GrubDAC Step-by-Step Build Guide

[tomb]

Cobaltmute's new GrubDAC was originally designed in conjunction with Joneeboi's new Carrie USB-powered portable amp.  However, it has all of the same uses as the BantamDAC - stand-alone, CableDAC, Millett MAX and MOSFET-MAX.  The PCB's have been shipping for awhile and we hope to have kits available soon, so perhaps a step-by-step build thread will prove useful.

 
To many of you, SMD soldering is something new and perhaps a little bit scary.
However, with the right preparation and strategy, you may find it easier in some
respects than through-hole soldering.  Like any Do-It-Yourself exercise, success
depends a great deal on proper preparation and tools.  SMD has some special traits that make a few tools (one in particular) absolutely necessary.
 
Here's a list of things you may want, with the first couple almost a necessity:

1.  tweezers
2.  flux pen
3.  helping hands
4.  small-diameter solder
5.  small-tip soldering iron

 
Tweezers cannot be over-emphasized.  Without a good pair, I have trouble imagining how manual SMD-soldering can even be accomplished. Get a good pair!  I use a pair of  ESD-safe, bent tip tweezers that I bought at Fry's, but all the good DIY-suppliers have them in stock - Mouser, DigiKey, Allied, Newark, etc.  Here's the pair that I use:

 
Another device that you may find useful is the flux pen.  The flux goes down as a
thick liquid and gets sticky very quickly - almost sticky enough to hold a part in
place.  The pen form lets you apply the flux where you want it, even on a very small
SMD PCB.  I like Kester and stick with their products.  The one I use is the Kester
#186.  It contains regular activating flux that works great (no water-soluable or
no-clean flux for me).
 
Finally, and this varies with the individual, I use a Hakko 936 soldering station, set
at 325 deg.C. with a 0.8D chisel tip - 900M-T-0.8D.  This is a half-size chisel tip of
the standard tip that comes with the Hakko 936 (900M-T-1.6D).  Solder is the same
solder that I use for all my DIY stuff: Kester 44 eutectic, 63/37, 0.025" diameter. 
Larger is too big for SMD IMHO, and smaller stuff breaks all the time.
 
Be sure to review the GrubDAC website and please check out Tangent's excellent
tutorial on SMD soldering: http://www.tangentsoft.net/elec/movies/tt03.html .

 

[tomb]

Here we have the grubDAC PCB all set up in a cheap pair of helping hands ($1.99 atHarbor Freight).  A couple of small strips of electrical tape cover the ends of the board, on top and bottom, to keep the alligator clips from scratching the PCB.  I know many of you think these helping hands are worthless, but with a little care, they will do the job.

 
This pic illustrates what I mean. Using the magnifying glass as a counter-balance, it should be possible to set up the helping hands so that the grubDAC PCB is level and will actually support weight without collapsing. This will let you apply the needed pressure when soldering without having the whole thing collapse on you. Properly set up, the helping hands should never fall during the entire time you're soldering the PCB. 

 
For the first step, we want to solder the PCM2706/7 chip. That's because you need to be able to reach it to solder on all four sides of the chip. That makes it the first candidate to solder, over anything else that might get in the way. In the pic, you should be able to make out the flux pen flux on the left and right side of pads - those are the ones we'll solder first. 

 
The standard technique for soldering SMD is to apply solder to a single pad. Then with the tweezers holding the part in one hand, melt the already applied solder on the pad with the soldering iron in your right hand. While the solder is melted and you continue to hold the soldering iron in place, move the part into position on that pad. When the part is straight, remove the soldering iron and let the solder cool. The part should be anchored in place, ready for final soldering of the other pads. This is common for all SMD parts, even the multi-pin IC chips. Here we see a small bit of solder applied to the right-most bottom pads for the PCM chip. I picked this pad for a couple of reasons - 1) it's not attached to the ground plane (those are harder to solder and not good choices for an anchor), and 2) we'll be soldering the right and left side pads first, so this one won't get touched until two entire sides of the chip are soldered. 

 
A small caveat to that is that the large chips will still move with only one corner anchored, so here we see that I also tack-soldered the left-most top pad, too. Now the chip is anchored with two opposite corner pins and will not move while we solder the left and right sides. 
NOTE: Be certain you orient the chip correctly!  Place the dot where the silkscreen dot is on the PCB!  You can't be too careful about this!!

