Please note that this portion of the website is long since out of date and does not contain information that is critical to the proper construction of the amplifier.  Please see the forumsfor the most current information including how to remedy several errors on the last run of these pcbs.  This section of the website will be remain for those few boards that have not been built but will not be updated now that the MAX has been launched!

Pete Millett's Hybrid Headphone Amplifier

Last update: March 20, 2006

A few years ago, tube amplifier guru Pete Millett created a safe, reasonably priced tube amplifier
based on the 12AE6A automotive radio tubes.  They're designed to run on 12 volts instead of the
hundreds of volts of standard tubes.  From a practical point of view, that means that you can stick
your hands into this circuit and not run the risk of dying from electrocution.

The amplifier circuit that Pete created is a hybrid tube/solid state design.  It uses the tubes as
votage gain devices and a Burr Brown OPA634 buffer as the output stage.  It's an excellent idea
and one that was common in the brief period between the introduction of car radios and low-cost
transistors.  Back in the day, those radios used a tube for voltage gain and a germanium
transistor as the output stage.  In the case of the Hybrid Headphone Amp, the BUF634 serves
two purposes: to provide a current source for the headphones and to drop the output impedance
to a value low enough so that the amplifier does not need output transformers.

This implementation of the Hybrid Headphone Amplifier was inspired by member
n_maher.  Based on suggestions from Nate and others, member drewd
adapted Pete's original design to fit on a standard double Eurocard form factor.

The result:

More Information 

(Latest info comes first)

Some forums are available
here if you'd like to discuss the Millett Hybrid or other DIY projects.
You may need to refresh this page to make sure that the correct link loads.

I'm no longer selling the Millett Hybrid PCB, but the Eagle CAD files as well as a full set of
Gerber and Excellon drill files is available for anyone to have their own boards built.  These files
represent the latest version of the PCB and you are free to use them for any purpose.

The Eagle CAD file is available here.  An Eagle library with most of the parts used in the CAD file is
available here.  A zipped archive of the Gerber and Excellon files is available here.

If you make changes to the PCB layout, I'd appreciate hearing about it and I'd prefer that you
make those changes freely available to everyone.

There's a nice article that features the Millett Hybrid project at RedHat Magazine.

The Headwize group buy of diamond buffers is over, but Nate has a few boards left over if you missed out.  
Please read his post at Headwize to find out how you can pick one up from him.  Please remember that as
of the beginning of Februrary Nate is working seven days a week, so he may not get back to you right away.

A group buy has been started at Headwize for the diamond buffer module discussed in the forums.
You'll need to register as a member at Headwize to take advantage of it.  Besides getting in on the
group buy, Headwize is a great source of information and help for DIY headphone amps.  Head over
to the forum in the Headwize Marketplace and place your order.

There are a few threads on Head-Fi mentioning that the BUF634 is out of stock with many
suppliers.  If you do not have a version of the PCB with pads for the Intersil buffer, you
can substitute an OPA551 if you connect pins 2 and 6.  It will not affect the performance of
the BUF634 if you decide to use one later.

Eagle .brd and PCB manufacturing files are available here.  You don't need any special permission
to manufacture the PCB, although I'm sure that Pete would appreciate an email to let him know that
you've made some.  Further down the page is a link to the custom Eagle libraries that were used to
design the board.

The build instructions are a little out of date, but mostly missing just details.  A new set of
instructions is being developed.  In the meantime, if you have a problem or a question, you can
post in the forums or send an email to Drew.

The web-based BOM has been updated to provide part numbers for the new terminal blocks
for power and inputs/outputs.  Part numbers are listed for the OPA551.  A Neutrik headphone
jack can be directly mounted to the PCB.  Circuit boards are in stock!

Other improvements to the board:
The board is still the same size - double Eurocard - as before.  The image above  is a 3D render
of the latest board.

