Auto de FET

An all-analog 15 W guitar amp head with every parameter of its analog circuit under software control. The guitar's signal never meets a converter: JFETs, op amps and an analog VCA make the sound, while a microcontroller sets each of them. It plays like an old-fashioned analog amp, and stores and recalls its sounds like an old car radio.

Software control of an analog circuit. The knobs on the front panel aren't in the signal path. The microcontroller reads each one through its analog-to-digital converter, and turns the reading into settings for the circuit: SPI digital potentiometers (MCP41010 and MCP41100) set the bass, middle, treble, level, reverb and master; a current from its digital-to-analog converter into an LM13700 transconductance amplifier sets the gain; and its logic outputs switch the bright and gain-range networks. Turn a knob and the amp answers as an analog amp would, but every setting is a number the microcontroller holds.

Presets in an all-analog amp. Because the whole state of the circuit is those numbers, a sound can be stored as a preset and recalled instantly, from eight preset buttons on the front panel, from a footswitch, or over MIDI, with nothing in the audio path changed but the settings themselves. That is the point of the design: the tone and feel of an analog amplifier with the recall of a digital one.

The digital board. The microcontroller is a Cypress PSoC 4, an ARM Cortex-M0, on a board of its own joined to the analog board by a flat cable. A 74HC138 decodes the chip selects for the digital potentiometers, 74HC595 shift registers drive the red and green status LEDs, and a key matrix reads the preset buttons and the mode switches. There are MIDI in and out, a footswitch input, and USB for in-system programming and debugging. Most of the hard work was the interface itself: connecting high-gain analog guitar circuits to a microcontroller without the one disturbing the other.

The analog board. J113 JFET gain stages with the LM13700 VCA and an overdrive; the three-band EQ; an effects loop and a reverb module (BTDR-2H); and a 15 W power amplifier on TIP31C and TIP32C output transistors, run from an 18-0-18 V, 25 VA transformer.