 
Here we see all four sides soldered. I won't go into the details of soldering SMD IC chip pins - you can refer to the GrubDAC or BantamDAC websites, or the Tangent tutorial referenced earlier. Suffice to say that for the chips used on the grubDAC, it's possible to solder individual pins with the solder and tip I specified earlier. That sort of makes it a lot easier than down-and-wipe, flood-and-suck, etc. I still cheat, though, and go against all the norms of soldering. I apply a tiny bit of solder to the tip of the iron, then touch it to each pin, repeating as necessary. It works fine. Use more flux as necessary - it burns off fast. For the top and bottom sides, I simply rotated the helping hands. Note however, that some of the electrical tape got melted doing that. This is because even though I was almost soldering pins individually, I was still holding the iron horizontal and using the down-and-wipe method to finish off the solder on the pins. 

 
A quick check against the light bulb to see if there are any bridges - 

Note the pins at left top, bottom left, and top right (in the pic) - this is the way Cobaltmute's PCB design "relieves" the pins attached to the ground plane - a trace runs perpendicular to the pins underneath. Those are not bridges!
 
Next up is the Wolfson chip! For this one, I re-position the alligator clips and helping hands to get the PCB as near-vertical as possible. That way, I'll have an unobstructed soldering path for the pins: 

 
Here we see flux applied to the both rows of pins, with solder applied to an anchor pad at the bottom right: 

 
Both rows of pins have been soldered in this pic. Note that I continued with the anchor pins at opposite corners and soldered the top left pin before I began to solder the rest of the pins. Down-and-wipe finished off the pins for a smoother appearance. 
NOTE: Again - be absolutely certain about orientation!  Unlike the TI-Burr-Brown chip, the Wolfson's orientating dot is part of the chip's top silkscreen labeling.  There are some molded-in flashing "dots" on the bottom , but these are not the orientation dot!  Be careful!! 

 
At this point, I clean the PCB of residual flux. Some 91% isopropyl alcohol and an old toothbrush work fine. This way, a clean PCB lets you do a better inspection of the soldering job and to spot errors. A final check against the light bulb for bridges and we're done with the IC chips! 

 
Here we see the PCB, all clean, both IC chips soldered and ready for the rest. 

More later ...

 

[tomb]

For soldering the SMD passives, the standard SMD manual soldering method applies:

• Apply a bit of solder to one of the pads.
• With the tweezers in the left hand, pick up the part so that you place it on the pads on the PCB.
• With the soldering iron in the right hand (I'm right handed, you can reverse this if you're not), melt the previously applied solder on the pad while placing the part on the pad.
• Go back and solder the other end/pad.

 
Some pointers on this -

• Apply flux with the flux pen to the pads you want to solder.
• Being right-handed, I apply solder to all of the right-side pads for the parts I'm going to solder.
• For parts that are oriented vertically on the PCB, apply solder to the bottom pad. That way, you can rotate the helping hands up so that the helping hands are on the left, and the soldered pad is on the right - allowing free access.
• Watch out for ground plane pads. Deviate from steps 1), 2), and 3) if needed.
• With through-hole soldering, it's best to do the smallest parts first. With SMD, though, it's better to do the inside parts first, then work outwards to the edges of the PCB. This gives you the best access.

 
In the pic, you can see that I picked the smallest 805 capacitors that are mostly toward the center/inside of the PCB. The right pads have been soldered and then re-melted to place and anchor the small capacitors.

 
In this pic, I've soldered the left side of the 805 capacitors and added the 1206 resistors and ferrites. Next will be the oscillator.

 
The oscillator is shown soldered here. Like one of the IC chips, I anchored the top right point, first, then proceeded to solder the other four corners. Most of the pads on the oscillator are on the bottom, but there are small sections of copper on the sides. I try to make sure the solder is contacting that copper on the sides. I don't know if that's important, but I've built four so far and they all worked the first time out.

 
Next up is the back side of the PCB -
Here we need to solder the TPS regulator, first. It's a very small 5-pin IC chip, two pins on one side spread apart, then three pins on the other side that are close together. I deviate a little from the SMD soldering strategy with this chip and anchor the center pin on the side with the three pins (at the arrow). This gets the hardest pin out of the way first, because you can come at the other pins from any corner unobstructed. These pins get soldered individually. The only problem I've ever had is that it seems these chips are more susceptible to SMD Tiddly Winks than any other. I've shot several across the room with my tweezers, never to be found again. So, be careful!

 
Now that we have the TPS chip soldered, all the rest of the parts on the bottom are simple passives. As described before, I flux all the pads, then place solder on either the right-hand pads or bottom pads, depending on whether the part is horizontal or vertical. The arrows illustrate this strategy at a couple of positions.
 
As before on the top side, here we see all the small 805 capacitors soldered.

 
Next up are the 1206 parts. This goes very fast when you use the flux/right & bottom/solder-to-the-pad technique.

 
Now we're done with the SMD parts! Since all of the SMD parts are sealed, I like to immerse the entire PCB in the good old 91% alcohol. This gets things cleaned very fast.

