There same PCB may be used as either the active buffered mult or the passive mult. It’s a very quick build, but you need to pay attention to a couple of small points to make sure you don’t miss a small detail that could compromise the functionality.
Before we get started with the solder, let’s make sure we’ve oriented the PCB correctly. The bottom of the PCB is the side which faces the rear of the case – away from the front panel. For the nw2s::m, the back is the side that has the nw2s logo silkscreen. The top of the PCB (the side which the jacks will eventually be soldered to) is the opposite – it has outlines marking the jack placements.
Once you’ve sorted out front-to back, you can figure out up and down. Up is the direction that will be facing up when it’s mounted in a rack. Looking at the back of the PCB, up is the side with the logo and down is the side with the power header. When looking at the front of the PCB, the top is the side with a resistor outline and the bottom is the side with the power headers showing through.
Note! There are four solder-bridge jumpers on the back-side of the PCB. If you are building an active/buffered mult, DO NOT solder these jumpers. These bypass the op amp and are only to be used when building the passive mult. If you are building the passive mult, you can skip to the Jacks section.
The next step is to solder the SMT ICS. There is a small SOIC-8 which is the dual MOSFET reverse voltage. The other IC is an SOIC-14 which is the audio op amp. The ICs are mounted to the back of the PCB with the pin 1 side of the IC facing toward the top.
If you haven’t worked with SMT ICs before, you need to determine where pin 1 is. The ICs are small and it’s sometimes difficult to see the pin 1 indicator, though it may be fairly clear on the op amp. The SMT ICs also have a sloped side along the same side as pin 1. This slope should face towards the top of the PCB (toward the logo).
If you don’t have experience soldering SMT IC’s, then you should watch a couple of Youtube videos to get comfortable.
One key is that you need a good soldering iron that is well tinned and a high enough wattage to make the solder flow easily. A separate container of flux is really the secret (but messy) ingredient. You should invest in some good (lead free!) solder and be VERY stingy with it on these pads. You will also need a decent pair of anti-static tweezers.
The basic steps are as follows:
- Place a small dab of solder on ONE of the corner pads of the PCB
- Smear a little flux across the pads on the PCB
- Using the tweezers, hold the IC in place against the PCB and simultaneously heat the pad that you pre-tinned.
- Make sure the IC is centered on the pads and remove the heat, but leave the tweezers in place until the solder cools.
- Double check the IC placement and very conservatively, heat each pad until the flux starts to flow and then dab a bit of solder and watch it flow up the IC pin.
- Move to the next pin, and so on.
The power header goes on next. It’s a shrouded header. The notch lines up with the tab that is silkscreened on the PCB.
Back on the front, there are five passive components: two electrolytics, two ceramics, and a resistor. The electrolytics are polarized, so be sure that the long pin lines up with the ‘+’ marking and the stripe on the outside of the capacitor lines up opposite of the positive mark. The ceramics and the resistor can go on in any direction.
The jacks are the last components to go on. The way I assemble jacks is to put the first four corners on first. Once those are soldered, then you can place the remaining jacks, place the panel over all of them, and use two nuts to hold the whole thing together while soldering.
When using the panel at this step, don’t yet remove the protective layer of plastic. Once soldered, remove the panel and set aside so that we can clean the PCB.
Passive Solder Bridges
ONLY if you are building a passive model, you should locate the four solder bridges and fill them with solder so that it spans the gap, connecting the two sides.
Once you’re done, and especially if you used flux to help you with the SMT assembly, you should give the PCB ad its components a nice bath in some isopropyl alchohol meant for this purpose. You can find some nice 99.8% anhydrous, non-denatured stuff at the major parts houses in varying sizes of bottles. I keep a pint-size container filled for this purpose. Let the boards soak a bit, scrub with a toothbrush (not the one you will be using after your own bath), give it a second dip to rinse, and then remove from the solvent.
Once removed from the bath, use some Kimwipes (paper towels will do in a pinch) to wick away as much of the alcohol as possible – particularly on the bottom of the PCB. This will suck any remaining solids away from the PCB so that as the alcohol dries, it won’t leave residue. When you’ve dried as much as you can, then leave it to air dry for another couple of minutes.
Now put the panel back on. First, remove the plastic covering, then place the panel back over the jacks. When you’re putting it on, make sure the power connector points toward the bottom.
By hand, thread all of the nuts over the jacks and finger tighten.
There are two things that you have to be careful of when finishing this step.
The first is that you need to protect the panel surface from the nut driver. The aluminum panel is realtively soft and will scratch very easily. I use a paper towel cut into one inch squares. Others use masking tape on the nut driver edges. It’s up to you what you prefer.
The second is that you do not want to over-tighten the nuts. This is extremely important. If you over tighten them, the metal can break free from the plastic and then the thread will move freely. Once the jack makes contact with the panel, you only need to turn about another quarter turn… gently firm up the jack. That’s all you need and they will stay in place.
And finally, we will test the circuit. Testing the mult is fairly easy due to the simplicity of the circuit. First, apply power for the active mult. Second, patch in a VCO signal into the first jack. Lastly, take another patch cable that’s routed to an audio interface or other monitoring module and patch it in around the other three jacks on that group. Once that’s complete, move to the three outputs of the second, third, and fourth groups. With audio coming into the first jack, and no other input jacks in place to break the normalized connections, you will hear audio at each point. If not, it’s time to troubleshoot.
If you have built an active module, and there is no signal coming through the outputs, then power is likely an issue. Double check the orientation of the ICs and using a magnifying glass, double check the soldering of the leads of both ICs.
If one channel is misbehaving, then it could be the soldering of one of the op amp leads or it could be the soldering of one of the jacks.
If there is humming when no signal is present on jack 1, then there is something wrong with the lone resistor. Otherwise, it should be pretty straightforward!