Some background:
These last weeks i have been thinking on building a headphone amplifier as part of a larger project (USB interface). I thought that i could use the CS43131 inside my Meizu Hifi Non Pro. But i wanted to do it with output opamps to have more power available, as this is going to be a desktop interface. However, i could not find any schematics of any of the CS43131 dongles with opamps and it is well beyond my knowledge to design one from scratch.
In the meantime, @jkim posted his excellent thread on the analysis of distortion issues some of the CS43131 dongles show. There is no consensus as what is causing the issues, but they are not present in the same degree in all the dongles. Having full control of the i2c programming of the DAC chip, I thought that my DIY amp would offer the chance to play with the different settings and maybe see what is going on with the different implementations of the chip.
Before moving forward with the PCB design, I needed to know which of the versions of the chip (solder ball or qfn) the Meizu non pro has, so peeked through its little window.... and no surprises, what i could see there inside was very similar to the Meizu Pro.
The Meizu Pro is, to my knowledge, one of the only modern dongles for which a detailed teardown exists. The bad news is that both the USB bridge (CS46L41) and DAC have a solder ball package, so the i2c communication between them is through inner layer tracks, and cannot be tapped.... Additionally, this package of CS43131 is tiny (3x2.7 mm) and has 42 pins, and I don't think i am capable of reballing and soldering such a part with any significant chance of success. On top of everything, i could see what seemed to be a conformal coating on the PCB: I have access to all kinds of solvents and i am confident i could get rid of it, but it might not be a clean easy process.
All this made me almost abandon the project, but i though.... what if the Non Pro is just a Pro, but without the opamp populated?? This would solve all my problems at once: i would have the schematic i was looking for + i could use the existing PCB without having to solder any microscopic multi pin IC.
With all this, the curiosity was too strong and this is what i found:
The dongle has an aluminium case, and the natural way of removing it, is to pull in the direction of the cable. I tried this but it didn't work, and when i noticed i was destroying the USB connector, i abandoned this route and used the dremel to cut open the case. At this point, i wanted the dongle to keep working, so i cut the sides carefully. Aluminium is relatively easy to cut. This was an easy process overall:
Once done, i could peel off the case, and the first good surprise was already visible. Unpopulated footprints!:
There is a second metallic shield inside, exactly like the Pro version, that is easy to peel.
The other side is where the bridge and DAC are, covered by the transparent window, that is a sort of methacrylate like hard plastic:
DAC, USB bridge, SPI flash memory, crystal, and pasives. This side is exactly the same as the Pro, even the PCB marking is the same. I believe the PCB is 6 layers, and yes, conformal coating covering both main chips. I wonder if it is to prevent them to fall apart because of the vibration and shocks a pocket device will have to stand:
The analog side is also the same than the Pro, but without the OPA1622, LM27762 and that other IC marked "9F HD" on the lower right corner that i don't know yet what is, a regulator maybe? if anyone has a guess please let me know. The larger rectangle is for a metallic shield encapsulating the output stage in the Pro version, and the best yet is that there is no coating here, one could solder directly on the pads:
Pro (from online teardown):
hifigo.com
Looking closely, some of the resistors that divide the PCB in two are missing in one or the other version. The ones in the non Pro that are not populated in the Pro are 0 Ohm, what tells me that they are just the connectors to bypass or not the opamps.
Well, the good news is that the dongle still works in this state. I will now have to reverse engineer the circuit. As I won't have access to the output pins of the DAC, i will have to do it backwards, starting from the headphone terminals. What i would not be able to know is the value of the resistors around the OPA1622 and the LM27762.
Help needed: if someone has an idea of how the schematic could be and what the 9F HD IC is, i would appreciate very much the information. I will publish here the schematic as i read it, it does not seem too complicated, i think it is doable, but any help will facilitate things to me, I am a complete noob in this sort of circuits. Thanks!