 
You may still need to touch up with the toothbrush and pat dry with a paper towel. The paper towel will help to soak up the flux that the alcohol has dissolved.

 

[tomb]

Next up are the through-hole parts. As with any PCB and through-hole parts, it's best to do the shortest parts first. Here we see the V-D CM50 resistors in their positions. You'll want to solder on the backside, allowing the solder to wick back up to the top. The holes in the PCB are pretty large compared to the resistor leads. This was to allow compatibility with other resistors, if you want. However, in the case of the CM50's, it may be better to cheat a little bit and tack-solder them on top at first.

 
Here we see the bottom side where I've applied solder to the leads/pads after tack-soldering the resistors in place on top.

 
... and a look at the resistors completely soldered. There may be a bit too much wicking go on there, but I wanted to make certain the joints were good through-and-through despite tack-soldering them on top at first.

 
On this PCB, I'm soldering the mini-USB connector (as opposed to a CableDAC). It's pretty simple and the mechanical tabs pretty much keep it in place while you solder everything on the back. Remember that the tabs form a mechanical joint, though, so be sure that you fill those holes completely with solder. This will help things last through all of those multiple connections/disconnections with a mini-USB cable.


 
Next comes the Wima box capacitors -

 
... and finally, the three electrolytics go in -

 
Here's the finished grubDAC, ready for casing and connecting to your favorite amp!


(Note - I'm going to use this one on the MOSFET-MAX, but the LED goes on the front panel.  I placed it in the holes, un-soldered, in this pic to give you an idea of what the finished DAC looks like.)
 
Next post - detailed step-by-step instructions on finishing a grubDAC for a CableDAC!!

 

[Ikarios]

Great tutorial as usual, TomB. I didn't know you used a Grub/Bantam in the MAX builds... I do believe I'll have to take a closer look at those.

 

[tomb]

grubCableDAC!
Assembling a grubCableDAC is a little more trouble than simply populating the PCB, but not as much trouble as casing one up without the CableDAC option, IMHO. The reasons are that it's not as simple as the paint-by-number convenience that the PCB gives you.
 
However, most of us have soldered leads, made cables, etc., before. The only difficult part is getting all of the parts right beforehand. Pictured below you see what's necessary to assemble the RCA leads for a CableDAC. With this, a USB pigtail, and the Hammond 1551HBTU case, I think you'll find it even easier and more flexible than the old-school method of casing up the DAC with regular case-mounted connetors.

What you see in the pic is the following:

• 4 pieces of teflon SPC 22ga wire, each 9 inches long,
• 2 quality, gold-plated RCA plugs,
• 2 6" lengths of 3/16" diameter heat shrink - one red, one white,
• 2 1" lengths of 1/8" diameter heat shrink - black,
• 2 1" lengths of 1/4" diameter heat shrink - black (at arrow),
• 1 1" length of 3/16" diameter heat shrink - black, and
• 3 1-1/2" length of 1/4" diameter heat shrink (only one shown in the pic).

 
The rest of this post will show you how to put all of the above together into a professional-looking set of output leads for a grubCableDAC.
 
First, cut a notch in the 1" long, 3/16" diameter heat shrink. It should look like it does in the pic. Later on, you'll see what this is used for.

 
We'll start out soldering the RCA plugs. Be careful with these - unless you have some teflon-filled ones, the nylon filler is very easy to melt when you're soldering. If that happens, it will mess up the plug connector, rendering it useless. If you haven't by now, change your soldering iron back to its regular tip (the 900M-T-1.6D for me). What you need is fast, high heat. That will enable you to quickly heat up the localized solder joint areas instead of heating up the entire plug, thereby melting the nylon filler.

Shown in the pic is the green ground wire (soldered first) and the white signal wire (left channel). Note that the ground wire is soldered into a hole in the long tab. The signal wire is soldered in the center, in a small channel cut into the center connector.

 
Next, we'll place one of the 1/8" x 1" long heat shrink pieces around the signal wire joint. Push this piece all the way up to the connector as shown with the white arrow. Heat and shrink into place.

 
Place the 6" white piece of heat shrink over both leads and push it up as far as it will go. It will only go to the ground wire's solder joint, but that's far enough (white arrow). It will be locked into place with the ground tab crimp on the end.

 
In this pic, the white heat shrink has be shrunk into place in the area around the crimp and the crimp has been crimped. This provides a strain relief for both leads and the RCA plug.

 
Next, take one of the three 1/4" x 1-1/2" long heat shrink pieces, slide over the entire assemby, and shrink into place. Be sure to push as far forward as you can (white arrow), before shrinking into place.

