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The Powered Wire: Class A Amp
My audio blog is called the powered wire, and this project is one of the reason why. This is the simplest Class A amp I have ever seen. Mark Houston published the "Class-A 2SK1058 MOSFET Amplifier" in 2007. (link to orginal project) After reading his post I started playing with the idea of building an amp that would be driven by my tube based headphone amp (Bravo Audio V2 Class A 12AU7 Tube Multi-Hybrid Headphone Amplifier), this should supply the needed voltage to drive the MOSFET.
Schematic for the Powered Wire project, the red line shows the single path, it passes thru only 4 devices (3 passive, one active):
This simple MOSFET amp was fun to build and tinker with. It lacks power to be usable, but it dove my Polk Audio bench speakers (8Ω / 89dB) to a listenable level. The voltage drop / current limiting resistor (R4) does get hot, and is completely inefficient. The simplicity is what makes it intriguing, and the sound is pure and uncolorized (yes I know that’s not a word). I also discovered there is no need for the headphone amp. I was able to drive it to saturation with a standard line level (1vpp) input.