Coupling capacitors

[jaymanaa]

Can someone explain how uf values for coupling caps are determined? How critical is this value (the uf)? I see lots of old designs that use .025 which are no longer readily available, so we use .022. Would it make any difference if we went the other way and used a .033? I never have understood this part of the coupling caps purpose. All I really get is that they are there to block DC, while letting the AC signal through. I'd really appreciate it if someone could help me understand. Thanks, Jay


and one more; I often wonder if lower voltage ratings could also be used many times for coupling. There may be several hundred volts from the cap connection to chassis ground, but if you measure "across" the cap, the voltage could be quite less could it not?:scratch2:

 

[EchoWars]

Capacitive reactance (1/2(pi)fc). Here's where you need to learn some math, and we've briefly discussed this. Here's a quick primer of sorts, but to understand the effect of the reactance on a circuit, you need to be able to understand everything else that is happening in the frequency domain, and be able to visualize AC and DC equivalent circuits.

As far as your second question, are you talking about AC or DC? Adding a cap between two points wll not change a DC voltage unless the cap is bad. It may change the AC voltage if the source impedance is high. Here also is where deeper theory, such as Thevenin, would help.

 

[battradio]

As long as its not in a feedback ,tone control , or timing circuit 50 to 200% more capacitance doesn't hurt anything but the pocket book .

 

[vinylkid58]

I see lots of old designs that use .025 which are no longer readily available, so we use .022. Would it make any difference if we went the other way and used a .033?

I went from .02 to .047 for the output coupling caps in my old HK 224. Replacing the old 400V paper caps just before the tone control section made a bigger difference, as they were getting pretty leaky.

Jeff