user 37518 said:
That is a great rule of thumb John, thanks!
That was my observation for a very specific circuit. After recognizing that capacitors are not ideal and have multiple moving parts, I worked to finesse or engineer the problems out.
In simple DC blocking circuits I set the pole frequency several octaves lower, so the terminal voltage is squat inside the audio band.
I addressed this in my old magazine column (Audio Mythology) back in the 80s. I won't repeat the entire column but a few highlights.
#1 non-ideal cap behaviors (like dielectric absorption and voltage coefficient ) can be minimized by keeping the terminal voltage across electrolytic caps small...(thus the low pole frequency example above). If you can't keep the terminal voltage small (like in band audio filters) use the best dielectric you can. These days NPO/COG ceramics are very good, back in my day I used good film caps.
#2 Capacitor characteristics like ESR and ESL (series resistance or inductance) express as a function of the current they are passing. The example I gave of a phono preamp was effectively loading a 22uF cap with 360 ohms. Passive loudspeaker crossovers are absolutely worst case for capacitors... If you must, and can, film caps like polypropylene suck less in that application... That said active crossovers completely avoid putting capacitors through that much pain.
A classic design approach to completely avoid electrolytic blocking caps is the DC coupled mic preamp topology.
Transformer mic preamp front ends trade one non-ideal component for another, while opinions surely vary about that.
For scratch designs we have the luxury of avoiding the obvious component weaknesses. To restore old designs I will repeat until people listen... measure, measure, measure... confirm the path performance, and if pursuing upgrades, measure the before and after so you know what actually changed or didn't.
There is a lot of expectation bias masquerading as empirical evidence around DIY tweaks. These were pretty popular decades ago and even called "Pooge" (Progressive Optimization of Generic Equipment a phrase attributed to Walt Jung). I am not a fan of second guessing design engineers, but some people make a career of that (just spend some time reading gearslutz).
JR