High noise level when using both DACs in parallel for stereo sound generation


Like so many I am trying to implement a synthesizer with the Arduino Due.
Perfectly, I would like to have stereo output using the two DACs.

Since I need fast processing I use direct writes to the DACs in the form of:

dacc_set_channel_selection(DACC_INTERFACE, 0);    // write to DAC0
dacc_write_conversion_data(DACC_INTERFACE, output);
dacc_set_channel_selection(DACC_INTERFACE, 1);    // write to DAC1
dacc_write_conversion_data(DACC_INTERFACE, output);

Each DAC has gotten a 500 ohm resistor in series and then goes to either tip or ring of a 6,5mm jack.
The base of the jack is connected to ground.

Now the problem:
Low volume, high noise.
If I physically disconnect one of the DACs, sound gets loud and crystal clear.
Unfortunately I have no idea why this is the case, and particularly not, how to have the stereo sound nice.

I checked all connections, they seem fine. Also it does not matter which DAC I disconnect, the sound immediately gets better.

Any ideas are much appreciated!

I wouldn't risk taking the DAC outputs any distance off board, they are very sensitive to
static damage, so an op-amp buffer is needed first.

Also the Due on-chip DAC unit is a poor performer, I wouldn't recommend using it,
stick a fast SPI or I2S DAC on a shield, you'll get much better performance (and 24bit
resolution if you want with I2S)

Your problem with volume sounds like you have driven the outputs in anti-phase...

Thanks MarkT, your hint with the outputs being in anti-phase was totally right.
I have corrected that and now the sound is crystal clear. Admittedly, I had not really understood balanced vs unbalanced cables before - now I do :slight_smile: Thanks a lot again!

I have read on several occasions that the DUE DACs are pretty sensitive to current draw. So was feeding them in mic preamps which I still had laying around. Do you think even that might be too much for the DACs?

I totally agree with you, that an I2S DAC would be much better performance wise. Actually that is the long term plan. However, you have to start somewhere, and for me that was the software so far. I can gerate sound now. In the not too distant future I plan to upgrade the DACs to good external ones. Maybe not audiophile grade, but I saw that there are quite nice ones between 10-20 USD already.