Pete Millett's Hybrid Headphone Amplifier (old history)

A few years ago, tube amplifier guru Pete Millett created a safe to build, reasonably priced tube headphone amplifier
based on the 12AE6A automotive radio tubes.  They're known as space-charge tubes and designed to run on 12 volts instead of the
hundreds of volts of standard tubes. They were made for automobile radios before transistors.   From a practical point of view, use of these tubes 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 DIYForums.org member
n_maher.  Based on suggestions from Nate and others, DIYForums.org member drewd
adapted Pete's original design to fit on a standard double Eurocard form factor.
NOTE from Beezar Audio's TomB: "Since the MAX, MiniMAX, and MOSFET-MAX, we refer to this particular Millett Hybrid design as the "revMH Millett Hybrid."

The result:

More Information 

(Latest info comes first)

• Additional buffer choice: Intersil's HA3-5002 buffer
• Positions for Kobiconn terminal blocks for input, output and power
• Removed alternate input, output and pot pads
• Reviewed and rerouted a few traces to tighten up the layout a bit

The web-based BOM has been updated to provide an alternative to the BUF634 (which is
sometimes in very tight supply).  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.

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!

Preliminary build instructions are here!

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.

Nate Maher has created a bill of materials, including part numbers and prices (in USD) 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 of the final prototypes in the picture above.


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.


The diamond buffer board (no longer available) was developed and plugs into the BUF634 sockets:







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 (100uF)	Elna Cerafine ROA104 (100uF)	Panasonic FC P10348-ND (270uF) P10352-ND (470uF)	Nichicon UPW 647-UPW1J271MHD (270uF) 647-UPW1J471MHD3 (470uF)	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 (100uF)	Elna Cerafine ROA104 (100uF	Panasonic FC P10348-ND (270uF) P10352-ND (470uF)	Nichicon UPW 647-UPW1J271MHD (270uF) 647-UPW1J471MHD3 (470uF)	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 P10346-ND	Nichicon UPW 647-UPW1H221MPD	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 505-M100.1/160/5 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			P6KE30ADICT	511-P6KE30A
D2L/R	2	1N5291 or J503 CRD				610-1N5291 J503	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 594-64W502
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 158-P02EK381V2	Optional for power and switch connections
Terminal Block	1, 2 or 3	3 Position Terminal Block				Kobiconn 158-P02EK381V3	Optional for input and output connections
Headphone Jack	1	1/4 inch				Neutrik NMJ6HCD2 550-22302
Buffers	2	8 Pin DIP Current Buffer			Burr Brown BUF634P-ND Burr Brown OPA551PA-ND	Burr Brown 595-BUF634P	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 575-113308	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- P6P T5020-P6P	Elpac FW1824-760 680-FW1824-760	24VDC - 30VDC, 400mA (min)
DC Jack	1	2.5mm barrel			CP-6-ND	Kobiconn 163-1100	There are a plethora of choices.  Make sure that your DC jack matches the power supply plug
Enclosure	1					Hammond 1455N1601 546-1455N1601	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
sockets.

Tube Data Sheets

Manufacturer	Part Number
Tung Sol	12AE6 12AE6A 12FK6 12FM6
General Electric	12AE6A 12FM6
Sylvania	12FK6

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.

Power

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.




Enclosure

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
Digi-Key
Mouser
TAW Electronics
Par-Metal
Tangent's Parts Shop

Purchasing the boards

Boards are available here.

---

Both drewd and n_maher can be contacted via PM on DIYForums.org

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

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

comabereni
bg4533
blip
MisterX
dsavitsk
Nisbeth
vksy
individual6891
slindeman
skyskraper
guzzler
sijosae
sbelyo
flecom
buzzerbro
amb
AtomBoy
Finch&Music