Rev 2 PCB - 400W amp project

Here is the design I used for REV 2:

So similar file structure to the previous design, but as you can see we use a fully digital amplifier stage now, which removes the DAC section that is previously there. The digital I2S audio signal is directly routed from the ADAU1466 DSP module straight to the amplifier chips. Since we have 1 less ADC, I exposed that extra I2S input line for external sources that could be added in later if I want (AES3/Dante decoder??) if I need. One of the I2S inputs are reserved for Bluetooth (provided by a QCC5125 module). 

Starting with the ADC design, the design changes over to the PCM1863 from the PCM1802. I still used a separate ground plane as I was concerned that the return currents (and ground bounce from both DSP and class D amps) are going to cause noise on the sensitive analog signals. I allowed for both single ended and differential inputs. Since the ADC is going to be a master clock slave, the crystal is not used.

Nothing changed on this circuit, it works really well!

I did change the above however. In fact this is the biggest change. As I mentioned earlier I am using a fully digital amp IC. In addition, each amp gets it's own PBTL bridged chip, which allows the amp to drive the output down to 3 ohms without stability issues, though I still stick with a 4 ohm rating since the inductors cannot handle such high current. As you can see, there is a significant reduction in component count here, which is good! There are 4 of these circuits. I allowed myself to use 0R resistors to set hardware settings and added DNP zobel networks just in case I need them. I still had clamp diodes to prevent damage from no-load conditions.

And now for the microcontroller! I am using a RP2350 microcontroller to drive the I2C (not I2S) to talk to the 4 AMP ICs, the ADC, and talking to the HMI using UART. The MCU enables and disables the PDN pin, sets the config, does PWM sync, and more. As I am still working on code, this part is still WIP.

Lastly, here is power. So the AMPs are powered off the same 3.3V as the DSP and other "dirty stuff". So you may ask yourself if I am crazy to do so especially for a low noise design. The answer is ... no. The reason why is that if you look at the power tree diagram for the TAS5830s, the 3.3V is actually just powering the onboard DSP. The power derived for the internal DAC comes from the PVDD power supply rails (the 29V or so of power). Since a DSP is digital, I don't foresee any issues with an increase in noise as it shouldn't care about ripple). In addition, my switcher switches at around 1.5 megahertz, so ripple should be low. 

However, what can't use dirty power is the 3.3V regulator for the ADC. This is critical as having stable power is important to maintaining a high SNR. Although the ADC has a PSRR of around 80dB, I still wanted the lowest possible noise. To achieve that, I needed to choose a 3.3V regulator with ultra low noise and very high PSRR numbers, and the regulator from Analog Devices should achieve that.

In addition, I lowered the first high current regulator from 12V to 5V. In addition to being able to remove one switching regulator, I also am using 5V PWM fans. Thus the need of a 12V power rail is not present and I used 5V instead.

Lastly, many would ask me, "Jason what are you going to do about the noisy spikes caused by a fan motor"? My response is.... decoupling capacitors! I once had issues with a HMI display causing a slight increase in noise in a PGA2311 board, and adding decoupling capacitors helped in that case. The same remedy can be used here as there was a whole paper I found online where large (47-100uf) decoupling capacitors near + and - power output to the fan reduced the fan's motor noise on power to nothing. I would be doing the same here.

PCB layout:

All layers

Layer 1 - no copper

Layer 1 - with copper

Layer 2 - (GND plane) - with copper

Layer 3 - Power plane (3.3V and 5V) - with copper

Layer 4 - rear signal layer - without copper

Layer 4 - with copper

And oh yeah, EVMs!

Thanks to Dr. Fayed, I got to have this project funded under the SCALE program. Because of this, I was able to order 2 TAS5830EVMs and a PCM1863EVM to test out my connections and code. With that I learned a ton about these chips and software quirks I had to deal with.

I also learned about the issue of ground bounce from him and because of that (and following EVM guidelines as best as I can, I added local bulk capacitors below the board to smoothen the lower frequancy di/dt spikes caused by the switching class D amps.

TAS5830:

Issue 1: No library for the TAS5830 chips (Arduino IDE) - this was my first time actually doing I2C stuff by using the datasheet and not the Arduino library someone already wrote for these chips. Because of that, it was more tedious and difficult from before but I managed to get it to work

Issue 2: PWM synchronization headaches! I can't express how annoying it was to get this to work. I got all the code right but the amp sometimes just jumped into high Z when nothing was playing. Turns out, it always worked when I had stuff playing into the XMOS on the EVM and didn't work if no stuff was playing before hand. Why? It turns out that the TAS needs valid I2S clocks for the sync to work. My guess for this is that it uses the I2S clock as a sort of oscillator clock to keep track of some sort of period. Think of the GPIO sync pin as the sound that plays when runners can start running in a race and the I2S clocks as like the number of laps around the track. So there is no sort of oscillator for the AMP to know where it is at in time without clocks. 

However, it took me a while to figure this out. But issues disappeared when I connected it to my DSP board because it outputs stable clocks, no matter with or without stuff playing. However, it still goes to high Z on reset, but I have a way around that

Issue 3: Pops and crackles caused by I2S signals. This was caused by my I2S lines being too long between the EVMs and the DSP, but was resolved with termination resistors on the DSP transmitter end and changing the slew rate for the DSP to lowest and drive strength to max.