Cap polarization question

[jonny]

Okay, so sometimes in effects you see a polarized cap in the signal path like the dod 250 for example. How come you can do this? I mean I thought you're not supposed to put ac through one because well you know.. bad things happen. Is the signal just too small to actually damage the electrolytic layer so it's okay to do this?


eg look at the 4u7 cap on the output of the 741. This has always confused me.

[CynicalMan]

The cap isn't damaged because of the DC bias on the output of the op amp. The op amp is biased at 4.5V, so the voltage on the output will always be 4.5V + the AC signal. The capacitor forms part of a high-pass filter that removes DC, but not AC signal to any noticeable extent. The voltage drop over the capacitor will always be 4.5V, regardless of the AC signal. So, the answer to your question is that when electrolytic capacitors are used in an well-designed audio circuit, the positive side is always more positive than the negative side. You can get away with small AC signals, but it's bad practice.

Another thing that should be noted is that electrolytic capacitors often have high values, of 1uF or more. So their reactance to audio signals is generally low, while their DC resistance is infinite. This means that, in most cases, electrolytic capacitors will have a DC voltage drop over them but only a small AC voltage drop.

[brett]

Hi
here, as most of the time, the output of an op-amp sits at about V+/2, so there's about 4.5V DC on the LHS of the cap (relative to gnd).  The RHS of cap is connected to the gnd pad (so it's at 0 V DC).  So the LHS of the cap is much more +ve than the RHS.  Sure enough, that's the way they oriented it.  There are lots of caps that isolate the bias voltage on op-amps (typically 4 or 5 volts) from the rest of the signal path.  That's coz DC through pots makes them scratchy, and has effects on feedback, and causes other problem.
cheers

[jonny]

aha  that makes sense. Thanks heaps