A novel design for an all-tube headphone amplifier

Above: prototype of a novel design for an economical all-tube headphone amplifier. I have been hunting for a way to build an all-tube cue amp into the @berlinettabrewing DJ console. I was forced to design this because no decent schematics exist anywhere online for something like this: something compact, simple, and inexpensive to make that can run either 600 ohm cans or modern low-impedance cans.

This design uses a basic voltage amp (1/2 a 12AX7, although a 12AT7 would work fine too) driving one paralleled cathode-follower 6CG7 per channel. In theory, with a big enough output cap and 300 plate volts, this SHOULD be able to drive 600 ohm headphones to a very high level. And it does. But here’s the twist: the “600 ohms” jack is a switching jack that, when blank, sends the signal to a pair of 600 ohm:8 ohm “70v” 5-watt line-to-voicecoil transformers: I used inexpensive old Stancors. These are the sort of transformers that you often find inside old schoolhouse baffle speakers.

At full rated level, the low end of these transformers is weak. But at the 1/2w that they see in this circuit, they sound absolutely fine. The amp sounds great with either variety of cans- i tried @akgaudio 600 ohm k240s and @audiotechnicausa ATHM50, which I suspect are 30 ohms-ish. Way way more then enough level, great fidelity and just great sound all around. A little experiment that paid off, coming soon to the world’s most esoteric DJ console.