I think there was a miscommunication on my part. I have two 1580 preamp cards soldered. The first one was not passing signal, or maybe extremely weak. I could not find the problem so I decided to try soldering the second one, which is working but has the possible low bass problems (Im not even sure at this point).
In an effort to salvage the first one, and also test whether the problem was caused by the 5171 i removed the 5171, and attached fixed gain resistors. This turned out to be a bust. So I think this card will be retired. Though I am looking at the costs adding up, and wondering if I should try another 1580 chip on there. Im always paranoid about too many reflows/heat damage, but also cheap and trying to save as many parts as I can which is probably not a smart idea when testing.
I tested the 2nd preamp card again with the new summing card (that seems to work), and the signal sounds actually pretty good. At this point it's too hard to trust my ears so a proper testing jig is high on the list. Noise was not bad either, just some very faint high frequency that could have been ambient, or radiated who knows. The motorboating from the rfduino was very much attenuated compared to last time, barely audible. I have no idea what's improving things.
I'm going to try for a better LCR panning circuit.
For the smt parts, the supposed hardest parts are the easiest if you get it just right. For the passives I'm using paste and hot air mainly, graduated from tormenting myself with an iron. I tried 3 different paste syringes and could not get any of them to work. The last one, a small edsysn syringe, I tried again and it worked well. It turns out it was a simple fix. I had been letting the end dry out too much, even with a cap placed. Just a few primer squeezes and the paste came out more liquid and easier to control the dose. The more liquid paste seems best to me.
This https://www.indiegogo.com/projects/pcbi ... older--4#/
helped tremendously. I was using a cheap alligator clip vise and it was terrible.
The bga initially I did with hot air, even though it did supposedly work (after 3 attempts) since I got good outputs on the ltm8049 I wasn't happy. Got a 10$ hotplate, did a few thermocouple measurements around the perimeter and tried another 8049. It sat down perfectly, slid it off onto a larger pcb, and that turned out the easiest of them all. Alignment was very easy since there was no air or movement while the reflow took place.
Soic, Sop's are pretty simple to do with paste, just a thin line and some hot air. Then touch up with an iron.
The qfn's were a little tricky, and I'm still sort of working that out. For the 5171 I tinned the pads, cleaned, preheated the board with hot air, then applied tacky gel flux, then placed the part with one hand and used the other for hot air, it reflowed fairly quickly and self-centered very well. 5171 Thermal pad was left floating, I guess next rev. I will connect to Vee?
I ASSumed the same would be true for the 1580, but no! I guess due to the pcb thermal pad underneath it dissipated too much heat and it would not self-center, or just barely so (just enough to plant a seed of doubt in your head as to whether it actually worked). I switched methods: tinned the underside thermal pad, tinned the top pads, cleaned, fluxed and then heated the thermad pad from below with a blob-tinned iron tip for better conduction. It self centers fairly quickly like this. I meant to ask you if this is dangerous and is more likely to damage the part. My past failures with qfn were all due to not enough heat. Depending on your answer to this I might switch over to the hotplate for both the 1580 and 5171. I'm leaning toward this anyway because both hands will be free and it's easier to poke the chip and test for centering.
I have a signal generator on the oscilloscope, just have to get a transformer and make the correct pad.
The ltm8049's are not cheap. I got two as samples, and from here on it looks like it's out of pocket, or maybe I'll try the lt rep again. The preamp cards as well are not so cheap as they are 4 layer, so I need to be a little more careful with revisions. I think for the p/s board so long as it's working I'm inclined to wait for something to blow up before I make a change. For noise issues though I would like to see if I can fit some more filtering on board.
I need to do another run on those as I messed up the summin card connecter pad spacing. I moved the two large caps on the p/s board to the chassis which allows larger values, maybe 1000uF for the bulk input and 330uF for the phantom cap - I know it doesn't do much, and need to revisit phantom for a possible rc/other filter there, but first the preamp has to work properly. There is space on the chassis around the rfduino board but not sure what I can do there. I also meant to ask you about this alternate power scheme. I updated the shutterfly page with some more images. You can see the chiclet sized ltc3265 card there, small and almost no depth to it. At one point I considered using 6 of these as plug in sub-boards on the the preamp cards for the bipolar rails. It would mean bussing Input supply which is why i didn't do it, but I suppose if the dc supply was filtered well, bussing it would not be so bad. That would make the only radiated noise source the phantom supply.