Confirmation on Bela Mini guitar pedal in/outs?

anteater

Hello,

I'm new to the Bela and trying to use it as a guitar pedal - there have been a few other questions to this effect, but just to be absolutely clear since I'm pretty new to this, I will likely have to put some sort of buffer on the input to prevent tone loss? And then from there that can connect directly to the audio in pins on the Bela Mini? And then, after processing (with Supercollider, in my case), I can send the audio out to something like an amplifier? Is that the correct way of doing this? So in sum:

guitar input -> buffer circuit -> audio pin -> Supercollider processing -> audio out pin -> amp

Is that right?

Thanks so much for the help.

giuliomoro

yes that looks pretty much right. The input impedance is around 10k if I remember correctly. Strictly speaking, you only need a dedicated buffer circuit if you connect your guitar's passive pickup directly to the Bela input. However, if you are using any non-true-bypass pedals before Bela, their buffer will do the job for you and so you won't need a dedicated buffer. For the audio output, you can use the line output that's present on the cape instead of the headphone output that's available on the Molex. That should be a bit quieter.

anteater

giuliomoro Thanks! What is the simplest possible recommended circuit to achieve the buffer? Especially if one would like to ideally use the 5V out from the Bela rather than a 9V battery? I see lots of simple JFET buffer circuits (ex. https://www.pedalpcb.com/product/jfetbuffer/) , but I'm not sure (as a electronics novice) if they would work with the 5V line.

giuliomoro

That won't work: you need a very quiet power supply line for that type of circuit to work because the resistor divider brings any noise from the power supply straight into your signal with just 6dB attenuation. You could try that circuit powered from 3.3V but that may still not be quiet enough. The quietest voltage source on BelaMini is 4.096V on the analog input header, that's labelled "REF", but I am unsure what its capabilities in terms of current are. It is the REFP pin of the ADS8166 ADC that is used for the analog inputs, so it's possible that using that may have some effect on the analog input performance, but that's unclear from the datasheet.

Yet another option would split the voltage reference from the JFET's power: use REF as a voltage reference, connecting two large valued resistors between that and GND, with the mid point connected to C1 and the JFET's gate. You should instead power the JFET from 3.3V. Resistor values to obtain the correct biasing would be 1M and 2.5M. Something like this:

         REF    3v3
          |      |
          R3     |
    100n  |    |--
IN --||---+----|--
          |      |
          R2     R1
          |------|    
         GND

with R1=3.3k, R2= 1M, R3= 2.5M