Arduino controlled Dual Mono AK4490 DAC (Part 2)

Following up on Part 1, it’s time to talk about the “brains of the operation”. Bare with me, this is going to be a rather long read.Hardware selection

The DAC needed to be controlled by a microcontroller so I looked into my options. I wanted something that would:

  • Be easy to program, so the Arduino IDE was a must.
  • Be able to drive a TFT.
  • Have enough storage capacity to store enough code & fonts for the TFT.
  • Be readily available.
  • Be relatively low cost, since it would have no influence on the dac’s SQ.
  • Be easy to build / integrate into a new design, even by a novice.

After some consideration, I decided to use an STM32F103 ready-made module. It would plug in to a “mainboard” of my design, along with the chosen TFT. It would be fast enough, have enough flash & ram, be easy to integrate and develop for and it would cost next to nothing.

Next up was the TFT. I’d seen on Ebay an interesting one that was relatively big, high resolution and inexpensive. It was this one:

It can be found on Ebay by searching for “3.5 tft uno 320 x 480”. Expect to pay 6-8€ inc. shipping.

After some searching I found a suitable library that (after some slight tinkering) would allow my tiny STM32 to drive it properly. (Note: do not download the library from this link. I will provide a customized version of the library together with my code).

I found a ready-made library for configuring the Si570 and modified it to run on my STM32, using one of its hardware I2C ports. I will also include this library in my code. To complete the recipe I also found working rotary encoder and IR receiver libraries.

The code

Next up was the prototyping work. I adapted my TFT HiFiDuino Pro code to run on the STM32 & TFT combo, with support for AK4490 dual mono operation. The end result had this feature list:

  • Support for either Dual Mono or single chip setups.
  • Support for the Amanero Combo384 USB to I2S module (must be set up as slave with MCLK/2 and F0,1,2,3 enabled).
  • Control with one rotary encoder with push-to-select functionality.
  • IR Remote support.
  • Support for software volume control, from -99dB to 0dB
  • Display incoming signal sampling rate and type, determined by “reading” the relevant I/O pins of the USB to I2S board.
  • Display and control of the AK4490’s digital filter.
  • Selection of the proper MCLK frequency according to incoming SR and type and programming of the Si570 accordingly.
  • Control of “DSD Direct” function of the AK4490s.
  • Control of the DSD Filter’s Frequency (50KHz or 150KHz).
  • Control of the Sound Mode of the AK4490.
  • Choice of either inverted or normal analog output for the AK4490s.
  • Choice of two sets of MCLK frequencies, either 22/24MHz or 45/49MHz.
  • Remote power on/off functionality (or always on – configurable in the code).

Software Requirements:

In the download I am including the modified versions of the libraries (as mentioned above) as well as the necessary font files. Be sure to extract the contents of “Libraries (place in Libraries folder)” to your Arduino IDE’s “libraries” folder.

Download it here: aKduino v2 (1024 downloads)

Here is the revision history:

v1.72 24/12/2017:

  • Minor changes to make compatible with current stm32duino core (changed HardWire.h to Wire.h and other minor stuff).
  • First public release as part of completed dual mono DAC project.

v1.66 10/10/2017:

  • Minor volume bugfix.
  • SuperSlow filter still problematic.
  • Enabled DAC synchronization feature (experimental..).

v1.64 30/09/2017:

  • Bugfixes.

v1.60 20/09/2017:

  • Added support of rotary encoder and IR remote control.
  • 3.5″ TFT support.

v1.50 07/01/2017:

  • Added support of rotary encoder for volume control.
  • Bugfixes related to DSD.

v1.41 06/01/2017:

  • Added support for dual mono mode.

v1.36 03/01/2017:

  • Added very basic TFT support.

v1.35 20/12/2016:

  • Code cleanup for first public release.

v1.33 19/12/2016:

  • Added full control of sound parameters through serial port.

v1.27 18/12/2016:

  • First functional version.
  • Automatic switching between PCM and DSD by monitoring DSDPIN.

The “motherboard”

After I was certain that everything related to the software was working the way it should, I designed a “motherboard” that would take care of the following:

  • Accept the STM32F106 board.
  • Accept the 3.5″ TFT.
  • Accommodate an 24LC256 EEPROM chip, used to store the DAC’s configurable settings.
  • Accommodate two sets of I2C signal isolators and I/O expanders.
  • Include headers for the encoder, IR receiver, power relay, non-isolated and isolated I2C communication, unused uC pins, etc.

