Thanks. Bob Katz uses one of THAT1646 boards for headphones. Quite a few made their way into mastering consoles to drive "color" transformers.
In 2019 I deconstructed the THAT1646/DRV134 design and made an op amp version. This permitted trimming even-order distortion and I added jumpers for the option of open-loop output, for Class-A power up to about 100 mW, or global feedback for Class-AB outputs in the 100 mW to 1W range.
At 500 mW into 30Ω at 1 kHz closed loop THD runs about -112 dBc or 0.00023%. Due to the way bias is controlled in both the 1646/134 design and the op amp version, no output servo is required. The only AC-coupling is for the wiper of the level pot. The board can be used in stereo for unbalanced HP drive or jumpered for single-channel balanced output. https://proaudiodesignforum.com/forum/php/viewtopic.php?t=825
Assembled: https://ka-electronics.com/shop/index.php?route=product/product&product_id=67
Bare boards: https://ka-electronics.com/shop/index.php?route=product/product&product_id=81
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