These last weeks i have been thinking on building a headphone amplifier as part of a larger project (USB interface). I thought that i could use the CS43131 inside my Meizu Hifi Non Pro. But i wanted to do it with output opamps to have more power available, as this is going to be a desktop interface. However, i could not find any schematics of any of the CS43131 dongles with opamps and it is well beyond my knowledge to design one from scratch.
In the meantime, @jkim posted his excellent thread on the analysis of distortion issues some of the CS43131 dongles show. There is no consensus as what is causing the issues, but they are not present in the same degree in all the dongles. Having full control of the i2c programming of the DAC chip, I thought that my DIY amp would offer the chance to play with the different settings and maybe see what is going on with the different implementations of the chip.
Before moving forward with the PCB design, I needed to know which of the versions of the chip (solder ball or qfn) the Meizu non pro has, so peeked through its little window.... and no surprises, what i could see there inside was very similar to the Meizu Pro.
The Meizu Pro is, to my knowledge, one of the only modern dongles for which a detailed teardown exists. The bad news is that both the USB bridge (CS46L41) and DAC have a solder ball package, so the i2c communication between them is through inner layer tracks, and cannot be tapped.... Additionally, this package of CS43131 is tiny (3x2.7 mm) and has 42 pins, and I don't think i am capable of reballing and soldering such a part with any significant chance of success. On top of everything, i could see what seemed to be a conformal coating on the PCB: I have access to all kinds of solvents and i am confident i could get rid of it, but it might not be a clean easy process.
All this made me almost abandon the project, but i though.... what if the Non Pro is just a Pro, but without the opamp populated?? This would solve all my problems at once: i would have the schematic i was looking for + i could use the existing PCB without having to solder any microscopic multi pin IC.
With all this, the curiosity was too strong and this is what i found:
The dongle has an aluminium case, and the natural way of removing it, is to pull in the direction of the cable. I tried this but it didn't work, and when i noticed i was destroying the USB connector, i abandoned this route and used the dremel to cut open the case. At this point, i wanted the dongle to keep working, so i cut the sides carefully. Aluminium is relatively easy to cut. This was an easy process overall:
Once done, i could peel off the case, and the first good surprise was already visible. Unpopulated footprints!:
There is a second metallic shield inside, exactly like the Pro version, that is easy to peel.
The other side is where the bridge and DAC are, covered by the transparent window, that is a sort of methacrylate like hard plastic:
DAC, USB bridge, SPI flash memory, crystal, and pasives. This side is exactly the same as the Pro, even the PCB marking is the same. I believe the PCB is 6 layers, and yes, conformal coating covering both main chips. I wonder if it is to prevent them to fall apart because of the vibration and shocks a pocket device will have to stand:
The analog side is also the same than the Pro, but without the OPA1622, LM27762 and that other IC marked "9F HD" on the lower right corner that i don't know yet what is, a regulator maybe? if anyone has a guess please let me know. The larger rectangle is for a metallic shield encapsulating the output stage in the Pro version, and the best yet is that there is no coating here, one could solder directly on the pads:
Pro (from online teardown):

Meizu Hifi pro DAC Amplifier Teardown | Hifigo
Now that 3.5mm headphone output is nearing extinction in smartphones, it's time to start compiling the list for the best USB-C to 3.5mm adapter DACs. Many phones don't even offer analog output anymore on their USB-C output, so the only option is to have a dongle/adapter with a built-in DAC.Meizu...

Looking closely, some of the resistors that divide the PCB in two are missing in one or the other version. The ones in the non Pro that are not populated in the Pro are 0 Ohm, what tells me that they are just the connectors to bypass or not the opamps.
Well, the good news is that the dongle still works in this state. I will now have to reverse engineer the circuit. As I won't have access to the output pins of the DAC, i will have to do it backwards, starting from the headphone terminals. What i would not be able to know is the value of the resistors around the OPA1622 and the LM27762.
Help needed: if someone has an idea of how the schematic could be and what the 9F HD IC is, i would appreciate very much the information. I will publish here the schematic as i read it, it does not seem too complicated, i think it is doable, but any help will facilitate things to me, I am a complete noob in this sort of circuits. Thanks!
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