Many own tube-based line-stage amplifier that can develop hug output voltage swings, but cannot drive low-impedance headphones. Actually, even 300-ohm and 600-ohm headphones are obscenely low-impedance loads for a tube line stage to drive. For example, to drive 300-ohm headphones requires an output coupling capacitor at 30uF in value, far bigger than the usual 0.22uf to 4uF coupling capacitors found line amplifiers.
The Triadtron solid-state buffer offers no gain, but does deliver high current into low-impedance lows, such as 16-ohm (and above) headphones. Many standalone DACs and CD players put out over 2V of peak output signal, which is plenty loud for most low-impedance headphones.
The Triadtron PCB hold two unity-gain power buffers. The PCB requires a monopolar power-supply voltage from 12Vdc to 24Vdc, with even 48Vdc possible. If you are like me, you own many orphaned switcher power supplies from now dead or superseded electronics. The current rating needed depends on the desired idle current, but a 1A or 2A switcher wallwart or desktop power supply works well. I run 200mA per channel as the idle current and use a 4A power supply, as that is what I had handy.
The way the circuit works is that the NPN output transistor runs under a constant-current flow (see the attached schematic), so it mimics a constant-current source, while the PNP transistor below it varies its current flow as needed to maintain the constant-current flow of the NPN transistor above it. When the PNP transistor ceases to conduct altogether, the NPN transistor must then increase its current flow. Once its current flow produces a great enough voltage drop across its collector resistor, the HER108 diode becomes forward biased and clamps the voltage drop, allowing bigger positive output voltage swings. The
1N5341B is a 6.2V zener that protects the PNP transistor.
The Triadtron circuit is fundamentally a single-ended one. Appearances to the contrary, the output stage is not a push-pull type—as long as the output current does not exceed the idle current flow. If forced past this threshold current, the circuit switches to push-pull output.
The PCB is 7 by 4 inches and the heatsinks are 1.5 inches tall. They only get quite warm, but never too hot to touch. Still, 400mA of total current consumption against the 24V B+ voltage results in 9.6W of heat. The big red capacitors are 10μF Wima polypropylene types, while the smaller red capacitors are 1μF in value and also Wima. The 10kμF center RC capacitor is a Nichicon audio-grade electrolytic shunted by a 2.2μF polypropylene capacitor. The schematic shows a polarized electrolytic 1kμF output coupling capacitor, but the kit includes a 50V non-polarized type from Nichicon. The 1kμF capacitors are Panasonic FC types.
The PCB come with a user guide and part list. The kit include all the part needed to populate the PCB, along with heatsinks and hex standoffs. The power supply is not included.