Nice walk through a distortion circuit

[culturejam]

Ive seen this in the "huminator" power filter, its a common tactic for power filtering (large electro + small film caps).  Not sure why there are 3 x 100nf caps though, I guess more "pipes" to filter the current, faster reaction maybe, but I'm just guessing here.

Yeah, even Brian has a 100n parallel with the larger filter cap in all his designs these days. But I agree that I'm not sure 3 of them are necessary, although I suppose if you place those very near the op amp's VCC pin, it might help reduce noise in some situations.

[sonnyboy27]

Sure, but there are 4 caps in parallel - 100uF and 3 100nF. Thus a total of 100.3uF and where these are physically placed will make no difference to this.

Placement actually can make a difference in some situations. And the smaller caps in parallel do add up as you say, but from what I've read in texts about PCB design, the smaller value caps can react "faster" to smaller changes in current than the larger caps. So if you have large and small caps in parallel, overall ripple noise smoothing is improved. From what I gather, it's sort of like improving the "slew rate" for power supply current transients. But I might have misunderstood some of the theory.

This kind of thing is more important with digital processing and things like power amplifiers (chip amps, etc) than it is with low-voltage / low-current analog stuff like dirt boxes. But it's just good practice to do it.

From what I remember in my board design course, it's basically what CJ said. It's a common practice to place a filtering cap between power and ground near each chip. In the original layout those 100nF caps are next to each IC. This is done to reduce any residual noise that could be introduced between the main filter cap(s) and reaching the chip itself. It's common practice in digital circuitry and so it crosses over to analog designs sometimes (most notably in higher gain circuits). It's one of those board layout things that shouldn't be an issue from a theoretical standpoint but it helps in reality.

[chromesphere]

This is good timing.  You can see the effect of different bypass caps on Daves oscilloscope:

https://www.youtube.com/watch?v=1xicZF9glH0&t=628s

[Philtre]

This is good timing.  You can see the effect of different bypass caps on Daves oscilloscope:

Brilliant! This video and Dave's other one on bypass caps explains thing nicely. Thanks for the heads up.

(Side note - some of the vero layouts at tagboardeffects remove the additional small-value bypass caps. Probably not a good idea then!)