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Alright, I tried without LM4780 by connecting OPA1632 outputs directly to 4780 outputs. I plugged in some headphones and I heard music :) DC offset about 2ma and sound similar to a headphone connected to an IVY. Now my best guess is a problem with traces around the LM4780? Russ I know you mentioned that previous batches had a manufacturer error, is it possible one still exists on these boards? I have full confidence that you guys would have check it thoroughly but something just doesn't add up  Let me know if there is anything else you want me to try.
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The only thing I can think of to test would be to use your DMM to see of any pads are connected to incorrect nets compared to the schematic. I too am quite puzzled since I have a pair of the same boards running at the moment.
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Specifically check that LM4780 pads are not incorrectly shorted to each other or a rail or GND.
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News...
I did some tinkering and was able to reproduce (I think) what the problem is. I removed the 220pF compensation caps (C1 and C2) and replaced them with some smaller values (22pF), which resulted in .250mVDC offset and about 3.6VAC at the speaker outputs (oscillation). Going back to 220pF and offset dropped to .002mV (meter limit) and 0VAC.
With both caps, the front-end test still works fine, so it would not be caught by my test rig which checks the front end only.
I think I must have accidentally installed the wrong caps on some of the boards I shipped.
I am willing to A) send out replacement caps for those who feel comfortable swapping them (I was able to do it with a soldering iron while the boards was installed in the case), or B) replace the caps for free and pay for return shipping.
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Brian,
I assume .250mVDC offset is between OUT+ and OUT-? Could you please also measure between OUT+ and GND?
Thanks
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The DC will be relative to the degree of oscillation.
The root of the issue is that with without the appropriate caps at C1 and C2 the amplifier is oscillating to the point where the control loop is ineffective.
I am sorry it took this long to track this down, and we very much appreciate your help and patience.
Cheers!
Russ
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Quote:I am sorry it took this long to track this down Per usual, I suspect my own error last.
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I managed to remove C1 from one of the PCBs and verified it was 26pf.
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Alright problem fixed. As Brian suggested it was the 22pF cap that was the cause of all these problems. I replaced the caps with 220pF and the amp started to sing. DC was 50mv above ground and all previous problems have disappeared. After a long time we have finally figured it out. I didn't have a SMD spare so I used a through hole ceramic. Thanks Russ, Brian and hkminn for your help. 
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Hello Russ,
Please help!
I haven been enjoyng Sympatico for about 6 mo, then yesterday one channel started to make high pitch noise and finally died. One channel only makes the beeping noise now while the other channel is OK.
Do you have any idea what the problem might be? Or is there any way to identify the failing parts?
Thank you.
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Sounds like a compensation cap was not doing its job.
You should start checking the voltages around each IC, both inputs and outputs.
Make sure you have good comp caps at the correct spots.
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