Input buffer and belamini powered from external power supply

giuliomoro

did you try an isolation transformer (see https://forum.bela.io/d/2330-uart-usb/9 ) ?

quintosardo

giuliomoro Uhm no, as the issues where there even without anything connected to the audio output (using the headphones, instead).

quintosardo

Several burnt units later...

Looks like the answer is:

a fourth order LC lowpass filter (two Ls, two Cs) on external power supply input.
another instance of the filter on Bela's power input.

The filters are relatively heavy (20mH inductors, 2000 uF capacitors), to go down to a 25Hz cutoff frequency.

The weird part of this is that all the literature talks about "radio frequencies" but discontinuous high frequencies can go down to audio.
All examples about switching supplies etc. show filters with high cutoff freqs (well beyond audio range).

I have a beautiful background silence, now, but still wondering if I'm missing something right.

giuliomoro

I meant to suggest an isolation transformer on the audio input, not output.

quintosardo

giuliomoro Oh, no, I didn't try as there's a guitar passive pickup there, so completely floating...

giuliomoro

I mean between the preamp and the BelaMini input

quintosardo

giuliomoro Right, good one, even though they share the ground and I imagine a transformer there to prevent connecting different grounds together.

giuliomoro

And then see if you can get away with a simpler power supply arrangement

quintosardo

Relatively relevant question: is codec's input gain passive-like? Can it be used to match input signal's level to codec's input range? (prevent distortion if input signal is too hot)

giuliomoro

There is an attenuator on there, 1.5dB steps up to 12dB, but I am unsure about its electrical characteristics. I'd assume it can be used for that purpose, or it would be pointless? You can enable that gain reduction by using the input gain setting in the IDE and setting it to a negative value. If it doesn't seem to work, try updating your Bela core code, because I think this was added 12-18 months ago.

quintosardo

giuliomoro Yes, I can see it and it works, i was wondering if it means that, when set low (negative) enough, we can send an input signal beyond A/D converter's voltage range (like if I have a passive attenuation in front of the input).
I think that probably not, that's one reason why I added higher voltage range for the active buffer at the input (to be able to potentially attenuate without hitting the voltage rails).

giuliomoro

I assume the input level will have to be within the specs of the codec (between 0V and 3.3V?) and then the attenuation should work the way you are describing it, otherwise what would be its purpose? In the latest version of our capes we are using a BAS70 dual schottky towards the power rails to protect the inputs, so that should protect the codec. You can find more details about how the attenuation works in the datasheet: https://www.ti.com/lit/ds/symlink/tlv320aic3104.pdf

quintosardo

giuliomoro Perfect, thanks! So I can't send a voltage beyond net input limits and expect it to become compatible thanks to programmable attenuation. I need an external attenuation (passive or active with higher range) before the input, like I did (not just to adapt the impedance).

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