The web-based BOM has been updated to provide an alternative to the BUF634 (which is
in very tight supply right now).  The OPA551, a high current output opamp can be used
as a unity gain buffer in the same spot.  You'll have to put a jumper between pins 6 and 2
to close the feedback loop.  This won't affect the performance of the BUF634 if you want
to try one out later because pin 2 on that part is unused.  Sonically, there doesn't seem
to be any difference between the chips.  I'll update the next batch of boards with a
connection between those pins.

If your PCB has a position for D3, please ignore it.  D3 on the PCB is not used.  There's no
need to put a diode there because D1 already fulfills that purpose.  It has been removed
from the latest board.

Ti Kan (AMB in the forums) has posted some rather extensive performance measurements of
his Millett Hybrid amplifier.  His comments at the end of the article provide a very
interesting contrast to the results of his measured data.  Thanks Ti!

A schematic specifically for the board is finally online.

These pages are now hosted at

We're starting a tips and tweaks page.  Please shoot us an email if you've got something
to add!

Preliminary build instructions are here!

The boards have shipped from the manufacturer - ahead of schedule!
Also, the BOM has been updated to fix a couple of errors - the quantites for C9 and C10
were incorrect and a different 470uF cap has been specified for the Mouser sheet because
the original will be out of stock for several weeks.

The boards were ordered on May 23, 2005, with a two week turnaround.

Nate Maher has created a bill of materials, including part numbers and prices (in US$) in a
Microsoft Excel spreadsheet.  You can download the spreadsheet here.

Along with the link to the board file below, a library file containing most of the parts used
in the board is available for download here.  The only really significant parts that you may
not be able to find elsewhere are the 7 pin mini sockets, the ALPS Blue Velvet and the
Panasonic EV-J pots.  The other parts are pretty common.

The final prototypes prior to production are in and being built.  See one in the picture above.

A detailed build guide will be ready this weekend (June 4th and 5th).  It will cover parts selection,
installation and bias settings.  With a little luck, some RMAA plots will be online as well.

Alternate placement positions for J503 CRDs (an alternate for D2R and D2L) are on the board.
D2 should be populated with a
1N5291 or a J503 but not both.

Drew is working on a plug-in replacement for the BUF634 output buffers, based on sijosae's
interpretation of Walt Jung's diamond buffers.  The preliminary design work is finished and
we will be ordering prototype boards on May 13, 2005.

Nate has opened the group buy for these boards.  The price will be determined
after all of the orders are received, but it will be less than US$20.00 per board.  If you're not
a Head-Fi member, please roll on over there and sign up and get in on this deal.  I'll be
purchasing a few extra boards for anybody who misses out, but not that many!  The open
period for the buy ends on May 20th, 2005.

The final version of the boards is finished and frozen.  Nate and Drew have ordered a set of ten
boards to check just to be sure that everything works.

The prototype boards are in and the first build is working.  There are a total of ten boards
and various people will be building at least six amps.  Once those are built
and the bugs are worked out, Nate will start the group buy.

Development on the circuit board has been frozen as of March 28, 2005.
Prototype boards were ordered on March 29, 2005.

Prototype boards include all input received up to 1:00PM MST, March 29th.   
The bypass capacitor changes that were suggested have been implemented..

n_maher will be organizing a group buy of production boards as soon as the prototypes are tested
and any bugs are corrected. 

Pete Millett's web page is here.  It includes links to the schematic for the board, the original
AudioXpress article discussing the board and some Eagle board layout files.

Please note that the BOM posted on Pete's site and in the AudioXpress article has a
couple of errors in the part selections for the electrolytic caps.  The proposed parts list
uses different parts from Handmade Electronics.

