When is 1F not 1F? When it's a resistor...
No component is a pure element. Indeed, necessarily so. Resistors, capacitors and inductors (the physical components) all have complex mixtures of resistance, capacitance and inductance. We merely call them what they are, because that property is dominant over a useful range of frequencies.
It's all about frequency range.
We must always be careful not to fool ourselves into thinking that property extends farther than it actually does!
A supercapacitor is useful at low frequencies: perhaps uHz to 10s of Hz. Below that frequency (down to DC), they look like crappy resistors (leakage current). Above that frequency, they look like crappy resistors again, smaller this time (ESR). At higher frequencies, they look like inductors (probably in the 20nH range).
There's also finer structure to each of these components:
- The leakage comes with long time constants across it, so that leakage starts out high, then decreases over time (in a way that the rated capacitance cannot account for).
- The capacitance itself, even in the range where it's being a "good capacitor", isn't ideal, and has some resistance associated with it, and this resistance varies with frequency. (Supercapacitors use the diffusion of ions in a solution, which is great for value -- you can get a lot of ions in solution; but awful for losses -- ions move slowly and with a lot of "friction". This is distributed over a wide frequency range, because ionic
diffusion is a very gradual process.)
- The inductance
- And this is all to say nothing of temperature dependency, or aging, which affect all these parameters to some extent. Most notable is ESR (and ionic diffusion losses) vary strongly with temperature, getting particularly bad at low temperatures (until, when the electrolyte freezes, the capacitor stops, uh, capacitating, at all... it's incapacitated, you might say..).
A regular electrolytic has the same model, but the frequency range is different -- reflecting the smaller capacitances, lower ESR, and somewhat lower ESL typical of them.
So, applying this to the video: the 1F is acting like a low value resistor, at the frequencies being measured. It is not doing a perfect job, by any means -- not nearly as well as a 1000uF electrolytic, or a 10uF film -- but at the scale shown, that's apparently enough.
The guy also appears to have much more test equipment than his experience and understanding would suggest to be necessary...
As for your case: you get two things with the pictured situation. When the capacitor is near the "AMP", it's able to supply power that the cable would otherwise drop. This is important at modest frequencies (over 10kHz, say), where the cable has equivalent inductance (because, again, nothing is ever just "what it says on the tin", and the cable has resistance, AND inductance, AND capacitance, all depending on frequency; and for frequencies above about 10kHz, the cable will be dominantly inductive).
But, this is probably not important to the AMP, because it will also have capacitors inside it already, enough to handle this.
So we move on to the next suspect: the battery. A battery isn't quite a capacitor: it doesn't store charge proportional to voltage, as a capacitor does; rather, the voltage tends to remain nearly constant*. But given that adjustment, a battery behaves very much the same as a capacitor, having that equivalent circuit with C, ESR and ESL. But the frequencies are even lower than for a supercap -- batteries are good for ~nHz to ~Hz.
(*Precisely, it follows an exponential relationship on the concentration of chemicals that are reacting to generate power. So the voltage falls gradually as you discharge the battery, then when it's pretty much empty, it just tanks (the exponential slope goes, well, exponential).)
So if you have a cold, or aging, battery (high ESR, low time constant) that isn't capable of supplying the AMP for short durations, the Capacitor helps.
Mostly, though, amplifiers don't consume nearly that much power, even under peaks, and it's a lot of marketing wank. This is reflected in the shitty construction of poorly verified consumer "capacitors", which may often be Matryoshka designs. Smaller example:
You
can get proper electrolytic or "super" type capacitors, in sizes and ratings suitable for supplying a very large amplifier (many kW) from a modest automotive electrical system (battery and alternator, both suitably sized -- at this rate, you may have two or more batteries wired in parallel, and your alternator will probably be upsized a few times from the usual). Beware of poor imitations, though; and, remember this: if you can't tell an objective difference (i.e., measuring the clipping point on loud music), you're better off saving the money.
Tim