How to hack a HiFiBerry DAC+ Pro by John Binns

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I thought I would demonstrate the usefulness of the Lincoln Binns Uniobox 66 enclosure for Raspberry Pi projects by undertaking one myself. I decided to make a one-off music system product we have called Pi-Player. We took it to the 2016 Electronics Design Show at Ricoh Arena in Coventry.
Pi-Player comprises a Raspberry Pi 2 model B with a HiFiBerry DAC+ Pro mounted on top, software by RuneAudio and an Edimax USB wifi adapter to connect to the network.

In common with all the PCBs we see, the Pi has components right to the edge, so I needed to mount it on a carrier PCB so that it could be supported in the slots of our enclosure. In this case, we mounted it on a 100 x 160mm PCB, slightly hanging over the front. This left room at the back of the PCB for a transformer based power supply. The need for the analogue power supply is explained on the Hans Beekhuyzen YouTube Channel here. Switch mode power supplies are not recommended.

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Figure 1

The photo Fig.1 shows just the DAC+ mounted on the Pi with the carrier PCB underneath. This is before I hacked the DAC. Notice the dual crystal oscillators at the top of the PCB, 44.1 KHz and 48 KHz, also the main chip is a Burr Brown PCM 5122 dual channel 32 bit 384KHz DAC. This is a fairly serious piece of kit. The DAC+ Pro has on-board digital volume control, and I guess the extra bits (32 instead of 24) will allow the reduction in volume without noticeable quantization error.

As supplied, the DAC is set up to draw power from the Pi underneath, which draws power from a cheap switch mode power supply via the micro-USB connector on the Pi. This micro-USB connector will be inside the box and not accessible, so we had to create a new supply for the Pi. I did not want to feed the DAC from the potentially noisy Pi supply, so I gave it a separate supply by removing the link resistor shown in the photo above. The grounds remain connected of course. The DAC+ has a ground plane underneath it which reduces electromagnetic radiation reaching the DAC circuit, but this also covers the on-board wi-fi of the Pi 3 model and reduces its range, hence our use of Pi 2 with external wi-fi dongle.

Ideally you will make your connections via 90 degree pinned headers which you will solder on the DAC board.  I did not do this, of course, and made my job a bit harder.  I soldered a 10uF tantalum in the empty capacitor socket as a matter of principle, and I fed the power to the boards via coax.  Incidentally, I did not try to connect the power input directly to the Pi.  I connected to the DAC board and allowed the connector to feed it down to the Pi because it made routing the cable easier.

I am not going to publish the circuit diagram for the power supply – if you want to undertake a project like this then you should be able to do it yourself. Some parts we had lying around, others were purchased. If I were to buy the IEC mains socket at the back, I would recommend one with integral fuse holder. I used a 3.15 A slow blow fuse in a fuse holder I already had. The metal stand-offs are M2.5 x 8 that we use for Raspberry Pi here at Lincoln Binns. The plastic ones come with the DAC. I had to purchase the beefier of the two regulators to power the Pi. I chose LT323AT which will deliver 3 amps. I put some LEDs with series resistors on the outputs of the regulators to make sure they are switched on and stable. I also put capacitors on the regulators according to their datasheets. Aim to draw 10-30 mA from the regulators via the LEDs. The bridge rectifiers are RS 707-2668 and the capacitors are RS 715-2619 – 10,000uF 16V. I only needed 10,000 uF for the Pi, but they are sold in pairs, so I used one for the DAC as well (can you have too much of a good thing?). I chose a brand that had low ESR, and quoted the values at 100KHz – Nichicon.

 

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Figure 2

I used a hot melt glue gun to secure the co-ax cables and to protect the capacitors from vibration. Also, underneath the board there are bare connections with mains voltage on them which I covered with the hot melt adhesive. As an additional safety feature I lasered the bottom panel from black acrylic rather than having a conductive aluminium one. When I assembled the Pi-Player I also used clear acrylic for the top of the unit, which I lasered with the Lincoln Binns logo. I put heatsink compound on the back of the regulators and pressed them on to the flat surface inside the Uniobox 66 extrusion, squeezing out as much compound as possible. They line up perfectly if they are 0.3” in from the edge of the board (see picture above). Note that the tabs on the regulators (which are ground) do not connect electrically to the box (and each other) due to the anodising and heatsink compound. The only ground connection is somewhere on the DAC board. The extrusion itself is a custom length (178mm) so that the Pi connectors are flush with the front. The front and rear panels are simply punched and printed. If I wanted to improve the quality of the front panel we could have run a mill round the outside to give a more expensive milled finish, or the front plate could have been made from thicker aluminium and milled from solid with inset cap headed screws. One feature of the Pi-Player is that the phono connectors are on the front. A “proper” product would have a 160 x 160mm base PCB, and it would be wide enough to put the phono connectors on the back. At the same time the transformer could be moved further away from the DAC circuit board.

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Figure 3

The music for the Pi-Player is stored in the Kingston DataTraveler Micro 3.1 USB drive inserted in the front panel. It is 128GB and will hold 5000 tracks. It is small and made of metal. I like it. If I want a backup copy of my music, I can get another one and put it on my key ring. Music on the USB drive comes from my cd collection ripped to FLAC format via dBpoweramp converter software.

Control of the Pi-Player is via RuneAudio installed on the Pi. It is programmed initially via Ethernet cable so that it has access to the wi-fi. When the unit is switched on it finds the network via the wi-fi, and then I can use the RuneAudio app on my iPhone to control it (because my iPhone is connected to the network). I can also use my browser to access RuneAudio on the Pi via the network. Theoretically, I could create a wi-fi hotspot on my iPhone and control the Pi-Player without the network. I believe it is even possible to get Pi to create its own wi-fi hotspot, in which case I simply need to find it with my iPhone.

The output of the Pi-Player is via phono sockets on the front and is fed into an Arcam FMJ A32 amp driving PMC GB1 speakers. What does it sound like? -It sounds a lot better than Naim’s entry-level CD5i CD player, which admittedly has been superseded now. It sounds slightly better than my 17 year old DPA Renaissance cd player designed by Rob Watts (who now works for Chord Electronics). I probably need a better amp and speakers to be able to make any further comment on it.

If a commercial company wanted to develop a Pi product similar to this, the enclosure printed front and back and with milled edges on the front plate would cost them about £60 for 5, £50 for 10, and £40 for 25, and maybe 25% less with plain pre-anodised aluminium panels on the top and bottom.

John Binns CEng MIMechE
Director – Lincoln Binns