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The analog part of the circuit, while functional, has some drawbacks. The authors already mentioned the noise. There are a number of unique requirements when connecting a guitar to a digital pedal like this. The “Teensy Guitar Audio Shield” was designed to solve all of these, so if you’re more interested in learning to code audio effects, and less interested in all the soldering, I designed the “Arduino Teensy Guitar Audio Shield” which might be a good alternative to consider. The free schematics are available under “Datasheet” if you want to build the analog parts yourself and use it with the board in this article.
Here are some special concerns for connecting a guitar to a Teensy (or any ADC really) that are handled by the shield I just mentioned.
- input opamp buffer must be very high impedance, and very low noise. JFET op-amps are bets for this. TL072 is a common choice.
- a wide range of adjust gain OR attenutation is required. The Codec ADC will want line level, 1 volt max. Lots of guitars put out more than 1 volt if pickups are active, you are using a bass, or another pedal in front of the Teensy. 9V pedals can easily get up into several volts of signal ampltitude. If this exceeds the 3.3V supply on the audio codec, you will fry it, so protection diodes are neeed. On the other hand regular single coil pickups are tens of millivolts. You need lots of gain to get the signal up to close to 1V for good quality ADC sampling. The audio shield should handle both gain and attenuation (padding) in a user controllable way.
- Analog audio signals must have supplies isolated from the digital ones, and their circuits must be kept shielded from the digital signals. The latter usually requires careful PCB design. Without these, noise will always be a problem.
I love the concept, but wanted more versatility. I inverted the enclosure (which required drilling a new hole for the 9V plug and another in the “base” of the enclosure for the stomp switch).
Now the “base” is a lid that can quickly be removed and I mounted a small breadboard in the middle of the pcb. I’m using jumpers to the +/- and the in/out connections from the pcb to the bread.
I can now build an easily transportable prototype circuit that I can adjust/experiment with on the fly in real time anywhere- and reuse the proto board for future builds.
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