Hybrid headphone amplifier

Hybrid headphone amplifier

A simple DIY hybrid headphone amplifier project. Rogers assembled a very simple hybrid headphone amplifier. This headphone amplifier uses 12AU7/ECC82 vacuum tubes as the voltage gain stage, and an IRF612 MOSFET follower biased in Class A with a passive constant current source. An advantage of this project is that both the vacuum tubes and MOSFETs are powered by a 24V external switch-mode power supply. Read on for more details on the 12AU7 vacuum tube/IRF610 MOSFET hybrid headphone amplifier.

I've always been interested in vacuum tube amplifiers, but most DIY kits are very expensive and use very high voltages. Therefore, I decided to build a low-cost, minimal-part amplifier to drive a pair of 32-ohm Grado headphones. After building several Yaha amplifiers and a Szekeres MOSFET follower based on a fa-schmidt design, I wanted to know how the two would sound combined. So, I built the schematic into TINA-TI, a free SPICE-based program for testing circuits before building, and the results were remarkable. I achieved nearly 20dB of gain from a 24VDC supply in the 20Hz-100kHz range.

As you can see in the schematic and parts list, there are fewer than 30 discrete components, which most DIY enthusiasts will have from other builds as spares. I chose the 12AU7/ECC82 vacuum tube because it can be driven at low voltages, and the filament voltage is 12.6 volts, so no further voltage adjustment is needed. I used 1/4W resistors in the first stage and 2W resistors in the second stage. 2W resistors may be too large, but I don't want to replace them later. 20-ohm resistors must be at least 5W, and wire-wound resistors should not be used because their inductive characteristics will distort the response curve.