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By-pass Capacitor?


kg4guy

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These IIRC are usually a smaller value cap in parallel with the main one. Often they are made of a different material.

I can only believe the theory is that somehow large value caps don't work properly at low voltage, maybe electrolytics (non polarized). So a small value (about ten percent of the main or less) which is not electrolytic (I've seen tantalium) somewhow helps.

I don't put any stock in it for three reasons.

One: it should be easy to see such a problem on an o-scope. I haven't seen that done.

Two: When caps are tested on meters-bridges low voltage is used. Yet they test to rated specs. If so, there should be no problem.

Three: If the big cap isn't working, the small one is only adding a small amount of capacitance. That is not solving the problem, assuming there is a problem.

Gil

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I stole this off of a thread on Audiogon, and after looking at other sources, think this is the usual reasoning:

Unfortunately, this is one of those topics where you first
have to define your terms. The term “bypass cap” is used quite
differently by audiophiles and electrical engineers. Audiophiles
typically use the term to refer to multiple capacitors wired in
parallel in the same physical location, or nearly so. The purpose of
this (at least within the realm of high-end audio) is to create a
single, conjugate capacitor that outperforms ("sounds better than") any
single cap in the same application. The mix of caps is usually arrived
at through a series of listening evaluations, so the process is mainly
subjective. For the purposes of high-end audio, bypass caps are almost
exclusively film types (polyester, polypropylene, polystyrene, teflon,
etc.) The ‘theory” is simple enough - smaller value caps may (for a
variety of reasons) have better high-frequency performance than larger
ones (especially when the larger caps are electrolytic), so combining
large and small caps may (when everything works well) result in a
single capacitance with outstanding full-range performance. As with
many things in audio, this is easier said than done, but it remains a
very popular practice among hobbyists.

Electrical engineers
usually use the term to refer to multiple capacitors connected in
parallel electrically, but distributed along the power supply path so
that the smaller caps with superior high-frequency performance are
located adjacent to critical circuit elements (particularly
wide-bandwidth gain blocks). The intent is to improve the stability of
these circuits by providing a superior low-impedance path to ground on
the power supply lines. Many modern wide-bandwidth circuits will not
operate properly (or at all) without these local bypass caps in place.

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In the speaker impedence, speaker voltage realm, "by pass capacitors" are a marketing ploy. Some believe that higher frequencies magically jump to the "better" cap instead flowing through of the "lesser" cap. No. No. No.

When caps are in combination, the result is a hyhbrid of both, but can never be better than the "worst" of the two. This has been proved a number of times.

Subjectivity sells expensive tweaks, science doesn't. If you have electrolytics (of any type) in your crossover, they should be replaced with good quality film units. The shift from lytics to film, even the much maligned mylar, is audible under most circumstances. The shift from one film type to another is more subtle and may not be consistently audible. If you think your present capacitors are suspect, they should be replaced with single, good quality film units of the appropriate value & voltage rating. Putting a teflon in parallel with a cheap mylar and thinking you have bypased the mylar and now have the equivalnt of a teflon cap just isn't so.

It's worth noting that many on this forum think Paper In Oil (PIO) caps are the cat's meow, and PIO are relativly inexpensive vis-a-vis the Capacitor De Jure the tweaks sites are agog about this week. I like PIO myself.

As noted so well, bypass caps have legitimate uses in power busses and at radio frequencies.

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I think I read somewhere, sometime, on this forum that Al said he bypassed the 6.2 uF with the 1 uF in order to get the resulting higher capacitance. I suspect that 7.2 uF capacitors may be hard to find. Of course, I could have dreamed I read that. I have no clue about bypassing the 39 uF capacitor, but it could be the same reason.

You might try researching this in the forum's search function if you haven't already.

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When caps are in combination, the result is a hyhbrid of both, but can never be better than the "worst" of the two. This has been proved a number of times.

Do you have a source showing the proof? [:o]

It's generally very hard to see hysterises on a scope with an audio signal, or any distortion for that matter...but I bet I could find a bad enough case where a lower ESR cap in parallel reduces the effects and can be seen on a scope. I've never actually done it though so I would be curious to see the results of anyone else that tried. But with that in mind, I still agree that it's better to just go with a single better capacitor (except in cases of noise filtering).

