DIY Audio Home

Wheatfield Audio HA-1 Headphone Amp

So, ya wanna build a headphone amp?

This is the Wheatfield Audio HA-1 tube headphone amp, which  I designed it to be an "affordable" alternative to the more expensive HA-2.  I manufactured it for a couple of years, and HeadRoom sold it.  When I finally came to the realization that I was perhaps one of the world's worst businessmen, I sold it - the design, rights, and inventory - to HeadRoom.

Headroom built and sold some more, but eventually discontinued it.

So a couple of years later, on a whim, I asked Tyll (the chief HeadRoomer) if he would care if I put the design into the public domain, since neither of us had any plans to do anything commercial with it.  "Sure, no problem" was his response... so here it is!


PLEASE READ - Commercial usage of information on this site:

I consider all the information that I post here to be in the public domain.  So, you can use it however you want, for commercial or non-commercial use.

That said, I would appreciate it if you at least let me know if you are going to use any of the circuits or especially PCB Gerber files to make commercial products, or to sell bare PCB's.  

There are some cases where products are being sold not only with my permission, but active involvement.  The "Millett Hybrid" effort and others at HeadFi are examples (and excellent models of how the DIY community should work, in my opinion).  There are other cases where I have asked vendors to sell PCB's as a service to hobbyists.  And there are other cases where companies are manufacturing and selling PCB's, chassis, etc. without contacting me at all.

In ALL of these cases, I make no profit from any of the sales.  Zero, zip, nada.  I have a normal "day job" that pays the bills, this is strictly a hobby with me.  So please do not expect me to provide the level of technical support that you might expect when buying a product.  I try and help, but it sometimes takes me days - even weeks if I'm traveling for work - to respond.

Thanks for your indulgence in reading this!


The HA-1 is similar to the HA-2 in that it is a 2-stage tube amp, using a regular common-cathode voltage amp stage directly coupled to a cathode follower.  The HA-1 uses a 12AU7 as the voltage amp tubes, and a pair of 7044 dual triodes (parallel connected) for the cathode follower.  The result is quite a low output impedance for a tube OTL, and this can drive even 32 ohm headphones.

I'm posting all of the original design data that I found, including data to make the PCB, the costed bill of materials including vendor data, and a mechanical CAD file that has all of the individual metal case parts included in it (on separate layers).

I don't have any parts or PCB's for sale.  If you, or somebody you know, wants to fab and sell PCB's, or even kits, to the DIY community, please contact me at:


So here lies everything you need to build them for yourself:

The schematic: 37k PDF file

BOM (Bill Of Materials): 20k XLS spreadsheet

NEW 4/30/08 - Custom power transformer specification: 46k PDF file

Schematic and PCB source files (OrCAD .DSN and PADS-PCB .JOB files): 44kB ZIP archive (see the CAD page for tools)

Or, Gerber files to fab PCB's: 66kB ZIP archive

Full mechanical drawing: 271kB AutoCAD .DWG file, or 626kB R12 .DXF file

Transformer spec, 447kB PDF file

User's Manual, 115kB PDF file


More pictures...

This is an inside view showing the original custom EI-core transformer (which came out at too high a voltage, hence the big brown resistor you see sitting on the transformer).  In this small case, I had a hard time getting the noise out, so I eventually switched to a toroidal power transformer.

Here's the inside with the custom transformer made by Plitron.  If you want to order it, they should still have the design - you might have to buy a bunch of them though.  The Plitron design number is 7410-X0-01.  They cost me about $45 each in lots of 25 pieces - expensive - but with this the amp was very quiet.

The 7044 tube (GE mfg, this one branded Penta), and JJ ECC82, which is what shipped in the amp...


Here are the specs published at the time:

General 

Input impedance: 50k ohms

Input connections: Unbalanced, gold-plated RCA

Tube complement: 1x ECC82 (12AU7), 2x 7044

Power supply: Silicon rectifier, 3-stage RC filter

Power requirement: 117V, 60 Hz, 50W

Dimensions: 9.5 W x 3.6 H x 7.1 D

Weight: TBD

Warranty: One year on all parts, including tubes

As a Headphone Amplifier

Frequency Response :

600 ohm load: <9Hz - 250kHz +/- 3 dB

                32 ohm load:  12Hz 250kHz +/- 3 dB

THD+N (1kHz, 1V RMS out, 100 ohm load): <0.12%

Noise: TBD

Maximum output voltage (5% THD): 

                600 ohm load: 13V RMS

                32 ohm load: 1.5V RMS

Output impedance:  Approx. 35 ohms

Output connection: Standard stereo headphone jack

As a Pre-Amp

Frequency Response (10k ohm load) :

<9Hz - 250kHz +/- 3 dB

<9Hz - 95kHz +/- 1 dB

THD+N (1kHz, 1V RMS out, 10k ohm load): <0.18%

Noise: TBD

Maximum gain: 28 dB

Maximum output voltage (5% THD):

10k ohm load: 15V RMS

600 ohm load: 8V RMS

Output impedance:  Approx. 35 ohms

Output connections: Unbalanced, gold-plated RCA