So the TPA325x series of chip amps came out about a decade ago, yes really that long! My datasheets for the TPA3251D2 are dated June 2015 and that's where we happen to be now only 10 years later, so, happy birthday TPA325x series. 
🥳🎂
Over the years I've seen it mentioned a couple of times that one could go composite with the TPA325x using an ncore style integrator only I'd never seen it actually done. Now my main amplifiers, for my active speakers, have been based on a PFFB implementation of the TPA3251. The performance is more than acceptable but I'd always wanted to try and go that bit further so figured I'd give the integrator a go. It wasn't my plan for this to coincide with the chips ten year anniversary but there you go.
As you can imagine it took a considerable amount of trial and error, plus simulation time, to get things working correctly. This is especially so as there are no models provided for any of the TPA325x series so everything required a large dose of guestimation.
4 layer PCB from China with the TPA mounted on the bottom side of the board. The PCB being bolted to a bar of aluminium to get the heat out. The bar then being bolted to a case panel once 6 channels have been built.
So some performance graphs.
First up frequency response into no load, 9.4 ohms, 4.7 ohms, and 2.35 ohms.
Next as ASR has made popular 1kHz 5 watts into 4.7 ohm load 48kHz bandwidth unweighted.
Next the same thing but 51 watts.
Here's a two tone 18500Hz + 19500Hz, as Bruno liked to show off with the Ncore amps, at 5 watts into the 4.7 ohm load.
And now at 51 watts.
Next up is a distortion sweep at 5 watts into the 4.7 ohm load 96kHz bandwidth just so we can see what the 3rd harmonic really does towards 10kHz.
And finally a distortion sweep at 51 watts 96kHz bandwidth.
Anyone who is familiar with the TPA325x will immediately notice the significant improvement in high frequency distortion. The main area the TPA325x has issues is at the high frequencies. I'm assuming that, for stability, TI needed to start compensating the amplifier pretty low down in frequency as can be seen from distortion Vs frequency sweeps of the standard implementation.
Here's the datasheet distortion at 80kHz bandwidth.
At higher power levels it can be seen that the distortion starts to rise as low as 100Hz! The composite version completely does away with that and reduces the overall magnitude significantly. So happy 10th year TPA325x and enjoy some higher performance.
Now I wonder when Chi-Fi will come up with something similar. Complete schematics not shown for obvious reasons (at least yet). If China want to copy it they are welcome to, and possibly improve upon it, however they will at least have to figure it out for themselves! Just knowing it can be done should be enough encouragement for them to try. Topping are you there? 🤣
I need to build 4 more channels and then I finally get to listen to something.
By the way TI hurry up and come out with a replacement for the TPA325x series using GaN FETs. You've already done an integrated GaN motor driver gimme some class D. I want the improvement in efficiency and idle power consumption. Not to mention that the ability to switch faster should reduce the needed deadtime and improve performance.
As to the clipping of the TPA325x composite. The usual ncore clipping detector and limiter is used although I haven't tried to really fine tune things. Nothing, so far, has blown up. The TPA325x amps are very good at shutting themselves down if anything untoward happens, and that happened a lot when trying to get this stable. The window of stability being quite narrow. Not enough feedback and you oscillate, too much feedback and you also oscillate. You need to find the Goldilocks zone of stability. One nice thing about the TPA chips is they have a clipping detector so should it be necessary you can shut the amplifier down at the onset of clipping.
🥳🎂Over the years I've seen it mentioned a couple of times that one could go composite with the TPA325x using an ncore style integrator only I'd never seen it actually done. Now my main amplifiers, for my active speakers, have been based on a PFFB implementation of the TPA3251. The performance is more than acceptable but I'd always wanted to try and go that bit further so figured I'd give the integrator a go. It wasn't my plan for this to coincide with the chips ten year anniversary but there you go.
