfet Headphone Amp
Photograph 10: Chassis
Preparation for 12AU7 Tube / IRF510 Mosfet Headphone Amp

The input signal is fed into the right side of
the enclosure via a set of RCA jack or can use a 3.5 mm
stereo jack if you prefer. The signal comes in through
the jack and to the potentiometer which allows level
control and then to the circuit for amplification. The
output exits the amplifier via the output jack. If your
signal source allows you to control the signal level,
you can omit the potentiometer. The Radio Shack
potentiometer can be used, but the fit will be tight so
to prevent a short, you may want to use a smaller Alps
10k audio pot (part number RK0971221Z05)

Alps
Potentiometer for 12AU7 / IRF510 Headphone Amp

I
have drafted a routing PCB (see the previous PCB
section), but three small gauge wires will work fine
too. Solder all connections before mounting. Mount the
jacks and potentiometer and leave the amp board free for
testing.

Putting 12AU7 / IRF510 Headphone Amp
Together

The positive lead from C3 is the same
potential as the MOSFET source pin. It is best to check
voltage at the capacitor, because the MOSFET can be
damaged if the pins are shorted. Also, check that both
heaters glow.

Check that 12AU7 Tube Heaters are
Working
Photograph 13: Check that 12AU7 Tube Heaters
are Working
Setting the Bias

The bias is set
by adjusting the 50k trim potemtiometer (P1) until the
output side of the MOSFET (Source) is at one-half of the
supply voltage (Drain). Adjusting the two trim
potemtiometers to one-half of your supply voltage, 6
volts since we are using a 12v supply. You will want to
check and reset the bias a few times in the first few
hours of use as it will drift while everything settles
in.

Setting the Bias
Check for DC Offset

Next check for DC offset at the headphone output
using a DMM. The DC offset should be less than 10 mV.
After a number of prototypes I measured 8 mV max and
typically about 3 mV.

Check for DC Offset at the
Output
Photograph 15: Check for DC Offset at the
Output

Now its time to hook up the cheap
headphones for a quick listen.

Check Headphone
Amplifier with Cheap Headphones

If all is good,
secure the amp to the case, one more check with the
cheap headphones, then try out your new amp.

Finished 12AU7 Tube / IRF510 Mosfet Headphone
Amplifier

Now its time to listen to audiophile
tube sound. Enjoy!

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Class-A 12AU7 Tube Headphone Amplifier -
Link

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