Great to see this.But according to khadas people, it seems the hump is not easy to solve, I also created a thread on ktb forum to track this issue,
https://forum.khadas.com/t/ess-hump-solution/3746/18
Hi All:
Our guys go to ESS Lab here in Shenzhen, and the hump issue do exist in there DAC chips, they also still don’t have a good solution at the moment.
They will update this issue to ESS Lab in USA, and yes, we will try to improve the issue on current KTB:
Didn´t notice this, as it looks like very short leads/pins on the board. But I guess soldering will be the same:Also KTB uses DFN packaging opamp which I think it's very difficult to change...
The suggested OPA1642 comes in 8-pin SOIC and VSSOP packages, and AD8397 is 8-pin SOIC only. These are not compatible with the 10-pin DFN footprint on the PCB. You'll have to find something else for this project.Didn´t notice this, as it looks like very short leads/pins on the board. But I guess soldering will be the same:
Heating the solder until it becomes fluid and expect it makes a bond to the chip. But removing the
original semiteh opams by an average DIYers means will probably destroy them...
The suggested OPA1642 comes in 8-pin SOIC and VSSOP packages, and AD8397 is 8-pin SOIC only. These are not compatible with the 10-pin DFN footprint on the PCB. You'll have to find something else for this project.
DFN/QFN packages often have an exposed pad in the centre. Working with these requires a hot air station or reflow oven.
The calculated resistor values is based on Rs(Rdac) = 774ohm as listed in the spec. Since its tolerance is 11%, correct value for your board might be different. To verify it, you can measure voltages at RT6862D outputs --- the easiest access point will be from R32, R34, R45, R48 --- all should be around 46.77mV while not playing music. Basically the application note suggests that after correction all four voltages should be around 0V.... So it will be only changing the resistors ...
Or using one for two? . I guess phase inversion will be more complicated.You'd need to sync the clocks of 2 boards. Soldering and scope required.
If my calculation is correct, the needed virtual ground voltage is 1.65*(806/(806+774))=0.84171V, and the current value is 3.3*3.3/(10+3.3)=0.8188V. To make it right, you can either change R40 from 3.3k to 3.424k or change R38 from 10k to 9.638k. The closest 1% resistor to 3.424k is 3.48k & for 9.638k is 9.53k. The other option is paralleling a 267k 1% resistor to 10k to make it 9.639k. Just my 2c.
@HououinKyoma
How precisely should the virtual ground match the recommended value? Within 1%? The tone board already have it within 3% and still have THD hump. And Rs (Rdac) of ES9038Q2M is specified to have 11% tolerance, so even if correct resistors value are installed individual board still can be far off your recommended value unless calibration on individual board is performed. Seriously I think this is a chip design issue from ESS's side to have such a big tolerance and also have critical design parameters based on this very loose value.