What are the impedances, levels etc. on audio inputs and outputs?

Jetrye

hi
I'm new on this.

I am considering buying a Bela and searching information about the audio connections.
I found some information in https://learn.bela.io/products/bela-boards/bela/#bela-tech-specs, but there are some information I failed to find:

Audio input and output channels.
- What are the levels (dBu), impedances (Ohm), SNR (dB)?
On-board speaker amplifiers.
- What is the Power (W), impedances (Ohm)?
Balanced audio lineout.
- Does it exist? I failed to find it in the pin diagram: https://learn.bela.io/pin-diagram/.

giuliomoro

Jetrye What are the levels (dBu), impedances (Ohm), SNR (dB)?
Some more details could be found here: https://learn.bela.io/using-bela/about-bela/bela-hardware/#tech-specs

balanced line outs are on J9, see https://learn.bela.io/using-bela/about-bela/bela-hardware/#unpopulated-pads

From the codec's datasheet (https://www.ti.com/lit/ds/symlink/tlv320aic3104.pdf) we get the following specs:

SNR for the DAC is 102dBA and for the ADC is 92 dBA. As on Bela the word width is 16 bit and the noise floor on the board is higher than in the tests performed by the chip manufacturer, real-world figures are closer to 84 dB (unweighted) roundtrip SNR
input impedance is 20k
balanced line outputs have 10k minimum load, output voltage is 0.707Vrms (unbalanced) or 1.414Vrms(balanced). Voltage swing is between 0.4V and 2.3V
headphone outs can drive down to 16 ohms, 0.707Vrms (unbalanced)

The power amp on Bela cape Rev C is a PAM8406 https://www.diodes.com/assets/Datasheets/PAM8406.pdf. By default it is configured in AB mode, powered from 5V, which allows down to 3.1W on 4 ohm at 10% THD. By cutting the AB/D solder bridge, it can be set to class-D, thus reaching 5W on 2 ohm at 10% THD.

Jetrye

giuliomoro
Thank you for the fast answer. (Sorry for my slow response.)

I have some more questions.

Have I got this right: Input level = 0.707Vrms => about -0,8 dBu?
codec's data-sheet mentions an "input mix attenuation=12dB". Is that possible to set in Bela? (It would make it better suited for guitar input: 4Vpeak and 80k impedance.)
When you write Headphone outs, is that the three-way Molex output?
Is it possible to reach the Bela IDE through an Ethernet connection?
Is it possible to find the schematics on Bela, as I did for CTAG? https://github.com/ctag-fh-kiel/bela-ctag/tree/master/pcb-design/schematics-pdf

Would it be possible to add a link to https://learn.bela.io/using-bela/about-bela/bela-hardware/#tech-specs
in the page https://learn.bela.io/products/bela-boards/bela/#bela-tech-specs ?

giuliomoro

Jetrye Have I got this right: Input level = 0.707Vrms => about -0,8 dBu?

so it says https://www.analog.com/en/design-center/interactive-design-tools/dbconvert.html

Jetrye codec's data-sheet mentions an "input mix attenuation=12dB". Is that possible to set in Bela? (It would make it better suited for guitar input: 4Vpeak and 80k impedance.)

Yes, you can set that setting the input gain in the IDE to a negative value. That will set the PGA to 0dB and work on the attenuator.

Jetrye When you write Headphone outs, is that the three-way Molex output?

yes

Jetrye Is it possible to reach the Bela IDE through an Ethernet connection?

yes. It's set to look for a DHCP server on it by default, but it can be set to have a static IP if that's preferred.

Jetrye Is it possible to find the schematics on Bela, as I did for CTAG

http://github.com/BelaPlatform/bela-hardware

Jetrye Would it be possible to add a link

makes sense