I have this, which is good for verifying that there's very little difference between opamps:

Then there's this, which I built to try out the TPA6120A2:

Now I'm building this, which is a variant on the previous, but with a more sophisticated feedback arrangement optimised for minimal distortion, and which can be built to be powered from an onboard Tx or by batteries with an external Tx for charging. The relay and zener are mounted on the front panel...

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