Maybe someone could make a chart of minimum capacitor values for different input impedance's.
If you are asking about coupling cap values here is a nice calculator.
http://www.sengpielaudio.com/calculator-XLC.htm Simply put in the input impedance in the third box where it says "reactance in ohms", put in the desired low frequency in Hz in the last box and it will give you the capacitance for 3 db down. If you want to be only 0.1 db down at 20 Hz put in 2 Hz.
Since we don't necessarily want to be 3 dB down we might want to find a lesser down point. If you go an octave away from the -3 dB point you will be down about 1 dB. Another octave away about 0.5 dB. A full decade away will be 0.1 dB down. This is approximate, if you like formulas try this.
http://en.wikipedia.org/wiki/Roll-offHere is an example of a typical calculation. A preamp needs an output cap to couple to a 100K ohm input power amp. Lets assume the preamp is of low output impedance compared to the 100K on the power amp and there is no feedback around the cap. The answer is 0.795 uF and you would use 1 uf. I put in 2 Hz for the frequency because that will be the -3 point.
If the output impedance of the driving device is significant it has to be added to the input impedance. Say your preamp is 10K output and your power amp is 47 K input. The total is 57 K ohms. So that number goes into the calculator.
This also works for cable loading at high frequencies. Put in the output impedance of your preamp in the ohms box, the cable capacitance in the cap box and you will get the 3 dB down point for cable loading. This is often significant as some cables have high capacitance even in one meter. You will need to know the capacitance of your cable or be able to measure it. Many DVMs now have a capacitance function. Here is a bit of an extreme but plausible example. Say the preamp is 10,000 ohms (you can't use K ohms in the calculator) and the cable is 1000 pF (0.001 uF, you have to use uF). The -3 dB down point will be 15.9 kHz which will be quite audible. Many cables that people say sound less bright are doing just this. You could do the same thing with a small capacitor across the cable at a much lower cost.
If there is feedback around the coupling cap it will go much lower by the amount of feedback. Our little $450 preamp has a 4 uF output cap with about 12 dB of feedback around it. Into a 47 K ohm load that is 3 dB down at 0.85 Hz. The feedback lowers the 3 dB down point by 4 times to 0.2 Hz. So at 2 Hz ( 10 times higher frequency) we are down only 0.1 dB and at 20 Hz only 0.01 dB. The RM-5 is very close to these values also. Many preamps I have measured are down more than 1 dB at 20 Hz which is not so good.
Any Questions?
