Basic Metal: A FET-tastic dirt circuit
After much tweaking and some setbacks, I’m pleased to announce that my take on the classic “hard-rock / vintage metal” sound is, at long last, ready for public consumption. This one is 100% “discrete” in the sense that there are no integrated circuit components (operation amplifiers or other ICs). It’s all field-effect transistors doing the amplifying and clipping…there aren’t even any clipping diodes in this circuit. There ARE diodes in the circuit, but they are for polarity and over-voltage protection (not in the audio path).
What does it sound like?
To me, it sounds sort of like a pushed Marshall JCM800 or an Orange Rockverb cranked up. With the GAIN set high, it’s got boatloads of compression and sustain and also a lightly fuzzy low end (but still fairly tight). At lower gain settings, the bass cleans up. It does NOT sound like the classic Mesa high-gain tone.
Bands I think it sounds appropriate for: Black Sabbath, AC/DC (lower gain settings), Guns n’ Roses, Ghost, Faith No More, Iron Maiden, Van Halen, Ratt, Poison, and a bunch of other hair metal bands. This very subject, of course.
Circuit Details – Basic Metal
The Basic Metal is a total of five cascaded / series gain stages: a single JFET input amplifier (boost) driving four MosFET amplifiers, followed by a simple variable low-pass filter tone control. The GAIN control sits between the JFET boost and the rest of the circuit. There is also a high/low gain switch that toggles a bypass capacitor on the JFET’s source pin. The TONE control is taken directly from the venerable ProCo Rat (why mess with simplistic perfection?).
Additionally, there is an output gain trimmer that sets the gain of the final MosFET amplifier. This is to fine-tune the max gain available and allow for compromise between gain and noise. And finally, there is a bias trimmer to dial in the JFET and a bias pad to easily read the voltage with a multimeter.
Availability
I will be building some fully finished pedals using the Basic Metal circuit, with laser-etched wood faceplates. I will also be making the boards available so folks can build their own. I will post an update when things are progressing. Final boards have just been ordered, so it shouldn’t be long.
