Audio HP Amp TI LM4811 - am I getting crazy?

[Saimoun]

Thanks tooki for the summary.

I tested with my last chip (number 4) and the 4th PCB. I can now get one of the two channels to work. The other channel is fried I think - I get a 42Ohm resistance between input and output.

I think I basically fried all 3 other chips, which explains the very strange behavior on all of them.

My guess about what happened: the chip must be really sensitive to changes on the CLOCK input, since each rising edge changes the volume, so I think leaving it floating just for a second makes too many changes and the chip fries. And as for the second channel of the 4th chip - I had the input tied to GND while I was testing the other channel, I guess that killed it?

I also noticed how sensitive the voltage on the bypass pin is - just probing it with my oscilloscope is enough to make it drop by a few 100mV!

[Saimoun]

Anyways - thanks all for the help, I guess the issue is sorted. I'll buy a few more and be even more careful this time.

[OM222O]

the main problem that I suspect is the very odd shaped pad which you use to solder. Adding large amounts of solder, overheats the chip for a long period of time, burning the circuitry. Most reliable method for hand soldering SSOP packages:

1)Add solder using a normal iron to all the pads excessively
2)Add flux and go over the pads with an iron, removing excess solder, leaving just enough on the pads
3)Heat the pads at the same time using hot air, hot plate etc, enough to melt to solder on the pads
4)Grab the part with the tweezers, place it on the pads, keep pressing down and remove the heat source

This way there is minimal heating of the chip, practically no chance of shorts (if you don't mess up the placing step and move the chips around) and reliable joints on the first try.

[Saimoun]

Thanks for the tip about amount of solder - but then I guess I can still use my technique just be very careful with the amount of solder? I am also really quick with the iron, leaving it less than a second on the pad usually.


I do not have a hot plate or hot air gun so for now this might be my only option. And the large pads *really* help.

[tooki]

Honestly, I doubt heat is the problem. It could be, but I don't think it's likely. (And honestly, you don't need those giant pads. SMD is about technique, and one of the things to unlearn is that the soldering iron needs to be as small as the pad. It absolutely doesn't.)

Anyhow, I listed a whole bunch of things for you to look at, but you didn't respond to any of them. In particular, we still haven't isolated whether the issue is in the IC or in the PCB...

[Saimoun]

Honestly, I doubt heat is the problem. It could be, but I don't think it's likely. (And honestly, you don't need those giant pads. SMD is about technique, and one of the things to unlearn is that the soldering iron needs to be as small as the pad. It absolutely doesn't.)

Anyhow, I listed a whole bunch of things for you to look at, but you didn't respond to any of them. In particular, we still haven't isolated whether the issue is in the IC or in the PCB...

Thanks for the info, I clearly have a lot to learn about soldering - this was my first SMD soldering though.

As I said earlier - IMO - I think the issue was in the IC - the first 3 were fried because of bad usage (floating the CLOCK pin), and one channel of the 4th one also (because setting the input to GND - my guess at least). On the other channel of the 4th one - everything works perfectly fine, including the volume change.

To answer your list:

5. Does the schematic actually make sense and work in theory? -> I think so, can you guys confirm?
6. Does the PCB match the schematic? -> yes - checked with software also checked the physical one a bit with the multimeter
7. Are all the support components good, and of the correct values? -> yes, they match the schematics
8. Was the PCB assembled properly? -> yes, everything is where it is meant to be
9. Was care taken to ensure the chip isn’t damaged during soldering, due to excessive heat or ESD damage?

Yes that's the question - I was careful about ESD and as quick as I can during soldering, but how can I tell if it has been damaged? So far the only direct tests I found were to measure the resistance between the pins - noticing how on the fried chips some of the resistances were completely off.

10. Is the device being tested under sensible conditions? -> as written above clearly the first 3 were not. I also found out that putting the oscilloscope probes on the bypass pin is a bad idea - now I only monitor input and output.

[magic]

Is there any chance that you applied some of the control voltages (shutdown, clock, up/down) before VDD? Perhaps that's what harmed them? I don't know, the whole thing looks very weird.

[Saimoun]

I dont think so but I'll keep it in mind in the future

[exe]

Can it be a ESD issue?