Hi, i am using pepper v2 and just realised that the audio inputs are ac coupled. is there an easy/safe way to change them to dc coupled like removing the capacitors that are in charge of ac coupling? could you point me to the schematic in pdf format? thanks.

The AC coupling is on the Bela cape itself and there's no easy way around it. You could DC-coupled them by shorting the two sides of C10 (and C11), or replacing each with a 0 ohm resistor.
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But then you'd need to add a DC-shift on Pepper to ensure that your inputs get to LINE1RP and LINE1LP centered around 1.8V. I am not recommending this modification if you are not an expert in reworking SMT components and it will void your warranty.
There may also be an AC-coupling setting in core/I2c_Codec.cpp that may need to be taken out.
Design files are available for both Pepper (https://github.com/BelaPlatform/bela-pepper/tree/master/Design%20Files , pdfs in "renders") and the Bela cape (https://github.com/BelaPlatform/bela-hardware/tree/master/cape, some old pdfs are there but for more recent versions, use Eagle (for revB) or KiCAD (for rev C).

  • x_x replied to this.

    giuliomoro
    thank you.
    it must be an old revision as i bought the cape october 2016.
    are these then C28, C29 ?
    the worst thing what could happen is clipping as there are voltage clamps 0 - 3.3v ?
    i probably will add ac/dc switches on peppers panel.

        x_x are these then C28, C29 ?

        On Rev B1, yes

        x_x the worst thing what could happen is clipping as there are voltage clamps 0 - 3.3v ?

        The Rev B1 cape as the one you have doesn't have clamping diodes, but only ESD protectors, so it's particularly fragile with the DC-coupled mod if used stand-alone. On the other hand, the 84k series resistor from Pepper should provide more than enough current protection.

        Note that the combined input circuit is:

        input jack       84K          15K
AUDIO_IN_R-------R16----+-----R18-----GND
                        |
                        | AUDIO_IN_R_SCALED
        PEPPER          |
======================= + ===================
       BELA CAPE        |
                        +-----+
                       C29    |
                        +-----+
                        |
                     LINE1RP codec input

        So once you bypass C29 you are connecting AUDIO_IN_R_SCALED directly to the codec's input LINE1RP. The common-mode voltage at LINE1RP is fixed at somewhere between 1.2V and 1.8V .. I never measured it and I can't find it on the datasheet, so it's up to you to figure it out ... measure it when the cape is not connected to Pepper.

        Keep in mind that R16, R18 and the input impedance of the codec (which is between 20k and 80k depending on the PGA and any attenuation/boost , see section 10.3.8 "Input Impedance and VCM Control" and Table 8.5 "Electrical characteristics" of the datasheet) will interact together and that whatever the common mode voltage is, that will be the value that corresponds to an audio input of 0 in the digital domain, so you may need to add an extra DC shift (e.g.: with a large resistor (> 10k) from Pepper's +10V VOLTAGE_SELECT_OUTPUT) in order for a 0V input to result in approx 0 in the digital domain.