input caps - time to ask the experts

[Altaic]

I've looked high and low for info on input coupling caps (google, diyaudio, etc.). I'm doing Elliott's (ESP) 3A project (the cap in question is C1, just to make clear), and the best info I could find was right there on the linked construction guide.

Well, this is what I gathered: It should be "large" between 1uF and 10uF, polypropylene or polysterene, although polyester is okay. So I'm thinking, input doesn't need to be more than a few volts, right? I mean on the official page, it said the bootstrap cap (C7) has to be 35V, but the rest of the polarized ones can be whatever is on hand. That is a bit non-specific, no? I guess if I knew what I was doing I'd know what is sensable. So I browse around the construction guide, and contrary to his own recommendations, Mr. Thornblade used a 2uF 400V oil/paper cap. So I have been looking through cap catalogs and such, and have found some 400VDC (275VAC) metalized polypropylene caps for ~$13 each.

But, I have a hunch it doesn't need to be more than a few volts, so if I get a reasonable cap, say 50VDC, I can get a better poly-film/foil cap for much less $$$.

So, what I'm asking is: what is the optimum voltage for an input coupling cap? And I'm guessing a 10uF input cap would be best.

Hmm, now that I'm thinking about it, it doesn't seem to give the recomended voltages for the nonpolarized caps either. I'd guess they should be >=35V (rail voltage), but I might be wrong.

[tiroth]

The cap size determines the low-frequency cutoff, so in that sense larger is better.

The only reasons I used the 400V oil/paper were

a) idiosyncrasy
b) DC blocker (150V) for my tube input stage

You can't go wrong with a PP film cap of 50-100V, although you are correct about the voltage rating not really mattering. Higher voltage caps just tend to be a little more linear (sometimes ^_^) Since film caps often start at 50V ratings, you can just use that rating or better for all your caps and have no worries.

[Altaic]

Thanks a bunch, tiroth! =)

[JensRasmussen]

Hello,

The cap C4 needs to be rated at: rails * 2 + a bít.

Since the entire voltageswing will be on it !

In your case go for a 100 V cap

\Jens

[NU_NRG]

Why should we use polyester or poly... polypropylene? Any reason we should use these and not eletrolytic ?


-Simon

[Altaic]

Polys are more stable than electrolytics, and emit less electromagnetic/electrostatic radiation.

[JensRasmussen]

In an eletrolytic cap there is (because of the way it's made) some inductance. This can (at the right/wrong freq depending how you see it) cause large not wanted effects in a curcuit.

An other effect is that eletrolytic caps are slower, and have a DC leaking current running through them.

A poly cap is closer to the "perfect" cap, if only they came bigger !

\Jens

[NU_NRG]

Can we mount these "little" caps outside the board with/out having noise problems?

[tiroth]

NU_NRG,

If you are talking about the bypass caps, they should really be installed as close as possible to the board. mounting them any distance away increases loop size (hum!) and series inductance (filtering efficiency, stability). These are your high-frequency bypass, so keep them close by.

Sorry for overlooking the miller cap voltage ratings. I'll add cap voltage notations to my site this weekend.

[Altaic]

Any idea why a 100V 10uF metalized PP cap is more expensive than a 200V or even 400V or the same capacitance? And is material (PP vs polyester) more important than type (metalized vs. film/foil)? 10uF film/foil PP caps are pretty hard to find. Also, is 10% cap tolerance for an input cap okay? all the 5% ones are too small. I guess I could look for oil/paper since they are 1%.