The preliminary build instructions

Current parts list:

Reference Quantity Description Handmade Electronics Welborne Labs Digi-Key Mouser Notes
C1, C9L/R, C10L/R 5 ≥100uF electrolytic cap Elna Silmic II 3-RFS-10001000
Elna Cerafine ROA104
Panasonic FC
Nichicon UPW
Voltage rating should be no less than 35V.  7.5mm pin spacing, 16mm diameter.  Alternate pin spacing is provided for 5mm pins.
C7L/R 2 ≥100uF electrolytic cap Elna Silmic II 3-RFS-10001000
Elna Cerafine ROA104
Panasonic FC
Nichicon UPW

The output coupling caps determine the low frequency cutoff point.  For 100uF is the lowest recommended value.  For headphones with impedances <150 ohms, up to 470uF is appropriate.  35V, 7.5mm pin spacing, 16mm diameter.  Alternate pin spacing is provided for 5mm pins.
C2L/R 2 ≥220uF electrolytic cap Nichicon Muze KZ 3-4030 Elna Cerafine ROA220 Panasonic FC
Nichicon UPW
Voltage rating should be no less than 16V. 5mm pin spacing, 10.5mm diameter
C3L/R, C4L/R, C5L/R, C8L/R 8 0.22uF film cap
Wima MKP10 WM210 (0.10uF) Panasonic Polypropylene P3224-ND Wima MKP10

Xicon Metalized Polypropylene 1429-2224
Placements provided for film capacitors with pin spacing of 5mm, 10mm and 15mm.  See note below for alternate supplier.

The Mouser Wima part is a 0.1uF capacitor.  That's fine - the exact value is not important.
C6L/R 2 0.01uF axial ceramic cap

1103PHCT 80-C410C103K5R
D1 1 P6KE30 Transient Supressor

D2L/R 2 1N5291 or J503 CRD


Locations are provided on the board for the 1N5291 and the J503.  Do not populate both diode types!  Use either the 1N5291 or the J503, but not both.  Also, do not mix and match - use two 1N5291s or two J503s, but not one of each.  You've been warned...
PF1 1
RXE050 PTC Fuse

Raychem RXE050 650-RXE050
RLED, R3R/L R5R/L 5 1KOhm, 1/4W Resistor
Vishay Dale V4-1.0K 1.00KXBK-ND Vishay Dale 71-RN60D-F-1.0K Any 1KOhm 1% 1/4W or 1/2W metal film resistor may be substituted
R2R/L 2
5KOhm Trimpot

Bourns 3296W Vishay/Spectrol

R4R/L 2 22 Ohm 1/4W Resistor
Vishay Dale V4-22.0 22.1XBK-ND Vishay Dale 71-RN60D-F-22.1 Any 22 Ohm 1% 1/4W or 1/2W metal film resistor may be substituted.  Take special care to be sure that you do not accidentally order a 22.1KOhm resistor from Mouser - the part numbers are nearly identical.
R6R/L 2 220 Ohm 1/4W Resistor Vishay Dale V4-221 221XBK-ND Vishay Dale 71-RN60D-F-221 This sets the bandwidth of the buffers. Theoretically, it can be jumpered, but this will increase the current demands of the buffers with little sonic benefit.  These resistors are optional.
VT1, VT2 2 Tube See notes below
Tube Sockets 2 7 Pin Mini Socket, PCB Mount See notes below
Terminal Block 2 2 Position Terminal Block Kobiconn
Optional for power and switch connections
Terminal Block 1, 2 or 3 3 Position Terminal Block Kobiconn
Optional for input and output connections
Headphone Jack 1 1/4 inch Neutrik NMJ6HCD2
Buffers 2 8 Pin DIP Current Buffer

Burr Brown BUF634P-ND
Burr Brown
Burr Brown

Pads are also provided for Intersil HA3-5002 buffers available from Arrow Electronics.  If you use the OPA551, you MUST connect pins 6 and 2.  Do not stack the OPA551.
Buffer Sockets
2 8 Pin DIP Socket

Assmann Electronics AE7313-ND Mill Max
Any standard 8 pin DIP socket will work here. This optional - you can solder the buffers to the board.
Pot 1 50KOhm Pot

