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6800uf across op amp PS pins?

70.213.7.1
Posted on January 3, 2017 at 08:34:32
Tweaker456
 In a recent post in Tweakers Asylum bobwire is suggesting 6800uf across the PS of op amp power supplies. He mentions the pin numbers so I'm assuming he means right at the op amp. In a previous post of mine I asked about using 10 to 20uf in the same location and was basically told it was dangerous. I'm blocked at Tweakers Asylum so I'm posting here . Don't want people doing this if it is incorrect. Tweaker456

 

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That's crazy nonsense IMHO...., posted on January 3, 2017 at 09:35:47
AbeCollins
Audiophile

Posts: 46302
Location: USA
Joined: June 22, 2001
Contributor
  Since: February 2, 2002
 That's way too much capacitance to place right at the opamp PS pins. You might see that much filter capacitance or more within the power supply itself but not typically right at the opamp +V and/or -V pins.

A modest amount of bypass capacitance can be added where the power supply leads enter the printed circuit board assembly, perhaps 10uF - 100uF. And then a much smaller amount of bypass capacitance right at the opamp PS pins, typically 0.1uF. More is not better because the larger capacitors and their internal inductance offset their effectiveness at bypassing higher frequencies. You'll sometimes see a slightly larger capacitor in parallel with a much smaller one as their undesired inductance is reduced while their respective capacitance values can cover a wider bandwidth for bypassing.

Power supply decoupling is similar to bypassing but the focus is on minimizing signal transfer between stages. A small amount of series inductance can be used comprised of a ferrite bead over a short length of buss wire that feeds -V & +V right at the opamp pins. Different ferrite materials are effective over different frequency ranges. Charts are available from ferrite manufacturers.

Of course audiophiles will go overboard thinking that more is better everywhere. For practical purposes none of this is all that critical until you're in the RF range, using very high bandwidth opamps that are inherently not very stable, and involving high transient currents.



 

RE: That's crazy nonsense IMHO...., posted on January 3, 2017 at 11:30:35
Tweaker456
 Abe, Maybe you can post about this at the Tweakers Asylum. The thread is near the top and has three op amp numbers in the title. I did just read a tutorial that said like 20 to 50uf with small bypass, so it looks like the person who told me 10-20 was a reasonable person and the person who said they would laugh when I saw smoke would not get a chance to laugh. My post on the subject is titled "10-20uf across op amp PS?" in tweakers asylum. Thanks, T456

 

RE: That's crazy nonsense IMHO...., posted on January 3, 2017 at 12:34:04
AbeCollins
Audiophile

Posts: 46302
Location: USA
Joined: June 22, 2001
Contributor
  Since: February 2, 2002
I learned a long time ago that one can be banned from a forum even if correcting a blatant mistake or the spreading of misinformation. I'm not talking audiophile opinion here, but absolute right vs. wrong. It's happened to me a few times. So for the most part, I just chuckle under my breath now and let some myths and morons live on. I did post a very short question to Tweakers, awaiting a response.

[Edit]: This sort of bypass shown below is not uncommon. But I have never seen anyone put anywhere near 6800uF at each opamp +V & -V. Silliness IMHO.



 

RE: That's crazy nonsense IMHO...., posted on January 3, 2017 at 22:54:04
Tweaker456
 Silliness or not I am guilty of being in the bigger is better crowd. It's clear that bobwire is getting away with it, it is not blowing up anything. That's what I'm worried about. Otherwise I would love to try it on the output of my dac. So, will or can it damage the equitpment, catch fire, blow up? If not I'd do it. T456

 

RE: That's crazy nonsense IMHO...., posted on January 4, 2017 at 10:00:43
AbeCollins
Audiophile

Posts: 46302
Location: USA
Joined: June 22, 2001
Contributor
  Since: February 2, 2002
 Putting huge capacitance directly on an opamp -V & +V pins will not damage it but the effectiveness of such a large cap in that location is questionable.

I'm not sure what you mean by "try it on the output of my DAC". If you're talking about the -V & +V pins on the opamp(s) in the output stage of your DAC, that's probably fine..... but not at the audio output jacks of the DAC.

Enjoy!



 

RE: That's crazy nonsense IMHO...., posted on January 4, 2017 at 14:12:59
Tweaker456
 V- V+ at the chip is what we are talking about. You and another person I believe says it is no problem. A suggestion of a high quality film cap has been made as opposed to a large electolytic. There is very little current there so it does seem another possible reason why it might not make much or any difference. Still, the proof is in the pudding, as us old timers say. Thanks, T456

 

RE: 6800uf across op amp PS pins?, posted on January 6, 2017 at 09:10:21
erik_sq
Audiophile

Posts: 150
Location: San Francisco Bay
Joined: September 28, 2016
 Is it possible you misread, and they suggested 6.8uF?

That makes the rest of your statement more normal.

Typically filter caps go down as they get closer to the destination.

An example for line-level audio (not speaker) would be 10,000uF at the power supply, then 10uF to 20uF before a regulator, 4.7uF after, and then 0.1uF right at the pins.

There's no hard/fast rule but this is all chosen to minimize high current and long runs. The larger the caps, the larger the charging current at turn on, and the bigger the voltage drop, so you'd like to have as big and fat a trace (if not copper/aluminum bus bar) near the power supply, and then decrease. If you put the PS caps at the op amp, or distant regulator, then the charging current would have to travel the distance from the transformer and bridge to there, causing a voltage drop proportional to the current.

Large caps also tend to be more inductive, so less able to filter high frequency noise.

By using a more staggered approach you get the best mix.

 

RE: 6800uf across op amp PS pins?, posted on January 6, 2017 at 19:04:09
Tweaker456
 It's 6800uf. No mistake. T456

 

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