Did you read the specifications on the op amp? It can drive 600 ohm loads no problem. If you also looked at the schematic you'd see that it was only driving the bases of two transistors directly, which is a very high impedance.
The only issue I can possibly see is that at very high frequencies beyond the pole of the feedback network (when the bypass cap shorts the large feedback resistor), the op amp will only be driving the lower value resistor. I don't know if this is a big problem though. The LME49720 is rated for driving 600 ohm loads. I would use 9k/1k at most. You can play with various values here:
http://sim.okawa-denshi.jp/en/opampkeisan.htm
With 9k/1k (gain of 10) you can use a 22pF bypass capacitor and make the first pole about 800kHz. The phase shift at 20kHz will be very small, maybe 1 or 2 degrees. It's hard to see from the graph on that page.
With 4k/1k (gain of 5) you can use a 47pF bypass capacitor and make the first pole about 850kHz. Phase shift at 20kHz is about the same as above.
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does the specification data show how the performance deteriorates as load Z is decreased?
How much does it deteriorate?
Many opamps show a very significant change in performance around about 2k resistive loading.
Add in a bit of capacitance and you may be asking the opamp to do more than what the specification is hiding from potential users.
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