A correct assumption as it turned out.
Sorry. Not. The issue had nothing to do with the case material. As I understand the explanation it was a result of a component manufacturing fault.
The best you might get in respect of the output inductor being wrong is that it was manufactured with the wrong material, gap or number of turns. That sort of mistake happens because production of such low volume items has humans in the loop. If it was the inductor then they probably build them and then test them at zero bias at a fixed frequency on a Simpson VOM. If so I guess that pre-production testing will have been tightened.
A self oscillating amplifier relies on operating on the point of instability. Part of that in terms of operating bandwidth will be the value of the output inductor.
Eigentakt drives that to it limits by including additional gain stages within the loop with those stages being dropped in and out according to operating conditions including output level. Even if you don't get it exactly right it can still be very forgiving to the extent that, in an early stage release, your product will 'look good' rather than immediately going futz before it gets out of the door.
Again rather than babbling about the case material being the issue you would be better to guess, and likely still be wrong, that level may have been the indicator given that the amplifier adjusts its loop gain to maintain oscillation at a particular, working/workable frequency without totally losing the plot.
Loss of loop gain, something got switched out prematurely, might go some way to explaining the higher distortion for the test that was being done. Cause and effect.... The amplifier exceeded its claimed distortion figures before it hit its claimed output power. Presumably if that was ignored and it was just driven harder then. assuming it did not hit thermal limiting, it would have met the power spec.
OK. An example of an early stage product as released for review did not hit the spec but they have been called for it, identified the issue and taken action to catch future mistakes before they get out of the door. Well done everyone. Thanks for the input. Good Catch. No we will not be using a different shade of blue on the cases unless you require that specific option.
In respect FTC requirements, which appears to be an aside to this discussion, assuming a manufacturer states the conditions under which they will claim specific performance for their product then they should be measured against such claims, as as happened here, rather than any poorly considered constraints placed upon them by a regulator trying to level the playing field. To me that just introduces a different opportunity for snake oil to the equation when it would be better to, rationally, challenge the snake oil that is already being used.
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