Direct-Coupled Tube Headphone Amp

Here’s an idea for a direct-coupled tube (or tube-MOSFET) headphone amplifier. It is a simple variant on a widely used circuit, so I’m sure something similar is already out there. Conceptual schematics for the tube-MOSFET and tube-tube versions are shown in Figs. 1 & 2. A DC level shifter consisting of zener diodes allows the circuit to be direct coupled, with the output at 0 V DC, while also allowing the tubes to operate at high voltages (200 V), where they work best. I can’t claim any creativity on the level-shifter idea, since similar techniques were used to direct-couple tube operational amplifiers back in the 1950s & 1960s. Back then, neon lamps were used instead of the zener diodes. But I haven’t seen this exact circuit for a headphone amplifier before, and it elegantly eliminates the low-frequency phase shift and distortion from an output coupling capacitor or transformer.

Note that a different method of direct-coupling is used in our Minotaur hybrid power amp (http://tavishdesign.com/products/minotaur), where level-shifted power supplies allow different portions of the circuit to operate at different voltages, but ultimately the concept is the same.

The output device in Figs. 1 and 2 could be a MOSFET, a tube, or an NPN. I use high impedance Sennheiser headphones, so the tube output would probably work fine for me, but I suspect the MOSFET or NPN output would have lower distortion. With a negative high-voltage rail, the output device could even be a 6AS7 (although that is getting a bit impractical for a headphone amp, assuming you want it to be compact).

Figs. 1 and 2 are conceptual schematics, drawn to emphasize the direct coupling, but the output DC offset stability is likely to be a problem. Fig. 3 is a more practical version, with a modified circuit to control DC offset. A DC blocking capacitor in the feedback loop (C28) reduces the gain to unity at DC, allowing the DC offset at the output to be stable to within ±50mV or less, which is more than good enough.

A direct-coupled circuit also requires DC offset protection circuits, in case a tube or transistor fails, or a tube is removed from its socket. Fig. 4 includes a window comparator which opens the output relay if a low-pass filtered version of the output offset voltage exceeds ±0.65 V or so. There is also a turn-on delay for the output relay.

I’m planning to develop a PC board for this design soon. Any suggestions before I do? I assume there are similar circuits out there? How much current is really needed for low impedance headphones? Is there a better output MOSFET than the IRF540? Thoughts about an NPN (or Sziklai) output version? And note, this basic topology could be used to build a direct-coupled hybrid power amplifier as well. I’ll update this website as the project proceeds, adding design updates and measurement results as I have them.

Fig. 1: Conceptual schematic of direct-coupled tube-MOSFET headphone amplifier

Fig. 2: Conceptual schematic of direct-coupled tube headphone amplifier with 6SN7 output stage

Fig. 3: Practical schematic of direct-coupled tube-MOSFET headphone amplifier (right channel shown).

Fig. 4: Turn-on delay and DC offset detection circuitry

Fig. 5: Power supply circuitry. An 18VAC at 1A, 50/60Hz input is assumed, such as from a wall transformer.

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