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NP-100v12: 12AU7 (ECC82) / IRF510 Headphone Amp

Rogers Gomez

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DIY 12AU7 (ECC82) Tube / IRF510 MOSFET Headphone Amplifier

The NP-100v12 is a simple headphone amplifier that 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.

The 12AU7 (ECC82) Vacuum Tube (Valve)

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
NP-100v12 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.

Photograph 1: Various 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.

Figure 1: 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.

Figure 2: 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 NP-100v12 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.

Circuit: 12AU7 (ECC82) / IRF510 Headphone Amp

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
voltage regulator IC configured as a constant
current source). Since most headphones are less than 50 ohms a little
current is necessary to run them efficiently.

Figure 3: NP-100v12 - 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
current 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
voltage regulator IC 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 troubleshooting. Similar to the ones you
use on the airplane, nothing like frying your $300 Sennheiser headphones to make
a project go upside down.

BOM: 12AU7 (ECC82) / IRF510 Headphone Amp

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
in Table 1.

Table 1: NP-100v12 BOM

Component

Value

QTY

Mouser

Radio
Shack

TubeDepot

Guitar
Center

Parts Express

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
Optional
Optional
Optional
Optional
12V
1.3Ah

2
2
2
2
2
2
2
2
2
2
2
1
2
1
4
4
1
1

660-MF1/4DCT52R1003F
660-MF1/4D52R1500F
660-MF1/4LCT52R472J
660-MF1/4DC10R0F

660-MF1/4LCT52R102G
512LM317AHVT
505-MKS42.2/63/10P15
647-UFG1C101MPM

647-UFG1E471MHM
844-IRFI510GPBF
N/A
N/A
3299W-1-503LF
N/A
N/A
N/A
N/A
N/A

*
*
*
*
*
276-1778
272-997
272-1028
272-1030
276-2072
N/A
N/A
*
270-1802
276-1373
276-1363
274-0249
23-9080

**
**
**
**
**
N/A
N/A
N/A
N/A
N/A
9pin PC
12AU7/ECC82
N/A
N/A
N/A
N/A
N/A
N/A

N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
ST-12AU7-C
N/A
N/A
N/A
N/A
N/A
N/A

***
***
***
***
***
LM317T
027-324
020-1664
020-1710
N/A
055-502
055-508
N/A
N/A
N/A
N/A
***
N/A

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.

PCB Files

NP-100v12 PCB Image - (PNG 50kB)

NP-100v12 Components-Layout - (PNG
23kB)

NP-100v12 PCB Image - (PDF 13kB)

NP-100v12 EAGLE Circuit Board File - (BRD
16kB)

Potentiometer Routing PCB Image - (PDF
3kB)

Potentiometer Routing EAGLE Circuit Board
File - (BRD 5kB)

Photograph 2: Toner Transfer Method - 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.

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.

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.

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.

Photograph 6: Populating the PCB

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

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.

Photograph 8: Populating the PCB

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

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.

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)

Photograph 11: Alps Potentiometer with 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.

Photograph 12: 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.

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.

Photograph 14: 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.

Photograph 15: Check for DC Offset at the Output

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

Photograph 16: 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.

Photograph 17: Finished 12AU7 Tube / IRF510 Mosfet Headphone
Amplifier

Now its time for tube rolling. Enjoy!

If you have any questions, feel free to ask them in the 12AU7 / IRF510 HeadAmp Support Thread in the message forum.

Last update 8 January 2014. Created 12 April 2010. © Rogers
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