Fixing analog audio on the $2.58 HDMI-to-VGA adapter

It seems like this person is incredibly intelligent and resourceful. Why not just design a new PCB for this instead of modding one? Surely the component cost of 2 connectors and a little PCB could still be in the < $20 range and actually output good quality analog signals? $20 is the same as $3 if you’re also buying CRTs just to play Switch games. This is such a hobby level activity I’m sure people would love a kit + a guidebook, could even provide a 3d printed case.

>> A photo revealed the chip was a NX3303X; I could not find any datasheets

Not much information out there but these are similar chips, possibly it’s a clone?

Capstone CS5213 datasheet:

https://file.elecfans.com/web2/M00/2B/BD/pYYBAGHVUbyARd_-AAU...

And also the AlgolTek AG6200 / AG6201 Series, datasheet:

https://assets.yoreparo.com/attachments/28SGHqLW2Z2MfSzEBJB4...

The weird edge cases that live around connectors never cease to amaze. I recall one, I think it was also HDMI (maybe displayport or VGA), that showed there's a very low resolution digital graphic or text signal and some connectors even have a screen embedded in them.

> Annoyingly the input pins had a constant DC offset of over 2 volts, which went away when shorted to ground but reappeared moments after I removed the short.

Mic inputs on audio codecs always have DC bias, because they are DC biasing the microphone inputs so that signals can pass. By default, you would expect ~2-2.7V of DC bias on each microphone channel. This is typically connected through a high impedance (~1kOhm) source impedance. The actual impedance of the input should be extremely high resistance, though, typically in excess of 50kOhm.

The proper way to connect this would have been to a line-in jack on your motherboard. There should be no DC bias on those jacks. Or use an audio input that is 'retaskable' to select the line-in function instead of the mic-in function.

> This large of capacitance is not easily available as C0G. The usual X7R MLCC capacitors are piezoelectric and will change voltage when bumped, which can add interference to audio. Electrolytic caps are an option, but will not fit well on pads meant for 0402 ceramics.

C0Gs are not needed for passing something like this. X5R/X7Rs are commonly what is used for DC blocking capacitors in ADC/DAC solutions and you can still keep extremely good audio performance even with the theoretical microphonic problems for Type 2 dielectric capacitors.

> If we wanted to improve audio filtering, we could design a replacement PCB (four-layer for impedance matching and signal integrity?) with footprints and traces to install a second-order Sallen-Key filter. This would be about the same difficulty as the HDMI2SCART, but likely reusing the existing case and screws unlike the HDMI2SCART's 3D-printed case.

Why not just add your filtering circuit externally via the 3.5mm jack? No limits to what you could do. Cascade as much as you want at that point. Might be a bit ugly, but it wouldn't require re-designing the board.

These chips appear to be 8051s with multi hundred MHz DACs.

Be fun to program them but I can’t find any programmable devboards with the Capstone CS5213.

What kind of mixer do you use?

If it has balanced xlr inputs a DI will completely eliminate the earth hum. That the signal is dropped to mic level doesn’t matter much as the impedance is correct, the connection balanced and the mixer preamp designed for the purpose. // I am somewhat professional sound tech :)

I love it when people go so far into the weeds to solve a problem on their own and then document the process for everyone.

I learned a lot from this writeup, and really enjoyed the read even though I know I would have made it about 10% of the way myself and chickened out and just bought a different product in hopes it would solve the problem.