Is there a situation where electrolytic capacitors are necessary?

[mybud]

I read thru that article, esp the bit on the different types of film caps was interesting. This part stood out to me as far as my original query about specific uses of elec caps.

I see both film caps and elec caps being used as decoupling/coupling capacitors in our pedal circuits. My EE background isn't as strong as I'd like it to be - what is it about a low impedance that makes an electrolytic preferrable?

Also, I see the terms coupling and decoupling used interchangeably sometimes depending on the source. Am I getting them confused in the previous question?

Coupling/decoupling and bypass caps explained

 

[falco_femoralis]

Whenever you have a coupling capacitor, i.e., a capacitor used to filter DC voltage between two parts of a circuit, you are introducing a high pass filter (which by definition filters low frequency including 0Hz which is DC). The most important feature of a high pass filter of this form (first order passive) is the cutoff frequency which is proportional to the inverse of the product of R and C (where R is typically the output resistance of the first stage in the coupling pair). You want the cutoff frequency to be low enough to avoid removing too much of the bass content, which means you want a high R*C product. If the circuit is already high impedance, you will need a smaller cap value. If it’s low impedance you will need a much higher value, which may force you to choose an electrolytic cap.

Edit: about coupling and decoupling, I’m not sure what decoupling typically refers to, do you have a reference or a quote?

Thank you for the clear explanation. What began this for me was trying to understand why something like the SHO has a low value film input coupling cap but a Fuzz Face has an electrolytic. Do pedal circuit designers measure the total impedance of the circuit, or calculate it, or do they just go by a rule of thumb and using their ear when choosing certain cap types?

Decoupling caps are mentioned in the second paragraph of post 9 on this thread, but I've heard it referenced to what is described here as a coupling cap perusing youtube. It's entirely possible either I understood it incorrectly or the youtuber mentioned the term in error

 

[giovanni]

Coupling/decoupling and bypass caps explained

Thank you! I’ve always called caps across DC filtering caps, TIL.

 

[giovanni]

Thank you for the clear explanation. What began this for me was trying to understand why something like the SHO has a low value film input coupling cap but a Fuzz Face has an electrolytic. Do pedal circuit designers measure the total impedance of the circuit, or calculate it, or do they just go by a rule of thumb and using their ear when choosing certain cap types?

You can probably calculate the optimal value based on the circuit characteristics, but that’s more of an academic/learning approach I think. My guess is that most pedal designers would just use their ears or maybe a Spice simulation.

 

[pricklyrobot]

Just for added confusion, if you delve into the world of modular synths at all, you’ll encounter the terms

• AC Coupled: meaning it has a DC-blocking cap in the signal path, and the module is intended to pass/process audio signals only
—and—
• DC Coupled: meaning no cap, and the module can pass/process both audio and control voltage (i.e. DC) signals

 

[YourGuitarist]

I believe MLCCs are also prone to microphonic. Correct me if I'm wrong though. Electrolytic are preferred for their capacitance/voltage rating compared to size.