I'm still playing with the subject mic amp and think I can present the best I've been able to come up with in LTspice.? This will be the version I'll build and test once I'm back from the scope rescue mission at the end of the month.? The LTspice schematic/model is attached.
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The original Q6 amp had a bit of distortion into a 1K Ohm load, with a 50 mV peak input signal resulting in +332.6 mV and -383.0 mV. The LTspice FFT showed the 10 kHz input to be at -32 dB with the 20 kHz harmonic at -58 dB.
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The attached model shows that R63 is retained, R64 changes from 1K to 130 Ω, and C62 changes from 47 uF to 0.047 uF.? This combo provides between 140 and 147 mV peak on the positive swing, and between 140-144 mV on the negative side? (it seems to vary between different spice runs. Not sure why.)? LTspice FFT shows the primary 10 kHz signal is now -20 dB and the 20 kHz harmonic is down 47 dB to -67.? The 10uF cap in the buffer provides a much better frequency response than a 1uF.
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FWIW...
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73, Andy, KG5RKP/MM
