It is standard practice today to add in parallel a small film capacitor across a much larger value electrolytic cap. That practice is called bypassing and doing so improves the high frequency performance of the larger electrolytic.
50 years ago, when we began our high-end audio journey, no one bypassed capacitors. It wasn't that the technique was unknown by engineers, it was simply that it made no sense to add the extra expense. Why? Well, the amplifier showed zero difference in its measured response. The frequency response, the distortion, the power output—all of it— were identical. Why waste money on adding components that offered zero measured benefits? Because the amplifier sounded dramatically better. More open, more depth, less hardness on the top end.
We're not getting into the measurement wormhole debate this morning, but still, this is an interesting bit of history.
Stan and I used to earn some side money adding our "secret sauce" to Phase Linear 400 amplifiers. Word got around to amp owners that for $50 they could get a huge boost in performance by letting us modify their amps and they lined up for the service. For us, we invested maybe $1 to make this magic. We didn't even have to solder as the Phase Linear 400 had screw terminals on the power supply capacitors. A handful of 0.1 film caps and a Phillips screwdriver did the trick.
As a side note, what's interesting is the measured performance of the capacitor is better when you add a bypass cap. This is especially true (and important) when designers used electrolytic caps on the input of the amplifier or preamplifier (interesting for the day but no longer since most of us have gone direct coupling).
Today I don't think there's a designer out there in the high-end world that wouldn't routinely just add a bypass cap to the product's electrolytics. It's easy and inexpensive. So, why not?
But, back then, it was a revelation.