Panasonic EV-J P2T3503-ND
Pads are also provided for ALPS RK27 "Blue Velvet" potentiometer
Power Supply
1 24VDC, 400mA

CUI-Stack DPD240040-
Elpac FW1824-760

24VDC - 30VDC, 400mA (min)
DC Jack
1 2.5mm barrel

CP-6-ND Kobiconn
There are a plethora of choices.  Make sure that your DC jack matches the power supply plug

1601 has metal end panels, 1602 has plastic end panels.  Add letters "BK" for a black anodized finish.  Also stocked by Newark and Allied

Additional parts are a headphone jack, input jack(s), power switch and volume knob (1/4" or
6mm shaft diameter for the recommended pots).

A slightly different, but every bit as good BOM is in a Microsoft Excel spreadsheet.

Notes on Tubes

Tubes that are known to work with this amplifier include the 12AE6A, 12FK6 and 12FM6.  Pete's
original design used the 12AE6A.  All three tubes are available online from various vendors,
including Tube Depot, The Tube Store and VacuumTubes.Net.  They also carry the miniature 7 pin PCB mount

Tube Data Sheets

Part Number
Tung Sol
General Electric

Buffer Data Sheet

Manufacturer Part Number
Burr Brown BUF634
Burr Brown OPA551
Intersil HA2-5003

Note on Capacitors

If you're on a budget, you may find that the Elna and Wima caps are a little pricey.  They are
considered boutique caps by many.  The Panasonic, Nichicon and Xicon caps that are also
listed are perfectly fine alternatives.  For a middle of the road configuration, consider using those
less expensive caps and use the boutique caps in positions C7L/R and C4L/R.  Those are the
audio output coupling caps and the audio signal passes through them.  The better the cap, the
(potentially) better the sound.  For C7L/R, the capacitance value, along with the headphone
impedance, sets the low frequency cutoff value.  The larger the capacitance, the lower the frequency.
100uF is the lowest recommended, 470uF is probably as high as you need to go.

For power ratings, 35V is sufficient for any of the electrolytic caps.  Higher power rating
gains no advantage.

An alternate source for the Wima capacitors is TAW Electronics.  Their prices are similar to Mouser's
They do not have a web ordering interface, but do take orders via email and phone.


An excellent choice for powering this amplifier is the STEPS linear, regulated power supply.
PCBs and instructions for building the power supply are available on Tangent's web site.
It fits the same size Hammond case as the amplifier.


The Hammond 1455N1601 enclosure is not tall enough to competely contain the tubes, so if
you are using it, you will have to drill holes in the top for the tubes to extend through.  Drew's prototype
amplifiers positioned the circuit board about four slots up from the bottom of the case so that
the volume control would be centered on the front panel and so that the tubes would be almost fully
visible outside of the case.  The best way to drill the holes is to use the "belly plate" of the case
as the top, position the circuit board on the plate and mark the hole locations with a pencil.
A 1" hole will allow clearance for the tubes.  A stepped drill bit is highly recommended.

As seen above, Hammond's 1455R2201 enclosure will also work, if you use the alternate
Panasonic FC caps - they're quite short, so they've fit in the 1.2" tall case.  An ALPS Blue
Velvet pot will not fit that case, though.

Other people have built these amplifiers in Par-Metal 20 series cases which are
large enough to contain both the amplifier and STEPS circuit boards.  Here's dsavitsk's:

Vendor Web Sites

Handmade Electronics
Welborne Labs
TAW Electronics
Tangent's Parts Shop

Purchasing the boards

Boards are available here.

Both drewd and n_maher can be contacted via PM on

Pete Millett graciously gave us permission to tear his original board to shreds - THANK YOU!
Nate Maher (n_maher on organized this whole thing!
Drew Dunn (drewd on redesigned Pete's work.  Blame him if it doesn't work!

A special thanks to those who contributed excellent ideas and suggestions: