Audio Sniffer Assembly

0. Your workspace

Find a good place to work – good lighting, clean space, and no ESD buildup. Don’t walk across a shag carpet in the middle of winter and pick up your PCB. Always touch something that is grounded before touching your board for the first time.

1. Inspect your parts

Make sure your kit came with all the parts you ordered, and that they aren’t damaged. Notify Open Music Labs immediately if something seems amiss. Your resistors might look different than those pictured below, as we decided to splurge on the 1% variety. You can check your parts against the parts list in the BOM (Bill of Materials).

2. Solder the TLC071 op-amp

This is the 8-pin DIP package, and goes in position U1. Make sure the dot on the chip matches with the notch on the silkscreen for U1 (see above). You can bend the corner pins over on the underside of the board to help hold the chip in place while soldering.

3. Solder the 1N4148 diodes

The diodes have a black stripe on them indicating the cathode side. This black stripe should align with the stripe on the diode silkscreen image (see above and below on D1).

4. Check your solder joints

You can compare them to the picture above (click for a larger image). They should fill in the pad, but not be large blobs (they should have a convex surface). If your joints don’t seem satisfactory, practice soldering on some old PCBs just to get the hang of it.

Some soldering tips:

  1. 1. Make sure your equipment is decent. Your iron’s tip should be tinned, and should accept solder when placed against it – The solder should not ball up and roll off.
  2. 2. Keep the temperature between 660F and 700F. Usually for this larger work, I keep it on the higher end, but it varies from iron to iron.
  3. 3. Place your iron to the juncture you wish to solder – wait 3 seconds for the juncture to heat up – apply solder until the juncture is adequately filled – wait 3 more seconds for the heat to spread – remove the iron and let the joint cool.
  4. 4. It is also a good idea to wear protective gear (respirator, gloves), or to use a fan to remove fumes. At the bare minimum, always blow on the joint while soldering, as this ensures you are not inhaling, and blows the fumes away from you.

5. Solder the .1uF capacitors

Place the 3 small yellow capacitors at positions C1, C2, and C5. The orientation of the capacitors does not matter.

6. Solder the 2M resistors

The 2 – 2M resistors are place at R2 and R3. If you are uncertain of the value of your resistors, you can measure them with a multimeter, or read the color coded bands on them: Measure twice – solder once! The resistors in this kit are placed vertically, so they will need to be bent in half first (see above).

Be sure to note the circle around one of the holes of the resistor silkscreen image (see above on R2 and R3). This circle shows which hole the body gets placed in, with the long leg going to the other hole. This does not matter from an electrical standpoint, but helps keep the bare leads from touching one another.

7. Solder the 1K resistors

There are 4 – 1K resistors placed at R1, R4, R5, and R6. These are placed vertically just like the other resistors. If you want a higher volume from your Audio Sniffer, replace R5 and R6 with 100ohm resistors. The 1K resistors limit the volume, so unexpected pops and clicks don’t get too loud.

8. Solder the 100uF capacitors

The 2 large blue capacitors are 100uF electrolytic capacitors and are placed at C3 and C4. These are polarized, which means they have a specific orientation, and can be damaged if placed in backwards. The black line along the side of the capacitor (see above) shows the negative side of the capacitor. The silkscreen image has a small + to indicate where the positive side of the capacitor goes. So be sure to place the side WITHOUT the black next to the + symbol.

9. Solder the jacks

The jacks need to be flush to fit in the faceplate. The 1/8″ jack should snap into place, but double check that it is fully seated before soldering it in place (see above).

The BNC jack is more difficult since it doesn’t snap in. You can tack a bit of solder to one of the small pins, and then squeeze the jack to the PCB while reheating this joint. As the solder warms up, the jack will move into place, and then you can remove the soldering iron, and let the solder cool. This should leave the jack flush to the board, but again, make sure its completely flush before you solder too many of the pins.

The pins on the BNC jack are also larger than most, and are connected to the ground plane, so they will take a little more time to make a good solder joint. Note the cold solder joint on the right hand side (see above). This happens when only the pin gets hot, and not the pad its connected to.

10. Solder the battery connector

Place the red wire into the + hole, and the black wire into the – hole.

11. Solder the pot

Again, make sure the pot is fully snapped into place and seated flush to the board before soldering.

The pots used here have a plastic conductor, which is sensitive to heat, so it is important not to damage them while soldering. The trick is to solder one pin quickly and properly, and let the device cool before soldering other pins, so the heat does not build up. But, it’s more important that you get a good joint on the first try, so that you don’t spend time rehitting the same joint. So take your time and make sure each pin is well soldered in one go.

Once all the pins are soldered, you can solder the legs down. These take a lot of heat, and a lot of solder. If you have a variable temperature iron, turn it up to 700F. Heat the leg and pad with the side of the iron, this will give the most surface area in contact. Begin filling in solder as soon as the iron will melt it, and continue until the hole is filled. You may have to move the iron to the other side of the hole. After the hole is filled, make sure the solder is completely melted (it will go from slightly concave to slightly convex, and look wet and shiny), and then quickly hit the second leg of the pot. It’s good to do both legs in sequence, as the other leg will already be hot.

12. Optional: wash the board

This is an entirely optional step. The reason you may want to wash your board, is that the soldering processs leaves flux on the board (see below). This flux is mildly conductive, with a resistance usually on the order of 10M to 500M. So if you’re using resistors in this range (which the Audio Sniffer gets close to with the 2M resistors) the flux will slightly alter how the circuit works. For the most part, you will probably not notice a difference.

If you do want to wash your board, and you don’t have a can of flux remover, follow the instructions below. If you do spray your board with flux remover, be extremely cautious around the pot and jacks. Flux remover is highly corrosive, and can damage plastic parts very easily.

Wet a toothbrush in isopropyl alchohol, and flick the excess off to keep just a bit on the brush.

Next, scrub the board until the flux is removed. Be careful to keep the alchohol on the bottom of the board only, as you don’t want it to seep into the pot on the other side and eat away at the lubrication inside.

Blow off the remaining alcohol, and your board should be clean as a whistle.

13. Assemble the case

Remove the nuts from the jacks, and place the faceplate on. Put the nuts back on, and tighten them up enough to hold everything in place.

Place the PCB into the case on the side closest to the screw posts inside. Then connect the battery and place it on the other side, keeping the wire wound up tightly so it doesn’t interfere with the case closing. Next, put the back panel in place, followed by the other half of the case

Tighten up the case screws, but do not overtighten them, as the plastic might strip or crack.

14. Congratulations, you are now done!