DAC for Raspberry Pi
I have developed an new dac for the raspberry pi. About a year ago I built a big version based on the PCM1794 with separate power supplies and I'm still pleased with it. Btw. I still like the BurrBrown sound signature, if there is any, and I prefer it over the newer 32bit dacs. In my ears its more musically and real.
I was wondering how small it can be without lots of compromises in sound quality and started developing a hat dac for the raspi. I chosed the PCM1793 with I had good results in the past. The schematic is nearly the same like the datasheet, except a few changes in the analog filter stage. Here the OPA1642 with Jfet inputs gave the best in my ears. Finally it is ready and with some changes in resistors and capacitors its damn near on his big brother.
A separate power supply is necessary, I'm using linear power supplies for the raspberry and the dac. Other tries with one supply for all failed. So an external power supply for the dac is a must for proper HiFi.
As player software I'm using Moode player. This all is running on a Raspi3.
Congratulations, great project. I have thought of doing this for so long but never did it!
I use a PCM1796 DAC for normal day to day usage and have bought the PCM1794 and done a "basic" HAT PCB outline with no components.
What drivers do you use for the PCM1973?
Superbly clean and neat build !
Are you making use of the internal filters or do you run it NOS (I'm not much of an expert so if this question makes no sense just say so !)
Great Project and very nicely executed!
1. what regulator are you using for PI power supply?
2. what is the power input module (can't read the brand and model #)
About the questions:
Interested if you ever do a group buy for bare boards!
Greg in Mississippi
a have got a couple of bare boards, if interested send me an pm.
There are kits and assembled dacs available here (at the moment in german only, for help send me a message):
For me thats more easy to manage than a group buy.
Thanks... PM on the way!
Greg in Mississippi
P.S. I forgot to add try some things on your Pi & DAC supplies... 1st, consider using the LT3083 regulator instead of the LM350 for the Pi supply... the more modern regulator made a worthwhile improvement in my setups versus using older LT1083 regulators.
2nd, I've been playing around with using .33F/5.5v Supercaps as local reservoirs as mentioned in Joe Rasmussen's thread here: http://www.diyaudio.com/forums/digit...+dac+filtering and used in this new R-Pi DAC here: http://www.diyaudio.com/forums/vendo...+dac+filtering and here: http://www.diyaudio.com/forums/digit...+dac+filtering. You also see Supercaps used in this thread I believe: http://www.diyaudio.com/forums/digit...+dac+filtering .
I've tried them on processor supplies (for R-Pi and for the processor/DSP in a Sony HAP Z1-ES) and on DAC Analog-side supplies (so far just on the PCM1795 in the Sony HAP Z1-ES). Results for me have been very positive, though they take several weeks to finally come on song. I suggest one at the input of the 5v to the R-Pi and at the PCM1793's Analog-side 3.3v supply.
Finally on powering the Pi, people had reported best results powering the Pi via the expansion pins, bypassing some of the protection devices on the micro-USB feed. I haven't tried this myself as I've only powered them via the expansion pins, but that's another thing worth trying.
I had always good results with the LM350 or even with 78S05 supplies. The most of the noise of their power rail comes the Pi itself. And I had also no problems while booting etc. The USB loads are SSD, USB-stick and a WLAN adapter. So I didn't consider another regulators.
The LT3083 is quite expensive. In which way there are improvements for the Pi?
Powering the Pi via the expansion pins I never tried. I heard about improvements for Pi1, especially for USB loads. Since the power managing at the Pi2 and Pi3 is better I don't believe in more improvements. But trial makes wise.
About the Supercaps: I have tried a lot of different capacitors in the supply line of the DAC and analog stage. Makes all a difference, most in the tonal matter and clarity of the high frequencies. In the end also by personal preferences. The Supercaps are not really designed for this application. In my opinion it can be also an another big capacitor.
I have to read Joe Rasmussen's thread later.....
On the LT3083 as a reg for the Pi, my experiences with this started during the period of 2008-2011 when I was running a motherboard-based player following the cMP/cPlay formula. I started with additional filter caps on the output of the ATX SMPS, then to a Peter Daniel-inspired linear-supplied PicoPSU (Linear-Hybrid supply), then to a fully Linear ATX24/P4 supply. Along the way I added separate linear supplies to all of the required peripherals... HDD/SSD, keyboard, mouse, monitor. I started with LT1084 regulators... moved to John Bau optimized versions of the same for the critical regulators, then to Belleson hi-current regulators. Each move improved perceived sound quality, leading me to the conclusion that digital circuits perform best with the widest-bandwidth, highest current, yet lowest noise power supplies.
Specifically on the Pi, I have tried 3 different supplies in my setups... On using the LT3083 regulator built-up on the K&K Audio power supply board, an Uptone Audio LPS-1, and the low-$ SMPS recommended by UltraFi in this post: http://www.diyaudio.com/forums/digit...ml#post4940489 . All sounded different, with the LPS-1 the cleanest yet least dynamic-sounding, the SMPS the most dynamic yet grundgy, and the mains-powered K&K supply in the middle on both parameters. This leads me to believe that the power supply is just as critical for the Pi setup as I found on the cMP/cPlay setup... and the LT3083 was my minimum choice, providing a quieter background and more detailed & dynamic sound back in those days over the LT1084.
AND from my experiences back in those days, there are also benefits to be gained by separately powering add-on peripherals such as WiFi adapters & USB-connected storage.
On the Supercap, where I've added one at the feed from a linear supply into the processing unit (either Pi or Sony HAP Z1-ES), it has lifted the sense of perceived dynamics to that of the switch-mode supplies without reducing cleanliness & clarity PLUS adding bass extension and solidity. A win all around in adding a <$5 USD part.
On powering the Pi via the expansion header, that is the only way I've done it in my setups, just by matter of convenience. BUT in the post referenced above, UltraFi mentioned his listening tests on that.
Finally, on the Joe Rasmussen thread, there isn't much useful after the first 100 posts or so, it quickly became a pile-on from the 'measurements are everything' crowd and no additional important information was added after that. That finally resulted in the thread being closed.
Greg in Mississippi
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