This is what I ended up with:

Basic Hardware Requirements:

  • STM32F106 module (a.k.a. “blue pill”, search Ebay for “stm32f106c8t6 board”)
  • 3.5″ TFT with resolution of 320 x 480 (Search Ebay for “3.5 tft uno 320 x 480”)
  • 24LC256 EEPROM chip
  • I2C Isolator ICs, I/O expanders, passive components, etc (see BoM)
  • Rotary Encoder
  • IR Receiver
  • Compatible IR remote control (Apple Remote or other – in any case you must edit the code and input the proper IR codes for your remote, see below)
STM32 TFT Motherboard Bill of Materials
PCB Part Value Notes
C1, C2, C3, C4, C5, C6, C7, C10, C11 100n, 0805
D1 1N4001, DO214BA
L2 Ferrite bead, low R, 1206/3216
LED1, LED2 LED 1206
Q1 AO3400, SOT23
Q2 BC808, SOT23
R1, R2, R3, R4, R12, R13 2.2 – 3.3Κ, 0805
R5, R7 2.2K, 0805
R6 5.1K, 0805
R8, R11 1K, 0805
R9, R10 10K, 0805
U1 24LC256, SO-08
U2, U4 SI8605, SOIC-16
U3, U5 MCP23008, SO-18W
DC_5V .100 (2.54mm) – 2 Pin
EXP .100 (2.54mm) – 2×4 Pin
I2C .100 (2.54mm) – 3 Pin
I2C_ISOL1A, I2C_ISOL1B .100 (2.54mm) – 6 Pin
I2C_ISOL2 .100 (2.54mm) – 3 Pin
IR .100 (2.54mm) – 3 Pin
JP1, JP2 .100 (2.54mm) female – 6 Pin
JP3, JP4 .100 (2.54mm) female – 8 Pin
UART .100 (2.54mm) – 3 Pin
MCP_ISOL1 .100 (2.54mm) – 10 Pin
MCP_ISOL2 .100 (2.54mm) – 2×6 Pin
POWER_RELAY .100 (2.54mm) – 2 Pin
ROTENC .100 (2.54mm) – 4 Pin
STM32_BLUE_PILL STM32F103C uC board

How do I make it work?

Power

You have to supply the board with 5VDC at ~300mA through header DC_5V.

DC_5V
Pin Function
+ 5VDC
- GND

Basic connectivity

Serial port:

UART
Pin No. Function
1 RXD (PA10)
2 TXD (PA9)
3 GND

Rotary encoder:

ROTENC
Pin No. Function
1 PB13
2 PB14
3 PB15
4 GND

IR control:

IR
Pin No. Function
1 PB9
2 GND
3 Vcc

If you will be controlling a power on/off relay, you can use the POWER_RELAY header:

POWER_RELAY
Pin Function
+ 5VDC
- GND

Expansion header:

EXP
Pin No. Function
1 3.3V
2 GND
3 PB3
4 PA8
5 PA15
6 PB1
7 PA12
8 PB0

I2C header (non-isolated):

I2C
Pin No. Function
1 GND
2 SCL (PB10)
3 SDA (PB11)

Isolated I2C ports

The board has provisions for two separately isolated I2C ports, complete with I/O expanders on their isolated sides. The idea is to connect the DAC board to one of the isolated ports (I2C_ISOL1 & MCP_ISOL1) and your USB-to-I2S board to the other isolated port (usually MCP_ISOL2)

I2C_ISOL1A(& B)
Pin No. Function
1 3.3V (isolated)
2 SDA (PB11)
3 OUT (PC13)
4 IN (PC14)
5 SCL (PB10)
6 GND (isolated)

MCP_ISOL1
Pin No. Function
1 3.3V (isolated)
2 GP0
3 GP1
4 GP2
5 GP3
6 GP4
7 GP5
8 GP6
9 GP7
10 GND (isolated)
I2C_ISOL2
Pin No. Function
1 SCL (PB10)
2 SDA (PB11)
3 GND (isolated)
MCP_ISOL2
Pin No. Function
1 3.3V (isolated)
2 PC15
3 PA11
4 GP0
5 GP1
6 GP2
7 GP3
8 GP4
9 GP5
10 GP6
11 GP7
12 GND (isolated)

That’s it for Part 2. Stay tuned for Part 3: The output stage.

Arduino controlled Dual Mono AK4490 DAC (Part 1)

For the better part of a year I’ve been busy developing what one would call a “respectable” DAC from scratch. It has been a team effort, with different people becoming more involved with specific aspects of the project, but pretty much everyone involved ended up learning a lot about DACs. Right now, the project is for the most part finished, in that it is fully functional with a USB input and a pair of single ended outputs (or balanced, if you use a passive transformer-based stage). It is controlled by an Arduino-compatible micro-controller, running my aKduino v2 code (more on that in a future post).

I will do a series of posts detailing the design and build process, with each post covering a specific PCB. When the series is complete, the contents of all of the posts will be concatenated into a project page.

So, without further ado, this is the schematic of the main DAC board:

(Right click, Save Image As.. to download it in full resolution)

This is the 4-layer PCB:

This is the parts placement diagram:

And this is the BoM (v1.9) in xls format: Dual AK4490 DAC (main board BoM) (940 downloads)

Design considerations

The design goal was to do a dual mono design so as to maximize SNR and channel separation. A 4-layer PCB design was chosen so as to have a very solid, low impedance ground plane as well as proper power and signal planes. The I2S, audio signals and power after the local LDO regulators are routed on the top layer, the 2 middle layers are ground and power planes, and the bottom layer serves to route I2C signals and some power lines.

