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Old 11th January 2005, 01:54 AM   #1
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Default Audio Grade Caps Selection Myth vs Reality

I am restoring a Harman Kardon A50K (kit version of A500) tube amp. A first tube project and first audio project for me. As I replace aged power supply caps and signal filter caps I am finding the art of cap selection mysterious. Armed with a Mouser catalog and online access to antique audio web sites, I just cant figure out the mistique of cap selection, and think maybe it is part real, part myth (bunk!). People advise that when replacing a .25 uf signal cap with an available .22 uf that there will be a significant change in frequency response. And that JJ caps will perform better for audio than another cap. I don't get it. I would think the thing to look for is max life at high temp, tolerance, capacitance coefficient to heat. And since the life is certified at voltage,one would tend to choose a much over-rated voltage to get more life.

How does one go about selecting caps for audio circuits, and how much is myth?
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Old 11th January 2005, 02:00 AM   #2
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My prediction is that at the end of this thread you will know a lot more about caps, but in the end you'll still have to decide for yourself.
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Old 11th January 2005, 02:33 AM   #3
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Changing the value of a cap, vs. the change of the frequency response, depends where in the circuit you put it. hint: pi = 3.14 is close enough for most rough calculations.
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Old 11th January 2005, 02:46 AM   #4
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Isnt a caps frequency response a function of its capacitance?
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Old 11th January 2005, 02:49 AM   #5
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Quote:
People advise that when replacing a .25 uf signal cap with an available .22 uf that there will be a significant change in frequency response

Hi

When you buy a capacitor of 0,25 uF , if the tolerance of the capacitor is 10% , the capacitor could be really a 0,22 uF capacitor and vice versa.

Don't worry , about this small differences ...
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Old 12th January 2005, 02:44 AM   #6
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It seems to me that the audiopholes in this forum can have two caps with identical published specs and say that the audio quality can be very different. If this is true, then there are characteristics that the specs do not identify. What is it that makes a cap sound better than another when it has the same published spec, assuming the spec is accurate?

I would like to understand this before I come to believe it is really smoke and mirrors, bragging rights, and misconceptions. I tend to believe there really is something more, but why is it that no one knows what it is?
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Old 12th January 2005, 02:56 AM   #7
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Quote:
Originally posted by ransom peek
It seems to me that the audiopholes in this forum can have two caps with identical published specs and say that the audio quality can be very different.
Published where? On the side of the cap?
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Old 12th January 2005, 03:28 AM   #8
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Datasheet will show cap value, tolerance, reliability rating for a given voltage/temp combination, a temperature coefficient rating to show consistency over temp range, dissipation factor (whit is this?), CV (dont know what this is), impedance/equivalent series resistance, leakage current, ESR (what is this?), DCL, an on and on.

So why in these many responses on this topic doesnt anyone offer any info with real value? There are a number of threads, but nothing of substance to indicate what makes a good audio cap - if it is indeed not something that can be captured by a spec.

Maybe in many cases people perceive differences that are not real.

The previous response: "published where - on the side of the caps" is not helpful. Of course, the manufacturers provide datasheets - online and easy to access.

I have very limited expertise here and am looking for someone to share some experience. I have not problem with electronics and am an engineer, but am frustrated by the fact that so many people pretend to know about things they dont understand and knowing that there are so many really smart, knowledgeable people that could help with this question am frustrated that no one is willing to serioulsy offer some quidance.

I am about to spend over $100 on parts to restore this amp - mostly for a few caps, and I have no idea if these "audio grade" caps are really any better than more common industrial ones, that can go head to head in terms of the detailed psecs notes above. I am learning that the material a cap is made of makes an audio difference, but again, this could be that the material impacts the published spec and the real performance can be understood from the spec. The datasheet has al ot to offer - does this only get us part of the way to understanding the audio quality, and if so, what is it that additional specs, say an audio spec, would say?
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Old 12th January 2005, 03:34 AM   #9
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Before you get too wound up, start by reading Walt Jung and Richard Marsh's capacitor selection articles. They were originally published in Audio magazine, but are available on the web. Try www.capacitors.com or Walt's website. He's a member here (WaltJ).
Yes, there are things that don't show up in the standard measurements. Jung & Marsh went a long ways towards proving what audiophiles already knew. There's more work to be done, but they laid a very strong foundation. That will go a long way towards clearing up some of the so-called 'myths' about capacitors.
The upshot is that caps make a tremendous difference in sound quality. The prices charged for some of the good caps are scandalous, but at least you can go in knowing what you're getting into. Do not let the sound quality question and the price issue get confused in your mind. They are two separate matters.
Once you've done your homework on capacitor selection, chosing values for DC blocking caps is a simple matter of plugging numbers into a simple formula and cranking the handle:

F=1/(2*PI*R*C)
where:
F= frequency
PI=3.14159
R=impedance in Ohms
C=capacitance in Farads (note that microfarads are 1e-6)

Most caps are 10% tolerance, so the difference between .22 and .25uF isn't going to be that big a problem. For DC blocking caps, bigger is usually better (low frequency response improves), although you will sometimes run across a circuit that acts funny if you put in too large a value. Don't fret about that, it's rare. Putting film bypasses on the power supply is a good idea, as electrolytics have lousy high frequency characteristics. Ignore those who scream that bypasses in power supplies are bad.
For that matter, ignore anyone who screams, period.

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Old 12th January 2005, 03:54 AM   #10
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Quote:
Originally posted by ransom peek
The datasheet has al ot to offer - does this only get us part of the way to understanding the audio quality, and if so, what is it that additional specs, say an audio spec, would say?
What does high ESR sound like?

My responses may not seem helpful to you, but my point is that either you trust your ears or you don't or you fall somewhere in between.

If "audio grade" caps seem like nonsense to you, don't buy them. If you're curious, investigate either by listening comparisons or by measuring. If you're willing to take someone's word for it, then ask people what they like to use and copy what they do.

You aren't going to convince anyone that they are delusional because they can't establish the existance of an "audio grade" cap through specifications, whether published or not. People have tried and nothing comes of it but arguing. I'm plenty guilty myself.

See the linked thread below for examples of useful measurements and pointless argument. Unfortunately, I have only contributed to the latter.

Carlos' snubberized Gainclone Power supply

Btw, I used to live in Ft Collins before I moved to Australia. Ft Collins is a brilliant place to live.
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