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. A DC level shifter 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.
The output device 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 little bit impractical for a headphone amp, assuming you want it to be compact).
Note that these 2 attachments are conceptual schematics, drawn to emphasize the direct coupling. I posted a more practical version on the “downloads” page of my website, with a modified circuit to control DC offset, along with the required DC offset protection circuits.
Downloads – Tavish Design
I haven’t built it yet, but I’m planning to develop a PCB and build it 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.
Scott
The output device 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 little bit impractical for a headphone amp, assuming you want it to be compact).
Note that these 2 attachments are conceptual schematics, drawn to emphasize the direct coupling. I posted a more practical version on the “downloads” page of my website, with a modified circuit to control DC offset, along with the required DC offset protection circuits.
Downloads – Tavish Design
I haven’t built it yet, but I’m planning to develop a PCB and build it 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.
Scott