Mods to your External Dac

October 1, 2007

For this specific circuit to work, your DAC needs to use: (a) 11.2896 MHz clock, (b) 5V and (c) i2s input. Please don’t attempt to implement this specific circuit unless these conditions exist. Before attempting these mods, please build the reclocker board. Furthermore, my implementation entailed defeating the SPDIF signal that routed to my dac chip from my external DAC’s digital receiver CS8414. I also defeated power to the CS8414 by lifting two resistors on my DAC pcb. I do not cover these two mods below because every DAC is different.

1. Determine where to fit the reclocking board. It should be as close as possible to the Dac input pins. I mounted mine vertically.

2. You’ll need a power supply to the board. The voltage range is dictated by the Tent Shunt Regulator. The operating range is 2-6V above the 5V output of the Tent Shunt Regulator. So 7-11 Volt power supply.

3. I used Belden CAT5 shielded FEP cable to connect the reclocking board to the DAC input pins.

4. Examine the datasheet for your DAC chip to identify the input pins for BCK, LRCK and SDATA and SYSCLK if your dac uses this last one. Mine did not use SYSCLK. Depending on your dac chip you may or may not need the sysclko. If you do, examine the Board Layout, then just solder a wire to the 47R connected to the clock. Specifically, the wire would connect to the 47R lead that is also connected to pin 1 of the 04 chip (not the lead soldered to the output pin of the clock).

5. You’ll need to find a handy ground reference on your DAC pcb. Twist all three negative leads of the CAT5 together and tin the leads. Now solder this ground bundle to a ground reference.

6. You should terminate the positive leads through resistors instead of going directly to the DAC input pins. My Dac uses 10 ohm resistors here. That’s it!

To illustrate some of the above steps, there is a link to a diagram of my external Dac’s PCB. This shows the interface between the reclocking board and my Dac’s PCB. There are two TDA1541A dac chips.

Mods to Squeezebox

October 1, 2007

I modified a Squeezebox 2. I have not examined the SB3 board or its schematic. If you have an SB3, you’ll have to ascertain which crystal on the SB3 board is the 11.2896MHz clock. This is the one you want to replace. These mods should be done after you finish building the reclocking board.

1. In order to get this project up and running, I decided to use inexpensive 30awg wrap wire to transmit the signals from the SB to my Dac. This is far from ideal, but the results in my system blow away using my $1,100 CDP as a transport. I plan to replace this shortly with shielded CAT5 using LVDS. Here is what I did as far as cabling. First, make up 4 runs of twisted pair 30awg wire. I created a “pigtail”, with these wires soldered to the SB board and then run directly to the reclocker board inside the dac. No connectors. The wire run should be as short as possible. But it will probably be difficult to keep these under 12-18” depending on the size of your Dac’s chassis and location of reclocking board. I used wrap wire and a drill to twist the pairs. You can get this wire in blue, red and white. Get at least two colors and use one colored wire and one white wire for each twisted pair. Designate the white wire as ground. Cut 20” lengths of this wire. After using a drill to make 4, twisted pairs cut them down to a minimum length without inducing strain between the components. You may want to add strain relief for the SB because these wires solder to the board directly. You can of course develop a connector interface or even add LVDS transmission. Even if you do this, you can cut the wrap wire and solder to the LVDS chip and, hence, you won’t have to desolder your existing work on the SB board.

2. Once you have 4 runs of twisted pair cable, twist all of the grounds together at each end. Tin the ground bundle with solder. On the SB board, now solder the ground bundle to a handy ground reference. I used the negative lead of the big capacitor. The ground bundle at the other end of the cable will fasten to the ground input on the reclocking board, but don’t do this yet. Make sure the positive wire of each cable can reach its reference point on the SB board.

3. You may want to tape a label onto each cable as MCK, BCK, LRCK and SDATA to keep things straight at both ends.

4. Once you remove the 11.2896MHz crystal you will see two solder pads. Now solder the MCK positive wire to the vacant pad that runs to pin 1 of the HC74U04 close by. On the SB2 board, this pad is the one on the right when holding the board with the I/O jacks facing up. It’s the pad with the trace to cap C15 (on the SB2).

5. Now locate the three solder traces that run between the XILINX and SB Dac chip. Note that the SB Dac chip is BB1748. Pins 1, 2 and 3 correspond to BCK, DATA and RLCK, respectively. BCK will be located in the corner of the chip, with DATA and LRCK next to it. Now find the 3 resistors that are located on the traces between the XILINX and BB1748 Dac chip. You will be soldering the positive lead of each of the three remaining twisted pair cables to the ***DAC SIDE*** of the leads of each of these resistors. You will need magnification to accomplish this.

6. Try to provide some strain relief for these delicate cables so that the wires cannot be pulled off (and possibly damage) the solder pads on the SB board. That’s it for the Squeezebox mods.

Tips: Fabrication & Tools

October 1, 2007

John Swenson provided some helpful build tips for this project. I’ve included these tips in John’s words:

I start by putting in the parts, using a fin point marker or pencil to mark the holes for the end pins of each chip (1, 8, 9, 16, etc) and the polarity of each cap. This step is very important, it very easy to get confused when you are wiring on the bottom of the board, because it’s backwards from the top!

Then run the ground bus. You want to make sure its “tied down” in a few places to the board so it doesn’t move around. I just loop some bare wire through a couple holes in the board and wrap it around the ground wire.

Next run the “taps” from the ground bus to the ground pins. I do this by tinning a spot on the bus wire first so when you are connecting to the pin you are soldering to the solder on the bus wire which melts a lot easier than trying to heat up the bus wire itself. For the taps I use 18 or 20 gauge bar wire.

Then put in the power wires. I usually use 18 gauge for this. Sometimes I use bare wire, sometimes I use insulated. The bare wire is a lot easier to build a network with. Take one piece of wire that is the longest piece and put it in first, stitching it through some holes in the board; at least at the beginning and end. Run the other main wire, and then put in the taps.

For the signal wiring I like to us wire wrap wire, primarily because the insulation doesn’t “melt back” when soldering. This is very critical when doing close quarter digital boards. You can use other types of wire, but it’s important to use a type with insulation that doesn’t melt. If using wire wrap wire, you will need a special wire wrap stripper, a regular one will not work with wrap wire. The stripper is just a simple metal blade with a slot in it. You can get the wire in a three pack of different colors so you can use one color for clocks a different for input signals etc. Radio Shack sells it. I strip the wire to a fairly long length (1/4″ or more) then trim it down to a very short piece of metal showing, you do NOT want the uninsulated part accidentally shorting between pins!

For solder to chip pins I usually tin each pin first so there is a solder coating on the pin, then when connecting the wire to the pin you put a little blob of solder on the iron tip, physically touch the wire to the pin, then bring the tip to the point where they touch and let it sit there for a second or so, no more, and you have a nice joint, no need to add any extra solder. That way you don’t have to hold the iron, the wire and the solder.

Required Tools:

There are a couple of indispensable tools you need for this project. If you use wrap wire you need a wrap wire stripper. Radio Shack sells a cheap one and it works perfectly. You also need a vice to hold the board while you are soldering. You don’t want it running around the table top as you are trying to solder pins. I picked up a Panavise base and circuit board holder (parts 300 and 315) which worked very well for this type of project. Finally, you will need some form of magnifier. I bought a magnifying visor from Harbor Freight at John’s suggestion for $6 and I couldn’t imagine completing the project without it. Besides this, you’ll need a soldering station with some fine tips, long-nose needlenose pliers and a small flat electronics screwdriver, wire strippers, wire cutters, etc