What are these capacitors for?

I want to say they did it to reduce the effective ESR of the caps but given that it's a consumer music-audio product it could be for any reason at all or no reason whatsoever, hard to say without pix.

0.1uF caps are often put next to ICs to clean up the supply rail to them. One big one further away wouldn't be as effective, there would be inductance & resistance in the track between cap & IC.

NT

One typically scatters a bunch of smallish ceramic caps around a board, for high frequency bypassing, so no opamp or logic gate is very far from a cap. They are not usually placed physically together on the PC board.

I use 1 uF 0805, usually, from 5 to maybe 50 on a board.

Most people use too many. It's like using garlic to keep away vampires. It works.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

Whats the equivelent of silver bullets? Ferrite beads ?

Wooden stake? Ground planes ?

Cheers

Some say that these foul beings can be destroyed by a stake through the heart, or exposure to sunlight. This does not suffice, however, as they may be brought back to life by means of a secret rite, which may be performed once a century, when the moon is in the Sixth House of Aquarius.

Kind of like Newt Gingrich

-----------------------------

** Nonsense.

** The purpose of distributed 0.1 caps is to prevent HF instability in the op-amp stages. Without them, oscillations may occur at certain control settings or as a permanent background.

Inductance in the DC rail tracks is the basic reason & much depends on which op-amp is being used.

..... Phil

They are high frequency decoupling caps, spread around the circuit in strategic places. The idiot who drew the schematic didn't give a hint where they belonged or what they were for, though.

Sure. It probably wouldn't work without them (and it's easier putting them in than figuring out why it doesn't work later). Eight decoupling caps for a supply isn't a lot.

OP gave the impression that they were all physically located together on the PCB as well, which wouldn't make a lot of sense in this application.

snipped-for-privacy@tubes.com wrote on 10/24/2017 7:09 PM:

This is very common for digital designs where noise in the MHz range is common. The larger caps and especially the caps that are not low inductance, like electrolytic and even tantalum caps won't filter the higher frequencies as well. Also, the power distribution can cause problems with noise coupling from one chip to the next. So they often use caps very close to the power and ground leads that have very little inductance.

However, this is of much less value in audio circuits where there should be little high frequency noise sources and amplifiers are less sensitive to high frequency noise. But if you are trying to design equipment that has very low noise levels and there are sources of high frequency noise, you might uses these caps near many of the chips.

Does this unit have much digital in it? Digital circuits are often a source of noise for analog circuits.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

They are grouped together on the schematic, but I did not find any cluster of them on the PCB. There are a lot of caps on the board, and I'm finding an occasional 0.1 here and there. Since the board does not have them marked as (C38) or whatever, I dont know which ones they are.

If I had to do any actual work on the circuits, I'd need a much more detailed schematic. But I found what I needed on this poorly made drawing to determine the resistance of the slider pot I have to replace.

This makes the most sense......

Parallel planes make great HF caps. If you have power pours over a ground plane, you don't need to put caps close to parts.

Big digital chips, CPUs and FPGAs, now have substantial bypassing on-chip. So don't need a bunch of bypasses nearby.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

That's basic common knowledge, but when I asked about mobo replacement caps, people said it's ok to use long leads if the cap doesn't fit.

It reminded me of the AC family with center power pins because the corner pins had too much inductance. Something about this isn't clear.

and it does that by cleaning up the psu rails lol.

NT

"cleaning up" is a vague term, it implies that the instability is caused by noise originating from the power supply or power line itself, which isn't why it happens.

John Larkin wrote on 10/24/2017 10:07 PM:

Most people don't understand that.

That is only true for certain chips. Many FPGAs and CPUs don't have bypassing on chip. There are still other signal integrity issues to worry about.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Rule of thumb...

Cheers

--------------------------

** Bollocks.

The distributed 0.1 caps LOWER the DC supply impedance at HF preventing the instability.

DC rails do not oscillate but wide band op-amps often do if not adequately bypassed.

..... Phil

"September 12, 1997"

A lot has changed since then. Here is another quote from that article.

"Your design is good for only one board".

But how many boards it the rule of thumb good for? Which ones are they?

If you are designing jelly bean sort of designs then sure, you can use this sort of overkill without problem. But how do you know when this approximation no longer works?

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998