What's the best cap 'TYPE' for DC filter at line input. - diyAudio
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Old 29th September 2003, 10:47 AM   #1
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Default What's the best cap 'TYPE' for DC filter at line input.

What's the best cap 'TYPE' for DC filter at line input.

Load across cap is around 25 KOhm, max 25v, I want flat to 1Hz. I figure it's be around 47uf-100uf.

This will be the only cap in the amp, I hate distortions created by caps & I want to minimize the damage. I don't mind spending a bit on it.
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Old 30th September 2003, 09:15 AM   #2
liz is offline liz  United Kingdom
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Hi Brian,

By "type" do you mean brand or dielectric type?

There's an article on cap selection here that might be of interest to you:

http://www.capacitors.com/picking_ca...rs/pickcap.htm
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Old 30th September 2003, 01:59 PM   #3
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Thanks for the great link.

When I say cap type, I mean:

glass,
silver mica,
electrolytic, (general crap...)
epoxy,
ceramic,
different foil types,
ect...

Brand is another issue, however, th inner cap technology is the first big step.
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Old 30th September 2003, 04:23 PM   #4
sam9 is offline sam9  United States
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I vote for metal film. About the largest reasonably sized value is 10uF. That should will be sufficient to have minimal effect on LF frequency response. Personally, in this application I don.t feel too happy with any electrolytic regardless of hoe pricy or exotic.

You also say no other caps in your amp. Uh... does that mean no power suppply bypass caps? If you are looking for "sonic purity" you might want to reconsider. Unless of course you like the sound of ripple. Clever remarks aside, power supply bypass caps prtect your amp from induced humm (100-120Hz depending on geography) entereing the input section where it is then amplified. It also protects it against any HF or RF that has somewho found it's way into the rails.
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Old 30th September 2003, 04:52 PM   #5
liz is offline liz  United Kingdom
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In my experience this is always a slightly contentious issue - I know experienced engineers who claim cap type makes no significant difference, and others who won't use anything but PPS.

Perhaps the main thing is to try to understand what it is that creates the distortion you wish to avoid.
I'm afraid I don't know an enormous amount about this (and am therefore more than willing to stand corrected on any of the following points ) , but one factor may be voltage-dependent capacitance. Since an audio signal in the analogue domain is a constantly varying voltage, a cap which is particularly sensitive to changes in voltage will not be a good choice. PPS caps are often selected for use in the signal path because of their low sensitivity to voltage change, but I doubt you'll find one large enough to suit your requirement.

As far as I'm aware, electrolytics have decent performance in this area within the audio bandwidth (another consideration), so, in this case would be perfectly adequate, and I wouldn't think you'd need to spend a great deal either.

In any case, if you work out what's causing the distortion, it should just be a case of checking the manufacturers data sheets to make sure the part you choose has suitable characteristics. Then maybe just try a few parts out and see which one you like the sound of. It is, IMHO, an almost entirely subjective issue.

As far as this being the only cap in your design, if you do mean that you have no supply decoupling then I can only concurr with Sam9 that you probably should have (well I can't think of a good reason why not to).
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Old 30th September 2003, 05:03 PM   #6
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Quote:
Originally posted by sam9
You also say no other caps in your amp. Uh... does that mean no power suppply bypass caps? If you are looking for "sonic purity" you might want to reconsider. Unless of course you like the sound of ripple. Clever remarks aside, power supply bypass caps prtect your amp from induced humm (100-120Hz depending on geography) entereing the input section where it is then amplified. It also protects it against any HF or RF that has somewho found it's way into the rails.

I meant in the gain & drive section. Only resistors and mosfets. Yes, the powersupply has a bank of computer grade caps.
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Old 30th September 2003, 05:21 PM   #7
sam9 is offline sam9  United States
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OK. Whithout knowing you are the details of your amp I thought I'd mention that. One reads a lot of extreme sounding stuff about caps that doesn't consider what the intended function of a perticular cap is.

By the way, there is often/usually a cap involved in the VA section that manipulates open-loop feed back. Are you including one or does your amp avoid that?
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Old 30th September 2003, 05:48 PM   #8
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I think we all can agree on that modern caps today are fair or better. No type is really bad.

25 kohms + 1 uF polyester is "minimum" = 6,3 Hz

Almost every other type (non-ceramic) is better in some or every respect, except for size.
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Old 30th September 2003, 05:58 PM   #9
sam9 is offline sam9  United States
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Even ceramic is an appropriate choice for some applications, though not the one in question.
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Old 30th September 2003, 07:00 PM   #10
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Siler Mica's are real nice, but good luck finding a 10uf one that isn't $$$$$. I think they might go up to 0.01 uf.
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