Power

All of the local power supplies are implemented using the currently top-of-the-line LT3042 LDOs. The VDDL & VDDR (analog power supplies) are set to 7.2V so as to maximize SNR and dynamic range. There is provision for providing separate pre-reg power supplies for the L and R channels (headers AVDDL and AVDDR) but I don’t consider that to be critical to SQ since there are local LDOs and the power draw is very very small. In my implementation I’m using a common pre-reg for both the AVDDL & AVDDR. The AVDDs and DVDDs are also supplied by LT3042 LDOs set to output 3.3V. The Si570 has its own dedicated 3.3V supply implemented with an LT3042 and features extra filtering.

Overall, the power requirements of the board are:
1) AVDDL: 8-10V DC at 40mA max
2) AVDDR: 8-10V DC at 40mA max
3) DVDD: 4-6V DC at 200mA max

Clocking & input signals

It was decided that the MCLK would be provided by a programmable low jitter oscillator, namely the Si570. This way we could select different MCLK frequencies at will, supporting different sampling rate families and different USB to I2S boards.

Speaking of USB to I2S boards, the DAC board has a very specific requirement: The USB board must be able to receive MCLK externally. In other words, the DAC board and USB board must be clocked from the same oscillator. This is due to the AK4490’s design. Unlike the ESS designs which by default run asynchronously, it needs to be on the same clock domain as its I2S sources.

So, the DAC board needs to output MCLK back to the USB board. There exist a number of USB boards that support that. The most popular ones are the Amanero Combo384 and JLsounds’ I2SoverUSB.

Since I had decided to do reclocking using flip-flops as close to the AK4490s as possible, and the flip-flops are clocked by the MCLK, its frequency needs to be sufficiently high in order to reclock signals corresponding to high sample rates. This translates to 49.152 MHz for sampling rates of 384KHz. If you’re content with going only up to 192KHz, you can use a MCLK of “just” 24.576 MHz. Of course you will also need the corresponding MCLKs of 45.1584 and 22.5792 MHz for the 44.1K families of SRs.

If you decide that you don’t want to do reclocking on the DAC board, you can just not solder on the flip-flops and just connect the proper pads together with some wire so as to bypass them. That way you can run with 22.5792 & 24.576 MHz clocks with SRs up to 384KHz (and probably beyond..).

If you decide that you would also prefer to not use the Si570 and just clock the DAC board directly from your USB to I2S board (or other suitable I2S source) you can also do that. You just don’t solder-on the Si570 and make a couple of changes to the Arduino code (to be implemented..).

But I urge you to try the Si570 & reclocking way first..

Construction hints

Start by first soldering on the power supply components (LDOs, resistors, caps, etc.) and testing that everything works the way that it should.

The LT3042s are pretty tiny and its easy to make a mistake while soldering them. I’ve found that the easiest method is by using a hot air rework station. First I use a regular soldering iron to tin the thermal pad and the pads with a small amount of solder. Then I apply a good amount of high quality solder paste, put the LDO on top of the pads and heat the area of the board until the solder melts. I set my hot air station to a relatively low temp of ~280 degrees C and the process takes less than a minute (per LDO). But you could also solder them with a soldering iron. To solder the heatpad, warm up the pad from the underside of the board and add solder.

After the power section, you should solder on the AK4490s and other low profile components. I do that with a low power (18W) soldering iron with a fine tip.

Next up is the Si570 programmable oscillator. I start by putting a little solder on one of the pads:

Then I add soldering paste and I place the Si570 on top of the pads. I use the fine tipped soldering iron to melt the solder on the tinned pad, effectively soldering the Si570 on the board. I then proceed to solder the rest of the pads by applying heat with the soldering iron to the side of the pads of the Si570 and adding solder.

You should finish up by soldering the electrolytic capacitors and other higher profile parts.
Beware that the spacing around the electrolytics is very tight. You should take that into account when selecting parts. The parts in the BoM are sure to fit in the available space.

That’s it for Part 1. Stay tuned for Part 2: The Controller.

Winter 2017-2018 teaser :-)

So many projects, so little time.. Here’s a teaser on what’s to come:

PGA2311-based preamp, with multiple inputs board, controlled by a custom Arduino, with an OLED screen.

Salas DCG3 preamp, powered by custom Salas Shunt Reg 1.2R.

MPD TFT display for Rasberry Pi, controlled by an STM32.

Ian’s Multichannel FIFO.

Allo.com’s DigiOne RPi HAT.

Dual Mono AK4490 DAC with on-board Si570 programmable oscillator and reclocking, Single ended Class A discreet output stage, Arduino controlled with STM32 controller and 3.5″ TFT screen.

Arduino controlled AK4118 based s/pdif receiver, with AK4137 SRC.