“Due Finn” is a real-time DSP guitar effects headphone amp. It is a personal project inspired by my desire to play electric guitar silently and portably. It is currently being prototyped on a breadboard. The project is based on an open source design by ElectroSmash called the pedalSHIELD
The input signal first passes through an active band pass filter, conditioning it to be read by the Due's ADC. A variable resistor on the feedback loop allows the gain to be adjusted manually. The output from this op amp is read by ADC0 however it also passes through an inverting amplifier. This serves two purposes. It allows the negative half of the audio waveform to be read by the ADC but it also increases the effective resolution of the ADC (from 12bit to 24bit) because the positive and negative halfs of the waveform will be read by seperate 12 bit ADCs. The seperated signals are combined later by the analogue output circuitry.
After being processed, the two seperate positive and negative halfs of the waveform are written out by DAC0 and DAC1. A differential amplifier is used to recombine the seperated signals. The following op amp is redundant in it's current configuration however in future versions it can be set up as a summing amplifier with one of the inputs connected to the output of the input stage, bypassing the Due all together. This setup can be used to provide a delay effect where the original signal is summed with a delayed signal output from the Due.
The schematic features a 5V buck converter, 3.3V linear regulator and a +/-5V voltage converter to provide the positive and negative supply rails for the op amps.
The audio quality can be assessed by listening to the input audio file and the output audio file from the device. The audio is crisp thanks to the 24bit resolution however there is a constant high pitch hiss which is yet to be fixed.
This hiss can be visualised by playing a clean 262Hz sinusoid through the device and taking the spectrum of the output. As you can see from the image below there are 2 spikes between 11kHz and 20kHz.
