This is the first of a few Millett Hybrids that I've built. It's based on the PC Board from
diyforums.org with pretty standard components. I used an Alps RK27 "Blue Velvet"
potentiometer for the volume control, all other resistors were Vishay Draloric. I used Xicon MKT 0.22uF caps for all the film
caps, and 50V Nichicon Muse for all the electrolytics, except for a pair of Panasonic FM's on the cathode bypass caps. The values are
pretty much bog standard, with the exception of the output coupling caps (470uF), the first PSU bypass cap (also 470uF), and the
cathode bypass caps (470uF/16V). At present, the tubes are Zenith 12FM6, output buffers are BUF634's.
I housed the whole thing in a the longer Hammond case. I powered it with a generic TREAD. Heatsinking of
the voltage regulator is provided by the case, and isolated by a mica pad (neither the regulator nor the case gets warm). I used
one of the nifty volume knobs from Apex Jr., and the LED is a high-brightness blue
that's been tamed down a bit. Input jacks are Tiffany style RCA's (also from Apex Jr.), output duties are handled
by a switchcraft 1/4". AC Power is supplied by a 24VAC 830ma wall-wart.
I have the PSU set to 26V, with the tubes biased at 16. I'm not sure how much of a difference the bias
makes, as I haven't been able to hear a huge difference between the standard 1/2 supply bias and the higher bias voltages.
I just sort of picked one inbetween what Pete Millett suggests in his article for lowest distortion versus the highest
drive.
The Millett is one of my favorite headphone amps. It tames the Grado's, so I use it extensively with my
HF-1's on my computer rig. At some point, I'm hoping to build a Millett-style amp using a +/- 12-15V power supply for the buffers
to get rid of the output coupling capacitor, perhaps taking the amp to the next level. The diyforums guys seem to be
revising their boards fairly regularly, so perhaps they'll include that as an option at some point.
