This auction is for ONE bare PCB.

I started this auction by selling extra boards from a personal project.  The reception was great so I had more made and made a few improvements.  The original goal of this project was to have a compact octal tube OTL headphone amplifier that would drive low impedance headphones (down to 32 Ohm).  I chose an Aikido input with a White Cathode Follower output (input/output stage design credit to John Broskie of TubeCad).  Sounds easy enough to pull off but the power supply requirements were a bit tricky.  I wanted to use a single standard power transformer with high voltage and filament supply.  I also wanted an external choke.  All those needs forced a custom PCB.  The tricky thing was the power supply design with DC heaters in mind.  To drive low impedance headphones with the White Cathode Follower circuit requires the use of 6BX7 or 6BL7 output tubes.  Those pull 1.5A each.  With 6SN7 input tubes the entire heater draw adds up to 4.2A of DC current.  That's not trivial.  After a few tries I made some great boards that work perfectly.  It sounds clean, loud, and amazing.  This will drive any pair of headphones you may own.

Heater Circuit:

• Regulated 6.3VDC from 6.3VAC winding with the use of schottky diodes and super low dropout regulator.  Trimpot adjustment to dial in the final voltage.
• Huge capacitance for ultra-low ripple.  45,000uF capacitance.
• Full 4.2 amps of DC heater current.  High current components and heat management designed in.  Heat sink on LDO and heat sinks on all rectifying diodes.  Remember you need 1.8X the DC current from your transformer.  A safer rule of thumb is 2.0X the DC current. The transformer I use puts out 8A @ 6.3VAC.   

High Voltage Circuit:

• Regulated output using Maida circuit.
• Huge capacitance for ultra-low ripple.  256uF capacitance.
• Tube rectified power.  I like the look of the extra tube and I like the slow start.  Use a 5AR4/GZ34.  Want to use a cool looking 5U4G instead, you just need more B+ headroom due to the additional voltage drop.  In the US, the Hammond 372JX does the trick.
• Designed for external choke filter (C-L-C).
• Ridiculously quiet power.

Additional Features:

• ¼ B+ reference circuit built in.  Elevates the heater ground to ¼ of the B+ voltage.  Provides for hum free power and max heater/cathode difference isn't exceeded.
• House ground isolation circuit built in.  House ground attach to the board for a hum free experience.
• Chassis ground tied through one of the mounting holes
• Designed for use with high end audio grade components (Wima MPK series, Nichicon KX and KW series, etc).
• 500uF coupling capacitance on output.  I'm using a 470uF electrolytic with a 30uF film capacitor in parallel.  Response well below 20Hz. 
• Small and compact.  All this circuitry is smartly arranged on a 6.50” x 7.75” board.
• Super thick PCB (2mm) with 2 ounce traces!!!
• Schematic shown above.
• I purchased a B18-HB case from Ebay a while ago for this project.  I most recently purchased a 1907e chassis from aliexpress that had the same dimensions.  194*70*311mm are the external dimensions.  This is the case I sized the PCB for.  Don't skimp on the airflow if you go custom.  CAD is shown to make cutouts simple.  If you choose the Hammond 372JX transformer instead of the 272JX, note that it's 0.50" longer and a few holes have to move.
• Uses (2) 6SN7 input tubes and (2) output tubes being either 6BX7 or 6BL7.
• 4 jumpers are provided to switch between the (2) 6BX7 and 6BL7 output tubes.  The White Cathode Follower is a little different than most output stages.  It's optimized for a given output impedance with a given tube.  The two output tubes that work in this application are similar, but have different operating points as a result.  Instead of a compromise I fixed two output points into the design.   Move the 4 jumpers and swap tubes.  Simple.
• Everything you need to build this is spoon fed.  Use this board, the BOM below, and find an appropriate case that has a volume pot and the jacks. 

As described above, I've designed this with jumpers so you can switch between precise operating points for 6BL7 tubes and 6BX7 tubes for headphone in the 32 Ohm range.  It's a simple task to use those jumpers to switch between 32 Ohm and 300 Ohm for one of those output tubes instead.  Many users love the 6BL7 tubes and uses this amp with 32 Ohm and 300 Ohm headphones.  The BOM below defaults to a 32 Ohm setup with tube swapping.  Make substitutions from this chart if you want the impedance and operating point options from the jumpers.  

What if you're headphones aren't 32 or 300 Ohms?  The formula and chart below will help you dial in the value for R17.  No need to be super exact, just get close.  For example, I have a pair of AKG K702 headphones that have an impedance of 45 ohms.  I use the 32 Ohm setting and it works fine, because it's literally a matter of a few ohms difference on R17.

A BOM from Mouser is below.  I used all high end components (Wima and Nichicon Audio caps, etc).  Full schematic and board layout are above.  Please know your skill level and knowledge of electronics before considering this PCB.  If Mouser is out of something try DigiKey.  You should be comfortable specifying substitutions, although I do my best to take care of that below.  For example, if a capacitor is out of stock try a slightly higher voltage or capacitance of the same lead spacing and series.  Or try another high quality capacitor with the same specs.  Also, there's some wiggle room with resistor values of the same wattage.  However, I do my best to keep the Mouser BOM up to date and also list alternatives below.

*** The Microchip LDO that's vital to this circuit is out at Mouser (and everywhere else I've looked) and has a long lead time.  I have some available that can be purchased with this PCB.

Quick way to load the BOM below is to do a web search for "Mouser Project & Cart Sharing".  Follow the first link and then enter this project ID: b3769f5416