Construction of an audio mixer?

I’d guess that the high-pass filter is to eliminate any line-frequency hum that leaks into the signal path? Also to totally eliminate DC currents that should not be amplified, especially if you are potentially mixing multiple offsets. Decoupling capacitors also help at blocking DC.

@gasman I’d guess that the high-pass filter is to eliminate any line-frequency hum

Line frequency is low, not high. The filter eliminates harmonics. Line frequency power supply noise is pretty rare noways, not like when it was all transformers and electrolytics. We use high pass filters to attenuate any harmonic distortion, though not in purpose built audio mixer, it is in very high amplification audio work. Similar – but not as many channels or as wide an audio range.

@DarkScribe I defer to your expertise—but high-pass filters block low frequencies, right? I understand that modern switching power supplies are less susceptible to 50— or 60-Hz ripple, but low-frequency hum is ubiquitous & can leak into the signal path through cables etc.—shouldn’t any mic pre-amp strongly filter it?