Just wanted to report back on this project of mine...
Wayne's 1570 stereo bus amp is performing wonderfully. The oscillation problem I was having was due to a ground loop from some not-so-neat flying leads to correct for a pcb error in the prototype.
As for the
DAC problem, it's come back to haunt me. The tracking CV bias supply has stopped the
DAC's
output amps from frying (when having to sink too much current before they're properly powered up), but I'm still having
DAC chips die on me. I've been able to monitor the I2C serial connection between the microcontroler and the
DAC, so I can see that the
DAC is no longer responding (dead). Today I'm going to monitor the communication with a known-good
DAC. I have three hunches:
1) supply voltage spike - but after 2 regulators in series (main +-18V regulator, then down to +5V with another regulator in series), with filter caps on each, I doubt this is it, but it's still a possibility. I'll probably need a better scope to know.
2) -12V bias frying the
DAC's
output amps - but we already addressed this with the bias supply tracking the
DAC's +5V supply. The
DAC's are no longer showing the same symptoms of fried
output amps (0 volts or full rail
output - nothing in between) so I don't think this is the problem either.
3) The microcontroler is powered by one 5V regulator (fed from the +7.5V digital supply) and tied to digital ground; while the
DAC is powered from a separate 5V regulator (fed from the +18V analog supply) and tied to analog ground. The two chips are tied together by the I2C bus, which I can't remember if it's biased with the +5V digital supply or the +5V analog supply. Perhaps there's some difference in potential that's causing current to flow where it shouldn't... On the schematic, the I2C bus is biased from the +5V analog, but in the hand built prototype, I can't remember. It's currently being beta tested at a studio at the moment, so I can't check for a few days.
4) Pin 4 of the
DAC, which is the LDAC pin, was left floating (as per the instructions in the data sheet), but in a design note they stated it should be grounded. When this pin goes from high to low, it's possible to change the
DAC's bus ID, and also power-down individual outputs. That's not something I want to do, so I've since grounded the LDAC pin, replaced all the dead
DAC's, and put it back into service. Crossing my fingers and waiting is not a very solid problem solving technique!
Any thoughts would be greatly appreciated, as usual!
Sorry for the huge image - smaller was unreadable. Might have to open it in another window