This design, which I completed in 2010, takes an LM4562 and marries it to a hefty single ended class A output stage, which can be biased up for either 50 mA or 110 mA. The output stage is included in the global feedback network and can achieve 1 ppm at 20 kHz into 600 Ohms at 3V Output, and about 10 ppm at 10V out into 600 Ohms at 20 kHz.
With the output bias set to 110 mA, this buffer also makes a fine high end headphone amplifier, allowing up to 3 V peak into 32 Ohms, whilst still remaining in class A with ultra low distortion.
A Universal Small Signal Class A Buffer V1.0.pdf
Attention: for a much simpler and more elegant way of forcing the opamp output into class A, see https://hifisonix.com/class-a-buffering-the-correct-way/
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