As you can imagine it took a considerable amount of trial and error, plus simulation time, to get things working correctly. This is especially so as there are no models provided for any of the TPA325x series so everything required a large dose of guestimation.
4 layer PCB from China with the TPA mounted on the bottom side of the board. The PCB being bolted to a bar of aluminium to get the heat out. The bar then being bolted to a case panel once 6 channels have been built.
So some performance graphs.
First up frequency response into no load, 9.4 ohms, 4.7 ohms, and 2.35 ohms.
Next as ASR has made popular 1kHz 5 watts into 4.7 ohm load 48kHz bandwidth unweighted.
Next the same thing but 51 watts.
Here's a two tone 18500Hz + 19500Hz, as Bruno liked to show off with the Ncore amps, at 5 watts into the 4.7 ohm load.
And now at 51 watts.
Next up is a distortion sweep at 5 watts into the 4.7 ohm load 96kHz bandwidth just so we can see what the 3rd harmonic really does towards 10kHz.
And finally a distortion sweep at 51 watts 96kHz bandwidth.
Anyone who is familiar with the TPA325x will immediately notice the significant improvement in high frequency distortion. The main area the TPA325x has issues is at the high frequencies. I'm assuming that, for stability, TI needed to start compensating the amplifier pretty low down in frequency as can be seen from distortion Vs frequency sweeps of the standard implementation.
Here's the datasheet distortion at 80kHz bandwidth.
At higher power levels it can be seen that the distortion starts to rise as low as 100Hz! The composite version completely does away with that and reduces the overall magnitude significantly. So happy 10th year TPA325x and enjoy some higher performance.
Now I wonder when Chi-Fi will come up with something similar. Complete schematics not shown for obvious reasons (at least yet). If China want to copy it they are welcome to, and possibly improve upon it, however they will at least have to figure it out for themselves! Just knowing it can be done should be enough encouragement for them to try. Topping are you there? 🤣
I need to build 4 more channels and then I finally get to listen to something.
By the way TI hurry up and come out with a replacement for the TPA325x series using GaN FETs. You've already done an integrated GaN motor driver gimme some class D. I want the improvement in efficiency and idle power consumption. Not to mention that the ability to switch faster should reduce the needed deadtime and improve performance.
As to the clipping of the TPA325x composite. The usual ncore clipping detector and limiter is used although I haven't tried to really fine tune things. Nothing, so far, has blown up. The TPA325x amps are very good at shutting themselves down if anything untoward happens, and that happened a lot when trying to get this stable. The window of stability being quite narrow. Not enough feedback and you oscillate, too much feedback and you also oscillate. You need to find the Goldilocks zone of stability. One nice thing about the TPA chips is they have a clipping detector so should it be necessary you can shut the amplifier down at the onset of clipping.
Really impressive values. Good job, congratulations.
Last ten years I have made many cards with TPA325xs . None of them were with PFFB. And I did not measure for any of them; But each of them has a background noise, especially in "hiss" style and high frequency. Since I could not solve it, I retired them all in a way.
However, I have recently made a PFFB project and PCBs have yet reached my hand. The PFFB line consists of only 18k single resistor. I also obtained the opamp supply from low noise symmetric regulators (+/-12v) , and I received the analog part of the TPA from the same symmetrical supplies + rail. I haven't installed yet, I hope I can get close results as yours.
Regards ..
By the way; which opamps you used for this project?
Last ten years I have made many cards with TPA325xs . None of them were with PFFB. And I did not measure for any of them; But each of them has a background noise, especially in "hiss" style and high frequency. Since I could not solve it, I retired them all in a way.
However, I have recently made a PFFB project and PCBs have yet reached my hand. The PFFB line consists of only 18k single resistor. I also obtained the opamp supply from low noise symmetric regulators (+/-12v) , and I received the analog part of the TPA from the same symmetrical supplies + rail. I haven't installed yet, I hope I can get close results as yours.
Regards ..
By the way; which opamps you used for this project?