I’ve seen my fair share of bad layouts for TDA1541. Some CD manufacturers just butchered the design, some are so so. I can’t remember seeing a single board layout that I really liked and even Philips own CD players are no exception.
So once for a change I tried to make things right and above is the result. It’s a 4 layer job, although I could’ve squeeze it in just 2. But with current Chinese PCB manufacturing costs I really see no reason for doing so. I wont expand on all the routing decisions here, but here are the most important things to watch-out for:
- All power supplies of +5V -5V and -15V must be decoupled to AGND pin 5
- Use additional decoupling between -5V and -15V rails
- Use a separate ground plain layer for all digital signals and connect it to DGND pin 14
- Only connect AGND and DGND ground plains with lots of vias right next to the AGND pin
- Take extreme care about DEM capacitor current return paths, they can’t mix with anything else!
And that’s more or less it. All other mixed domain PCB design best practices do also apply here. Shortest current paths possible, loop inductance, track impedance etc.
In case you are wondering what are those electrolytic capacitors doing there around TDA chip, I have an elaborate answer here. Only difference from the test setup is larger 680uF capacitor for MSB DEM current. It helps to attenuate modulation side-bands even further. On the back side we have I2S input attenuators for all digital signals except LE, which is directly coupled to the re-clocker.
I covered all I2S signal attenuators in this separate article here. For the final board I decided to feed LE signal without any bandwidth limiting. It’s still only 1.8V peak-to-peak and centered around 1.2V. This is a NOS DAC so at analog output it already has a massive energy spike at sampling frequency Fs. Any leakage directly from LE digital front won’t change a thing. I also made some listening tests with fast and BW limited LE signal and couldn’t figure out any difference.
All analog currents are handled by a gold-plated SMA connectors and RG316 coaxial cable. With proper tension this is a second best thing after direct wire soldering to a PCB. I learned this the hard way when I build my low noise measuring pre-amplifier. You can really appreciate a good connector only when measuring a nano-volt range. This is where you see how terrible BNC and RCA connectors really are.
All power supplies are regulated by a low-impedance shunt super-regs. This is where probably 30% of all the R&D time went to. Designing your own voltage regulator that is capable of sub-miliohm output impedance over audio range and capable of working with fast digital loads is not a trivial task at all. I will cover them in a separate article and will probably run a small PCB batch that I will offer in my e-shop. All I can say for now is that this is my “secret sauce”. No amount of low ESR cap decoupling sounded as good as these bad boys. And believe me I have tried some insane stuff. Like 50x1000uF Panasonic FM’s in series. Made a really good spot-welder, but didn’t come even close to how good these shunt’s can sing. Only downside is the heat output, and with all the solid ground planes on a PCB this was a ticket to a thermal nightmare.