Some audiophile friends who owned my discrete R2R DAC and satisfied with its sound quality, and recently, they suggest that if I can make a smaller board for raspberry pi. I tried and its quite impossible to squeeze those discrete parts in the small 65mm X 56mm raspberry pi board . After searching around for a while, I decided to make a batch of NOS DAC for raspberry Pi using AD1865 which I kept for quite a long while. The obsolete AD1865 are rare now, but still available in very small quantity if we search online. I love the sound quality of AD1865 and it had been used in Audio Note’s DAC, Wadia , Rockna Audio etc. which is quite expensive if we buy one of those.
I choose LM27762 low noise dual power supply , and internal LDO provide dropout voltage of just merely ~45mV. The drawback is its WSON package size, soldering this chip onto a 2oz copper clad seems a little challenging, especially with my old shaky hand. I choose X7R type of capacitors as suggested in manufacturer’s data sheet, and add another LC filter to further clean up the power rail, low ESR capacitor before the power supply and audio grade capacitors for audio bypass.
I include a small jumper in between raspberry pi’s 5V and the DAC 5V, so that I can remove the jumper if I plan to supply the raspberry Pi and the DAC using two separate 5V regulator. So far, I’m using a single 5V linear supply which connected to the DAC’s ext power terminal , also feed the raspberry pi as well. Thats means I have 3 option to supply the whole thing:
1) supply via Raspberry Pi’s micro USB and it supply the DAC as well.
2) connect a 5V power on DAC’s power terminal J4, and it supply the raspberry pi as well.
3) remove the jumper J5, use 2 section of 5V power supply , one connect to raspberry pi’s USB, and another one connect to DAC board J4.
The AD1865 using very old school standard R2R design , with segmented string at its last 4 bits to maintain its accuracy. I also built a discrete version of R2R DAC with segmented string, will post it here in my page once I tidy it up. For PCM1704 or PCM63 etc, they use signed magnitude method to increase its accuracy, and I found that there’s slightly different characteristics between segmented R2R and signed magnitude R2R. The AD1865 include a MSB trim pinout so that we can trim the MSB to its lowest distortion, but I found that almost all DAC manufacturer using this chip don”t care to adjust it and just leave the pin unconnected. I include a multiturn trimmer to trim it, take a while for me to trim into lowest distortion but worth the time I think. to make the trimming job easier, I created a WAV. file specially for MSB trimming, it just a low level sine wave tone.
The AD1865 can’t accept I2S data, so I put several logic gates and shift register to re-align the data into right justified format. This DAC accept bit rate up to 24bit and 384 Khz of sampling rate. A small relay circuit act as muting circuit by shunt the output signal to the ground, and a simple RC filter as output filter , just like others NOS DACs , to limit the output frequency at audible range.
I found that Nichicon KA and Nichicon Fine Gold capacitors are more easier to run-in than others audio grate capacitors. When I first power it up, the DAC sounds very hash and the sound getting smoother after the first 10 hours , so I decided to use KA and FG in this DAC, and the chemicals in new capacitors will totally settle after around 80 hours of run time.
After a few days of run-in and listening test, I found that it sounds quite similar to my discrete R2R, they share the same characteristics of good sound stage, very life-alike scale. Overall, it sound very musical and I can listen to it whole day without getting fatigue.
Just like most R2R DAC do, good recording files sounds even better, but bad recording sound even worse.
Here’s my youtube video regarding this project.
Recently, I noticed that every time when I post a new project, readers will contact me and wish to buy from me. I’ve made a small batch of this DAC and for those who wish to buy one, please do not hesitate to contact me via email.:
***Edit***
The latest Volumio having some problem and it happened in some sound card such as Allo and some PCM1794 based DAC. ( Thanks Chris in UK and O van Schaik in Netherlands for informing me about this !)
Here’s the solution :
“”Download latest Volumio and make a bootable sd card. The first start will be on setup wizard page , you select language, your the name of your volumio set , Volumio as default, and then when you come to select the I2S drivers , please choose ” R-PI DAC ” and then it works right after restart. If you set others drivers at first installation, you can’t actually change to other drivers afterwards, you need to format and re-install and it will stuck to the first driver you choose. “””
*** Update :
I wish to mentioned that this DAC can’t support software volume very well, In fact , all my audiophile friends knew that software volume can degrade the sound quality and they never use software volume control. AD1865 has 18 bit of hardware bit depth and software volume will throw away some bits , thats make the sound quality getting worse if software volume is used. ****
