KrIVIUM2323 said:
The choice depend of bandwidth/phase behavior you want.
The formulae is 1/(2*pi*r*c) where r is input impedance of next stage c capacitance in f.
Frequency/phase response is just one side of the coin. The sonic deterioration due to electrolytics in the signal path is proportional to the amount of signal developped across them. That's why "
Sometimes you'll see very low values (sometimes in the 1 or below hertz area): this is not really for bandwidth". For a given current circulating through the cap, the higher the capacitance, the lower the voltage.
Sometimes you'll see very low values (sometimes in the 1 or below hertz area): this is not really for bandwidth but phase related as hp filter generate phase modification up to two decade over the fc (eg: with 1hz fc up to 10hz (first decade) there is lot of phase shift and still some minor modifications until 100hz (second decade)-usually acceptable).
I won't start a debate on the audibility of phase-shift of signals that are below aural perception; unless there is something
very wrong in the transmission path (strong asymmetry of response, non-linearity of medium under very high pressure, low-frequency FIR filters), the amplitude perception always dominates whatever phase perception there may be. The actual reason for oversizing elcaps is a) high manufacturing tolerances: your 100uF cap can be as low as 70uF or as high as 150
b) indeed elcaps dry with age; increasing the nominal value increases the normal operation life expectancy
c) in systems with global NFB, the cumulated phase-shift due to coupling caps may cause stability issues; tube amp designers know it well
d) ancient "radio" designers noticed that using caps with just the calculated value did not sound right
All in all, this led to the "10x rule" (design for 2Hz) that was prevalent for several decades; now, with the cost going down and capacitance going up, one can go for the 100x rule (design for 0.2Hz).
About choice of technology: the real downside of electrolitycs is that they dry with age so be ready to change them in 5 to 10 years depending of your application , build and range of product you choose (+105°c are usually better).
Agreed; currently made elcaps are somewhat better, though. I would think properly used modern elcaps would last about 30-40 years. We have some hindsight now, with products manufactured in the 80's, that show no sign of need of recapping.
About their sound... yes they have a kind of sound but every component have one. Do you need to stay away from them? Just take a look at ANY schematic of say R.Neve (or SSL, or AMS/Neve, or...) products from the 80's till now and see/ear by yourself if it don't make 'good' sound, and how many are in the signal path...
These products were designed in view of the various concerns I expressed earlier, certainly Rupert Neve did care about audibility of elcaps and understood that it depends as much on the implementation as in the technology.