Parts came in for the "Factory Approved" cap mod a couple of days ago, and I've managed to do the upgrade.
I'm not a huge fan of stacking parts so I decided to remove the compensation cap and 10ohm resistor rather than just adding parts on top. I stacked the output cap as I though the additional capacitance would be beneficial.
The compensation cap has been replaced with a 0 ohm resistor, and the 10 ohm resistor changed to 0.1R as per Søren's recommendation.
Working with parts this small is a serious challenge for me, and it's getting close to the limits of my eyesight and equipment. The result isn't especially pretty but it works.
One of the "elephants in the room" with user testing is the quality of the test tone used.
I recently picked up a copy of the Denon Technical Audio CD thinking I would get a decent set of test ones and was surprised at just how bad the spectrum analysis of the DAM looked with the 1001Hz 0dB tones.
Looking at the spectrum of the original test tone made it clear that the DAM1021 was actually reproducing the test tone accurately, even if it wasn't quite what I'd expected.
Rather than dealing with this, I've made up some new test tracks using Faber Acoustics Signal Suite at 24bit, then dithered the result down to 16bit. Much nicer.
And testing the DAM1021 (0.2% version) using this file...
The DAM1021 volume was set to V-02 to avoid clipping the input of my audio interface. Peak level in the file is -1.1dB, and the 2nd harmonic is at -83.9dB, so -82.8dB relative to the 1kHz tone level.
Having gone back and compared the jitter tests I did with just the 470uF polymer caps added, I'm pretty disappointed at the outcome of the "factory" mods.
Immediately post "factory" mod I got the impression the sound quality had gone backwards, and the jitter test (above) points to Søren's suggestion not fully addressing the issue - artefacts are significantly reduced compared to stock but are not as clean as simply boosting capacitance.
Re-running the test with 470uF Polymers brings the spuriae levels almost back to where they were:
As a "reality check" this is the same test done on the unmodified board:
I've had a look at simulation of the buffer in LTSpice using the correct model for the OPA365.
The sim shows around 5.8mv ripple in the original stock buffer with a load stepping from 0 to 15mA at 44.1kHz.
Adding 470uF to the stock buffer drops the ripple down to 0.4mV.
Søren's reconfigured buffer has around 1.2mV ripple (with stacked 22uF and 47uF caps) which is a substantial improvement over the previous stock version.
The reconfigured buffer plus 470uF polymer drops the (simulated) ripple to 0.25mV.
The revised buffer has the added benefit of eliminating noise peaking.
It appears there is still benefit from added capacitance. I'm tempted to play around with increasing the 0.1R resistor to 1R which will bring the corner frequency of the filter down to 300Hz from 3KHz. This should help filter reference noise, and the noise contribution of the op-amp.