Input Capacitor Theory

[sonnyboy27]

Can anyone explain to me the theory of how input/output/coupling capacitors work? I know we all go with the "bigger means more bass" mantra but what actually makes that true? Is it forming a high pass filter with the input impedance of the next stage or something more complicated? Also, how much does reactance play into it if any?

[solderfumes]

I think you hit the nail on the head.  I imagine reactance plays into it only insofar as the capacitor forms a voltage divider with the input impedance of the next stage.

[reddesert]

The coupling capacitors form a RC high pass filter with whatever resistance R is available to ground or a reference voltage. This blocks DC voltage. The knee of the filter is at frequency f = 1 / (2*pi*R*C).  Calculating this frequency will tell you whether the capacitor is actually filtering out any bass.  For example, look at the Rat schematic/analysis - see http://www.electrosmash.com/proco-rat. At the input of a Rat, there is a 22nf capacitor followed by a 1M resistor to Vref, so the filter frequency is 7.2 Hz, well below audio frequencies. Making that 22nf capacitor any bigger wouldn't be useful. There are other places in the Rat circuit where one could usefully change the frequency response by altering capacitors or resistors - see sections 3.2 and 6 of the electrosmash discussion.

Reactance is just a means of modeling the capacitor as a frequency-dependent resistance; it's another way of calculating this RC network behavior.