The Millett Hybrid MOSFET-MAXed
The Millet MOSFET-MAX is the latest refinement in the fine Do It Yourself (DIY) tradition of the Millett Hybrid MAXed series of headphone amplifiers. All Millett Hybrids incorporate a hybrid single-ended vacuum tube gain stage combined with a solid state output stage. The Millett MOSFET-MAX is no different, but includes features and refinements that bring this design to a level of performance and convenience heretofore unattained.
The Millett MAX incorporates all of the following features and more:
- The basic Millett Hybrid circuit,
- onboard, STEPS-quality Power Supply,
- dual JFET-input hybrid MOSFET Diamond Buffer output stage, and
- onboard, e12 delay circuit (abbreviated for the Millett MOSFET-MAX)
- extruded TO-220 heat sink footprint for power supply
- extruded TO-220 heat sink footprints for all four output transistors
- options for MOSFETs on the output transistor positions
- CCS has been changed to a feedback transistor pair with selectable resistor sizes for different currents and superior tube regulation
- power resistor option for heater circuit, allowing higher voltage supplies without damage to tube heaters
- 16mm minimum cap diameter pads throughout, 18mm for the PS
(Note: This is spacing for 1000uF-1200uF caps and larger. The Panasonic FM's in the photo are 1000uF@50V.)
- 15mm lead spacing for film caps, even larger for cathode bypass films
- signal input pads front and rear, 5mm spacing for terminal blocks
- Noble and Alps adjustable pot positions
- onboard Neutrik output jack pads
- tube LED and panel LED pads included
- PS choice of Schottky rectifier diodes or generic axial diode positions, ceramic snubber caps
- board sized for Hammond 1455T1601 case, center post for stability around tubes
- board dimensions: 6.3" wide x 6.2" (160mm x 157mm), 2oz copper on an 0.062" (1.6mm) FR4 dielectric
- In addition, the MOSFET diamond buffer has been configured for dual JFET-input, allowing the maximum in linear response from the tube gain stage.
The Power Supply allows the use of a simple, cheap 24VAC walwart supply (1A minimum recommended for the MOSFET-MAX), but uses a high-quality, linear-regulated onboard power supply with a 4000uf capacitance bank. Capacitor pads allow use of very large boutique electrolytics and film caps. The CCS is superior to the old current-limiting diode, resulting in improved tube current regulation for superior performance.
The DB's use only one set of complementary output transistors for each channel, but with the board mounted heat sinks, biasing up to 100ma each and beyond is possible, allowing the MOSFETs to operate within their optimum power range, and lowering distortion for the very best in performance. In addition, the MOSFET Diamond Buffer has been specifically tuned for JFET input, presenting a very high input impedance to the tube. This results in a linear response by the buffer from the subsonic to hyper-sonic regions (see MAX History).
Finally, for the ultimate in convenience and protection of your headphones, a small subset version of Amb's (Ti Kan) e12 delay circuit is used, resulting in a ~30 sec power delay on turn-on and a quick cutoff. No need to unplug your phones upon power on or power off.
The Millett Hybrid MAX offers the most integrated board design yet available. After populating the board, all that is needed are a knob, input and power jacks, the enclosure, and 24VAC walwart (1A minimum recommended for the MOSFET-MAX).