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"The load will obviously influence the current draw in the output section. Since ours is class A, the constant quiescent current in both Fets is much higher. By raising our output current we drive down distortion which of course generates more heat. In our first proto we used just one heatsink. Current flowed not just through the Fets but also the voltage regs. Afterwards we decided to go dual mono. Now we use two heatsinks and channel-specific voltage regs. A final important drive characteristic is low output impedance which too is influenced by the fixed class A bias. Ours is always 3Ω. That's a direct reflection on true class A bias and the struggle between heat dissipation and impedance. In our scenario both heat sinks operate at a typical 55°C. That creates a safe ambient temperate to maintain long life for the capacitors.


"Were I to double the quiescent current, the chassis should really grow as big as the Hex or beyond to contain the larger heat radiators. Of course one could use feedback to drive down impedance further. But this also reduces gain to soon want additional gain devices. About driving every type of headphone, for most we can create continuous 115-120dB SPL depending on efficiency but the design is not made for very exotic types like the HE-6 and probably won't reach this level but will be around 100dB. These are safe margins as some more headroom is available.

"Protection becomes paramount when you drive very expensive headphones with a class A device where a DC error could blow out the voice coils. Hence a full 60% of our board is dedicated to protection. We use a servo offset control system to keep the output at zero volts and render it insensitive to thermal drift. Should DC voltage rise above a programmed threshold, the output relay interrupts the signal. Depending on subsonic content in the source signal which could create undue excursions for the headphone diaphragm, the same output relay gets triggered. This avoids in-rush current pops and clicks during start-up too.

Alps pot, front-panel logic board, 2 x 22'000uF Epcos caps.
"The power supply transformer is a 30VA unit whose 2 x 15V AC is rectified then stored in two huge 22'000uF capacitors. Thereafter power splits into dual mono with discrete voltage regulators and circuits to create constant current through the Fet followers. For a final curiosity stat, the main board and 10dB gain module have altogether 669 thru-holes."

Two front-panel switchable inputs, one output, 0/10dB gain toggle, power IEC.