 
Finally, add another piece of the 1/4" x 1" heat shrink and shrink into place. What we're doing is building up the diameter of the lead assembly so that it fits better into the connector. Finish it off by sliding the clear gasket (comes inside the RCA plug) over the assembly.

 
At this point, you can screw on the RCA plug body and shrink the remaining of the 6" heat shrink if you haven't already. Repeat this procedure for the other lead. Pictured below are the separate, finished RCA leads.

 
Now we want to combine the two separate leads into a single assembly for connecting to the grubDAC. We do this by using the heat shrink piece we cut that notch in earlier. Bend it in half at the notch and feed each lead assembly through one half of the heat shrink piece. Try to get the heat shrink close down to the end of the 6" heat shrink, but not onto the actual wires. Shrink into place.

 
Finish it off by placing the final piece of 1/4" x 1-1/2" heat shrink over the V-connection. Run the 1/4" heat shrink up to and just over the edge of the V-connector on the RCA plug side. The remainder to the other side will be shrunk down over the actual wire leads. This will be the point where the leads will exit the grubDAC case. Refer to the 2nd pic below for this final heat shrink and the white arrows for details.


 
Now, it may look like we've got a lot of wire leads left over (3"?). However, the actual exit from the case is off-center and the output connections are all the way over to the side. We need that extra lead length to give us enough to curve the leads around as shown in the pic below.

 
Now that we've got the wire curved around to the output pads, we need to make another twist and bend in different planes. In the piping industry, one would call this rolling the elbow.

I use a pair of hemostats, but some smooth nose pliers should work just as well. In the pic, we're going to cut all four leads just ahead of that clamp. Strip just a small bit off the ends and solder them into place. If you're lucky, you may be able to solder all four leads in at once. You should probably not tin the tips - or at least do it very lightly - because the pads are not much larger than the 22ga wire.

(Note: I built two grubDAC's at the same time.  This one has the LED soldered in place, but obviously, no mini-USB connector.

)
 
Next we'll do the USB pigtail - almost done with the CableDAC!

 

[tomb]

OK - next we need to attach the USB pigtail ...
 
The first thing you need to do is print the grubCableDAC page from the grubDAC website. It shows you a convenient legend for the wire colors of a standard shielded USB cable and where those wire go on the grubDAC PCB. Here's my printout below, with the grubDAC and the RCA leads attached.

 
What's nice about the USB pigtail cable is that it comes with the wire leads pre-tinned, including the shield and ground cable. There's no stripping, no cutting, etc. - just simply push them through the proper holes and solder.
Note: Only the right-most USB connector tab hole is grounded. Use that one for the shield/ground wire connection.

 
Cobaltmute designed the grubDAC specifically for the Hammond 1551HTBU plastic translucent case. The only thing you need to do is cut a slot in each end for the RCA leads and the USB cable, then screw the PCB into the case. Very simple! Don't look too closely at the pic below - I marked mine with holes that are way too big! I think the first prototype I made had much larger leads. So remember the old adage - measure twice, cut once. Still, this should give you an idea of how simple is the task. I found a good bit on my Dremel Moto-Tool and the slots were cut in few seconds.


 
Next - screw the PCB into the case using #2 x 1/4" self-tapping screws. Note the cable ties on both cables to act as strain reliefs.

 
Finally - screw the lid down with the two Hammond screws. FINISHED!

 

[Nebby]

Thanks for the guide and all the pictures, I'm sure it'll make things easy going when I get around to building one Any ETA on kits?

 

[viralbug]

Great! Many thanks for the guide with pictures. It will help me a lot in my build.

 

[tomb]

Quote:

Thanks for the guide and all the pictures, I'm sure it'll make things easy going when I get around to building one Any ETA on kits?

It'll have to be next week at the earliest, now - I'm packing for CanJam!

 

 

[nsx_23]

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[nsx_23]

 

[HiGHFLYiN9]

Great work Tomb! Excellent in-depth tutorial with crystal clear photos.

 

[Ikarios]

Tomb, I was wondering - what exactly is the "down-and-wipe" soldering method you mentioned? I started work on three AlienDACs today and had a heck of a time with the flood-and-suck method. Some of the bridges persisted after attempting to "suck" many many times; I ended up having to use desoldering braid and applying lots of heat to the bridged pins and praying that it comes apart.

 

[jdkJake]

Quote:

Tomb, I was wondering - what exactly is the "down-and-wipe" soldering method you mentioned? I started work on three AlienDACs today and had a heck of a time with the flood-and-suck method. Some of the bridges persisted after attempting to "suck" many many times; I ended up having to use desoldering braid and applying lots of heat to the bridged pins and praying that it comes apart.

Follow this link:
 
   http://www.diyforums.org/GrubDAC/GrubDACsolder.php