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Another simple buffer with 50kΩ input impedance -- audio line standard.
This time, I'm trying to reduce input noise. My main concern was the huge bias resistors. The higher-value the resistor, the more thermal noise it produces. Having a split power supply makes it possible to use a single bias resistor of half the value.
The Darlington pair reduces the effect of the load on input impedance, allowing the bias resistor to be smaller still.
Total bias network resistance is 51kΩ instead of 300kΩ (17%). Noise should now be 3/7 (43%) what it was, or a 7.4dB reduction. That's not what I hoped, but it's not insignificant either.
Exact noise figure unknown without knowing resistor characteristics. To determine the ratio between old and new values, I used a chart here:
http://sound.whsites.net/noise.htm
The greatest effect of decreasing the load now is output level reduction. This is very small, less than 2% with a 300Ω load.
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