The replacement Burr-Brown OPA2604AP opamps arrived - thank you very, very much! I popped them into the X-Mystique, fired everything back up, and was blown away! Remember the poor analog frequency response curve for the stock op-amps? As a reminder, here it is again:
Well, you're not going to believe what an enormous difference replacing the op-amps makes. Check this
I'm absolutely blown away by this! It's very obvious that the compensation on the board is wildly
incorrect for this part. Whoever in operations decided to cost-reduce the board by swapping the opamps out for cheap parts apparently chose not only a real POS, it's also clear that it isn't in any way a direct substitute, other than being pin-compatible. As a result, the message is quite clear: changing the opamps to the recommended part makes a huge
difference in frequency response!
What's particularly intriguing to me is the "60 Hz notch filter", which doesn't even exist! My earlier conjecture about it was obviously completely wrong, for I'd never expected that the notches we saw could be caused by swapping out a different opamp. Please accept my apologies for this mistake!
Now, unfortunately, the news isn't all good. Despite the huge improvement in frequency response, the distortion doesn't change significantly. Here's the original THD results from the stock opamps:
...and here's the THD with the Burr-Brown OPA2604AP opamps:
As you can see, the distortion levels are quite similar. Also, while I didn't bother posting it, the EFQPSK plots are essentially identical. While these results indicate that the digital outputs are in fact superior (at least with a good receiver), the results also indicate that if you're interested in the analog outputs, it's really
important to upgrade the opamps!
Thanks, Jerry, for giving me the opportunity to test this! Your perseverance certainly payed off, and the results were very surprising, indeed. Well done! Thank you again for the opportunity to test the updated parts - it really does make a big
By the way, are you sure that you don't need these back?