PCBs & Parts: Beezar
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The TooleAudio BantamDAC


The BantamCableDAC is a variation on building the BantamDAC. Essentially, the USB Type B jack is left unpopulated. The selected case, Box Enclosures #RX2KL07-S, is very small - 2.48x1.10x0.63" inside dimensions - and light. This allows one to wire the case directly into the ends of a USB cable and to terminate the other side with standard leads and RCA jacks. In one fell swoop, you've eliminated any
plug-in connection to the BantamDAC board, and eliminated any patch cords in connecting the BantamDAC to an amp.

How to Construct:
(Populate a BantamDAC board minus the Type "B" USB connector.)
Start by drilling the Box Enclosure in seven places:
  • Four holes for 4-40 screws on the bottom of the case for the board mounting holes
  • Two 1/4" holes on both ends at approx. mid-height on the case, and
  • One 3mm hole for the LED in the case lid.
The case itself is closed up with four tiny Phillips screws at the four corners on the top (provided with the Box Enclosure). On the ends, I put a rubber grommet in each hole to act as a strain relief. The cables on both ends are extremely tight. My guess is that the case will crack before the cables will pull out, despite any lack of hard clamp on the interior side of the grommets.
Wiring the USB cable into the board is a piece of cake. USB wiring is an international standard - all wiring positions are defined down to the color:

Note that the BantamDAC board is the same as the diagram for "B". A Type B jack simply turns those wire positions 90 degrees. Here is an easy reference on a web page that you can print out for your convenience:
USB pins and color codes
Here's the BantamDAC layout showing the actual wiring connections for the USB cable. Again, these connections and colors are a world-wide standard:
The trick becomes wiring the board with the cables routed through the grommets on the ends. Pulling slack was not too difficult, but taking the slack back out while pushing the board back down into the case onto the mounting screws was a bit difficult. Smaller wiring on the RCA Jack side would probably make this easier. Also, there's a 1/6" nylon washer underneath each mounting hole. This corrects for the thickness of the C8 SMD cap on the bottom of the board, which is the thickest part on the bottom of the board (you don't want to stress the board by clamping down over that SMD cap in the middle).
Finally, mark the hole in the lid for the LED (after soldering the LED at the proper height) and screw the lid on. Voila - BantamCableDAC!
Here's the bottom of the case - simply mark your holes using an unpopulated board on the bottom to mark the holes. Nylon screws will prevent marking up a counter top, but are not strictly necessary. USB cable is Molex #88728-3400. Be sure to choose a full-rated USB cable, not a sub-channel version. Sub-channel USB cables may not be shielded and will not support even the full USB-1 specification. Because the BantamDAC and BantamCableDAC are USB-powered, the quality of the cable is important: performance is sensitive to interference.
The pigtail cable datasheet: Molex #88728-3400
As mentioned, it is important that you select a full-rated shielded high-speed USB cable (as above). For reference, here's an excerpt from "Chapter 4 - Physical Environment, Cables" from the book USB System Architecture, by Don Anderson - USB Cable Spec

A closeup shows the cable routing and connections. Route the USB cable around the big power cap. The board has separate connections for OG (Output Ground) for each channel, so that you connect two separate leads for each channel. Note the twin Black Gate NX-HiQ's in this case (47uf 6.3V). They are very cheap in this size - about $2.50 each.
In this photo, you can see the connections to the USB pads. Simply connect the "Drain Wire" to one of the USB connector snap tab pads (the large black wire). The Drain Wire is much bigger than the other wire gauges by virtue of being the outside mesh of the wire bundlen twisted into a point. However, the USB connector snap tab pads are plenty big enough. You might consider some heat shrink on the interior side of the grommets, but if you size your grommets small enough, the cable will not move.

file last changed:Monday, October 13, 2008 7:00:00 AM
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