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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.
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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.
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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.
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