Jukkapoika But to power a simple transistor based booster needs 9 volts.
You can bias these to work off 5V if something like +/-2V (4V pk-pk) clean output is enough for the levels you're driving.
I think the first thing to do is to try a JFET booster biased for 5V operation and see if it gives you a hot enough clean output. Only look to more complicated solutions if you simply need more headroom than that.
As for the 5V booster, start with something like this:
** Add a 47 ohm from 5V to "V+" shown in the schematic (at R2). Connect a 100uF parallel 0.1uF capacitor from V+ to ground. This is to reject noise from the 5V supply.
** Change the value of R2 (smaller value) until the voltage measured between R2 and C2 is somewhere between 2.7V to 3.0V) . A value of 4.7k would be a good first guess.
** Remove C3 and the gain will come out near to 10 dB, which might be enough. If more gain is needed add a 1k in series with the 22uF (C3) to get closer to 12 dB.
** Don't use C23 without a resistor in series. The full gain of the booster will require the Bela DAC output to be reduced a lot for compatibility with the booster's dynamic range. The aim is only to make up the gain you lost, not to make a distortion pedal (at least, I think this is your goal).
** If you want to have a volume control pot, use a 10k pot feeding to IN, rather than at the output. This will allow you to maximize the Bela output level and then adapt the drive level into the booster to avoid driving it to distortion.
** Ideally this is a fixed-gain system. You find out how much the input needs to be attenuated and then adjust the output gain (using a resistor in series with C3) to perfectly make up for input attenuation. I can give you rules of thumb, but the actual gain depends a lot upon the JFET you select, but do not fear: you can measure the gain using Bela Analog inputs and scope feature:
ADC CH1 --> signal feeding Bela input
ADC CH2 --> Output from booster
Feed both ADC channels using 3 10k resistors and a 1uF capacitor:
10k + 1uF goes to ADC input.
ADC input will have a 10k to 5V and a 10k to ground.
(Above configuration creates a divide-by-three voltage divider so none of the signals will clip the ADC inputs).
Then feed a 1 kHz sine wave from an ADC output to the Bela audio input and adjust output booster gain until both ADC1 and ADC2 have the same amplitude.
If you need a schematic for calibrating this, let me know. I don't have much time right away, but maybe can carve out some time during a lunch break to make a diagram for this test setup.