1) Draw up the schematic as built> since the changes both boards should be almost identical.
2) still haven't seen any pictures as requested before, hopefully now with lots of twisted wires either shortened or beefed up..
I suspect construction issues and supply bypassing>> 0.1+220uF+1000uF for each chip (or shared depending on closer placement)
PS you might think about combining both amps on one board with added 2x 1000uF DC bypassing.
sorry, by that i meant removing those caps stopped the ringing on the square waves. I still get that crazy oscillation whenever the boards are connected.
with regards to bypassing, i have 2 220uF caps on each board right next to the LM3886's as well as 100nF caps soldered to the bottom of the boards at the supply pins.
If that was the problem, wouldn't i see it as severe hum even when the boards aren't connected?
It tell's me something I already knew from own experience. If you compensate each chip with unmatched capacitors, weather it is on the input or on the feedback-loop, each chip will output a slightly different frequency curve. Each chip try to compensate each other, and result in phase modulation (measurable). This doesn't always need to lead to oscillation but for sure it leads to increased distortion in the high frequency response. My paralleled chips also oscillated with the 220p. capacitors at the input. Not at a point that they self destruct or heat up, but oscillation in a lighter form you can clearly see in square-wave reproduction and the tops of the sine wave.
With kind regards,
Bas
Okay.
1: Did you tried to short C11 and C4 with a wire? For the sake of experiment, try that.
2: Instead of multiple output resistors, try to do it with just one.
With kind regards,
Bas
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They do have LED's as clamping diodes. They are not drawn in schematics posted by me on DIY audio, since that where pure simulations.
With kind regards,
Bas
sebastian,
i did remove C11 and C4 and replaced them with wire links. i'm pretty sure I destroyed the chips when i was messing around with it though, now when i power them up even when unconnected they work for about a second then their outputs become 0v. I have some more 3886's, i think i'll remake the circuit with both on the one board, hopefully that will help.
out of curiosity, why do you think using only one output resistor would help? the reason i didn't do it that way in the first place is that the only resistors i can get with resistances less than 1 ohm are wire wound and probably not the non-inductive type.
thanks, mitch.
i did remove C11 and C4 and replaced them with wire links. i'm pretty sure I destroyed the chips when i was messing around with it though, now when i power them up even when unconnected they work for about a second then their outputs become 0v. I have some more 3886's, i think i'll remake the circuit with both on the one board, hopefully that will help.
out of curiosity, why do you think using only one output resistor would help? the reason i didn't do it that way in the first place is that the only resistors i can get with resistances less than 1 ohm are wire wound and probably not the non-inductive type.
thanks, mitch.
Hi Bas - thanks for your answers to my questions - looking forward to inspecting the results when you've finished
I just today checked TI's new product blog and they've announced the OPA2141. In pretty much all of the important specs for a servo, this easily bests the OPA2604. Offset stability, input bias current and low frequency noise are considerably improved, supply current better than halved and its a bit cheaper too. What's not to like?
Edit - ah, there's always a fly in the ointment. SMT packages only.
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