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That was an old ALK design. He just uses higher quality caps without bypass caps now. I have had just about every one of Al's networks, still have some. Use Auricaps for the midrange and tweeter positions. Use the cheaper Solens for the bass. Those were the best combinations I ever came up with. I spent a fortune on that stuff. Thousands of dollars. I also tried using bypass caps at one point early on. I agree with everyone else's posts from my own practical experience. Bypassing caps are BS. Don't waste your time and money. Buy Auricaps. Just my humble opinion.......if it didn't sound humble, sorry.

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Okay,

I expect that Al K. is not advocating tweek benefits of funny helper caps. But I'll let him speak to that.

It is a real bear when you do a design and find that no standard value comes in on the money. Then you parallel. In an inductor, you can pick a bigger value which might be in your box and unwind.

The tweek thing still doesn't impress me. It seems that the problem can't be demonstrated with our objective machines and the solution can't be shown with our objective machines. So we have subjective problems and subjective solutions.

If only we had subjective money.

Gil

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Okay,

I expect that Al K. is not advocating tweek benefits of funny helper caps. But I'll let him speak to that.

It is a real bear when you do a design and find that no standard value comes in on the money. Then you parallel. In an inductor, you can pick a bigger value which might be in your box and unwind.

The tweek thing still doesn't impress me. It seems that the problem can't be demonstrated with our objective machines and the solution can't be shown with our objective machines. So we have subjective problems and subjective solutions.

If only we had subjective money.

Gil

Yeah...for the benefit of the less experienced, we should note that paralleling caps to get a given value is perfectly legitimate and often necessary...not at all the same thing as the bypass cap notion.

Also, D.B. Keele, in a paper some years ago, showed me a frequency selective attenuator, that is merely an L-pad with a cap bridinging over it. The idea is to flatten the falling responses of an HF horns by knocking down the response up to a point and letting the higher frequencies "bypass" the L-pad via the bridging cap.

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Did you ever hear the one about the drunk looking for his car keys?

He's looking for them in the gutter about closing time.

People start coming out of the bar and ask him what's he doing?

"Dropped my keys!"

They all start looking in the gutter, up and down the street.

Finally one of the searchers asks, where do you think you were when you dropped them?

"Around back in the alley."

Well why are we out front then?

"The light is better."

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

We measure many things in audio that don't seem to correlate to how it sounds, simply because we know how to measure them (the light is better).

An oscilloscope is a very poor device for measuring most things, you have a hard time even seeing 2% THD on one. But you can see a little something about caps.

Let's run a little 600hz through them (they should all be a straight line).

Posted Image

?action=view&current=Capbasics.jpg

(1) Man, the tantalum looks pretty bad (imagine that, they sound bad too). (2) Aluminum looks a little better. (3) Mylar (polyester) looks even better. (4) Polypropylene, even more so. (5) Polystyrene looks like a straight line, too bad they don't make them bigger than about 0.1µF. (6) Paper in oil doesn't look too shabby, but it isn't perfect.

Now let's do some tweak stuff.

Posted Image

?action=view&current=Capbasics2.jpg

7) A stock tantalum for reference. 8) 20V DC bias makes it look much better. 9) 100V DC bias makes it look very flat (wonder if JBL knew this when they designed their DC biased crossovers)! 10) Stock paper in oil for reference. 11) Paper in oil with a good bypass cap (looks pretty good, hmm). 12) Why we don't use ceramic caps in the signal path.

Note: the bypass cap in 11) was a polycarbonate, not available in larger values anymore. If you want to really do it up, use a small film-and-foil polypropylene and an even smaller polysteyrene for the bypass caps (like JBL does on their better stuff). Or you could go the DC bias route too (JBL does both on their top-of-the-line product).

Photos from Steve Bench.

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The Dayton's are an incredible value for about $1 each, other similar caps are so much money I won't use them.

Polystyrene is a problem. Rel-Cap is the only one I know making new high voltage polystyrene caps. Michael Percy sells them for about $4 each. New Old Stock (NOS) can sometimes be found for about half price. Make sure they are rated at at least 160V. Polystyrene can melt while soldering if you're not careful.

Xicon 50V polystyrene caps for line level circuits are dirt cheap, only about $0.09 each in low quantities. I don't use many of the $4 a piece ones for speakers.

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