This was my thinking, that they helped to prevent fluctuation/spikes of current when the draw suddenly changes. That being said, though, the bus board is really just a wire, and I think those kinds of power conditioning caps should go on the PSU or the module.
That as I understand it is pretty much the point of bypass caps, but they should be close to the component theyâre pushing current to. The ones on the bus board arenât, and there are (or should be) ones on the module PCBs that are.
They are probably copied from other schematics, as happens a lot.
If they were meant to buffer anything, then surely they should be dimensioned to a given load. And since that is unknown ⊠Now there are just a few 47mu caps which seems a random (low) value to me. I can however imaging that given that there are wires leading towards the bus board (which do have a resistance albeit small) from the power supply and depending on how much HF that produces if it is an SMPS some small caps (nano farads) could help get rid of RF spikes and help to produce cleaner DC-power to the modules connected to the bus board. But better then would probably be to replace the wiring or the SMPS by a traditional regulated (low efficiency) DC supply. My 2 cents.
OK, from the TI LM78xx datasheet:
Thatâs a cap to ground connected directly to a âvoltage sourceâ output, which they recommend to suppress high frequency noise. I donât see any further explanation so donât know exactly how itâs supposed to do that. But this is a 10 nF cap, not a 47 ”F, and itâs likely to be immediately next to the regulator, not on the next board downstream, so itâs very different. Still, it suggests a lone capacitor can under some circumstances do some useful filtering â though it presumably depends on either the component youâre trying to suppress noise from or the circuit youâre trying to keep noise away from, or both.
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This really cooks! Great stuff, would love to hear you put a full track together.
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I assume those are bypass capacitors to remove the low frequency ripple in the power rails as opposed to the bypass capacitors that remove the high frequency ripple near most ICs. Although, the PSU should have these bypass caps and most modules have the two 10u caps for this purpose.
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What they mean I think is that for higher frequencies C1 and C2 will progressively form a short. Nothing more, nothing less. The question remaining then is why C1 and C2 have those specific values. Another dose of 2 cents.
I went looking for commercial passive bus boards. Nearly all the ones I found have no capacitors on them, including:
- 4ms Bus Stick
- Befaco Passive Bus
- CGS CGS374
- Doepfer A100BUS
- Frequency Central Expandobus/Double Decker
- Low-Gain Electronics 4U Modular Power Distribution Board
- Meng Qi Flat Back
- Rakit Sticky
- TipTop Zeus Passive
I found a couple that do:
- ADDAC900B VS.2 has some sort of electrolytics and some passive SMD parts, no details found
- Nonlinear Circuits Covfefe has footprints for 100â470”F electrolytics and no resistors
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I wouldnât expect anything less from a module named covfefe 
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If you look at NLCâs PSU board, they did not include the low frequency decoupling caps, so it would be a good idea to include them on your bus board. How big they need to be depends on the load put on the voltage regulators and how much ripple exists in the supply. For the low frequency ripple, it really doesnât matter where the capacitors are in the power rails.
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My understanding of bypass capacitors comes mainly from this video
and is that the parasitic inductance of PCB traces inhibits fast current changes, so when a chip suddenly decides it needs more current, the PSU canât supply it immediately. So you put bypass caps close to the chip â close enough that the trace inductance is small. You can use multiple capacitor values to extend the frequency range (17:29 in the video) and the larger caps can be further from the chips.
Apparently the frequency range has to do with the ESL of the cap; at high enough frequency the rising inductive reactance overtakes the falling capacitive reactance. But the frequencies in question are way above audio frequency! At least so it appears from this graph I found (from here):
The 100 nF caps we usually use for bypass next to the chips have their minimum impedance at around 20 MHz! Even a 1000 ”F cap bottoms out at around 200 kHz. These are so far away from audio frequency I wonder if they really have any relevance at all. In the audio frequency range the total impedance is dominated by the regulator impedance (POL), not the bypass caps. As mentioned in the video, itâs not the signal frequency thatâs important, itâs the speed of the rising/falling edges which corresponds to much higher frequency components â but still, anything a lot above 20 kHz just isnât audible.
Anyway, to me that explanation suggests it doesnât make sense to have even very big bypass caps on the PSU â because theyâll see just the same inductance as the regulator does.
A bypass cap on the bus board will see less inductance than one on the PSU â probably roughly half, if itâs roughly halfway between the PSU and the module. But Iâd think youâd want a location with much better than a factor of 2 smaller inductance for a bypass cap there to make much sense.
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Tim Escobedoâs Bronx Cheer (an âenvelope waveshaper filterâ), essentially a transistor fuzz with a filter attached to it. The resonant fuzz filter frequency changes when one hits the notes harder, and changes again as the notes decay. The build includes a small transformer/inductor intended to mimick the impedance of a guitar pickup.
You can find the build notes and stripboard layout in the verified layouts thread.
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He doesnât really go into bulk bypass capacitors in that video. He does a little on the follow up on visualizing bypass capacitors.
The small bypass capacitors deal with current needs of individual ICs. The bulk bypass capacitors supplement the current needs of the entire system, supplying a more consistent voltage with changes in current while the voltage regulator catches up. Some of this demand is to recharge those small bypass capacitors. A more consistent voltage will keep circuits working as they should. The idea is more to prevent unwanted behavior from voltage drops than it is about removing audible noise, though in modular circuits the unwanted behavior could be causing audible noise.
That is how I understand it, anyway. As with many of these âbest practicesâ in component placement and selection, they donât always have an impact on the circuit. They are often a common problem prevention measure. Ainât nobody got time to actually test these things under all possible use cases to design what is actually required.
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well this is the latest " big " project nearly done .
I ordered PCBs and circuit boards of Analogoutputâs Hero and Sidekick modules
5 heroes and 10 sidekicks . only 4 sets here because I gave 1 set to my buddy .
still have euro size vcaâs and envelope gens going in the middle spaces but I just had to share.
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Superb! I love the pigs! Add a nicely sharpened quantizer to give each squeal that bacon rasher appeal!
This made my day Scott. Cheers!
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You seem to love symmetry, or at least an idiosyncratic version there of 
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And backwards France. 
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Or counter clockwise rotated Dutch.
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So, no oneâs going to mention @devicex 's shameless efforts to land that top marketing role in @analogoutput 's RICHâS-N-MORE conglomerate?
No? I see. Well played Scott, well played.
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actually red white and blue , I had the idea for project back around the 4th of July .
the gold piggies may be a statement to âŠ
yes I wouldnât have a problem with promoting the Analogoutput line , great designs easy builds. but not sure I would want to turn a fun hobby into job .
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