To compare with standard noise specifications of things like opamps.
400µV rms 20--20kHz (assuming a flat spectrum and a gain of x16) would be about 180nV/√Hz noise spectral density (input referred).
Most audio opamps are 5 to 10nV/√Hz, and well designed class B power amps can also approach such values, leading to output noise levels more like 20--30µV rms for a power amp - basically a whole 20--25dB quieter.
Take a sensitive speaker, lets say 8 ohm, 95dB at 1W 1m, give it 400µV of noise and that will be 18dB of audio noise at 1 metre, which is a clearly audible 38dB at 10cm... Give it only 20uV and it will be -8dB at 1m (completely silent) and a super quiet 12dB at 10cm...
Compare to using 100k pot on input to a gain 20 power amplifier. 100k pot gives a worst case voltage noise of 20nV/√Hz at input, so 400nV/√Hz at the output (which is about 56µV rms).
400µV rms 20--20kHz (assuming a flat spectrum and a gain of x16) would be about 180nV/√Hz noise spectral density (input referred).
Most audio opamps are 5 to 10nV/√Hz, and well designed class B power amps can also approach such values, leading to output noise levels more like 20--30µV rms for a power amp - basically a whole 20--25dB quieter.
Take a sensitive speaker, lets say 8 ohm, 95dB at 1W 1m, give it 400µV of noise and that will be 18dB of audio noise at 1 metre, which is a clearly audible 38dB at 10cm... Give it only 20uV and it will be -8dB at 1m (completely silent) and a super quiet 12dB at 10cm...
Compare to using 100k pot on input to a gain 20 power amplifier. 100k pot gives a worst case voltage noise of 20nV/√Hz at input, so 400nV/√Hz at the output (which is about 56µV rms).
