Bottlehead Kits > Smack

The usual question: cap "upgrades" (real or imagined)

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adamct:
I recently (well, relatively recently) picked up a Smack kit from someone who never got around to building his. I will build it in stock form first, but knowing me, I will want to roll in some film caps fairly soon after I have it up and running.

I see from the parts list that the Smack has the following caps:

2x 0.18uf 450V metallized polypropylene caps
2x 10uf 250V metallized polypropylene caps (Solen)
3x 220uf 250V electrolytic caps
2x 10,000uf 6.3V electrolytic caps
1x 10,000uf 10V electrolytic cap

Which of these are potential "upgrade" (i.e., replacement) or bypass (using a film cap with 1% of the capacitance) candidates?

For the 0.18uf / 450V caps, I haven't been able to find any replacements. Only 0.15uf, 0.2uf or 2.2uf. Don't know if any of those would work.

For the 10uf / 250V caps, I've found the following candidates:

corndog71:
While I'm not familiar with the Smack I can make a reasoned assumption that the best place for upgrades would be the .18uF and 10uF caps.

I therefore recommend Clarity ESA series in 0.1uF/630V and 10uF/250V. 

If you want to really supercharge it then consider the 220uF/700V Clarity TC caps but others might say it's not worth it.  Or you could just bypass the current ones with 0.1uF/250V film caps.  (2.2uF seems kinda big for a bypass in my opinion.)

adamct:
Yeah, I saw the 220uf/700V ClarityCaps, but at $90/each, and given that I would need 3 of them, that just isn't going to happen. I thought the rule of thumb was that you would bypass a high capacitance cap with one that has 1% of it's capacitance? That's how I came up with 2.2uf.

As for the .18uf, am I wrong in thinking that a .2uf would be a better choice than 0.1uf? Or at least 0.15uf?

Paul Joppa:
2x 0.18uf 450V metallized polypropylene caps - these are in the shunt regulator stability bypass RC network, and may have some effect. It will be very small though! They must have good high-frequency behavior, as in VHF radio frequencies - so forget mylar. In polypropylene, look for film and foil, or anything labelled "snubber". AC line rated ceramics may be a good choice.

2x 10uf 250V metallized polypropylene caps (Solen) - these are the parafeed output caps, and by far the most important sonically. Look in the power amp sub-forums to see what others have used in that function. Whatever you try, remember that breakin is especially audible with these caps, it will take usually 50-200 hours of actual, loud, music before you really know what they sound like.

3x 220uf 250V electrolytic caps - these are in the high voltage power supply, isolated from the audio by two C4Ss and a shunt regulator, and not going to be directly audible. There are two areas for possible improvement - you can look for caps of higher temperature rating (105 degrees C) and higher voltage (up to 450v) which will help the caps live longer, and you can bypass them in the hope that it will reduce switching noise at radio frequencies. For the latter, snubbers and AC line rated ceramics are good candidates.

*** for switching noise, consider replacing the UF4007 rectifiers with Cree silicon carbide Schottky diodes, 600v or greater; that will likely make more difference than bypassing the filter caps. ***

2x 10,000uf 6.3V electrolytic caps - these are "soft-start" time delay caps; their AC behavior is shunted by the LEDs so they are unlikely to have much sonic effect. Also, the location is pretty tight; the extended wires necessary to use any alternative would probably do more harm than any possible good.

1x 10,000uf 10V electrolytic cap - this is the heater power filter, not connected to any audio area. It already has Schottky rectifiers, so the least unlikely improvement would be a bypass for high frequencies. Low voltage ceramics in Z5U material would be a good candidate. I'd want 10uF at least to have any possible effect, and note that these caps lose a lot of their capacitance with DC voltage on them - use 16v or 25v parts, and you'll probably want the 22uF or greater nominal capacitance.

adamct:
Paul,

Thank you for the quick, and VERY detailed response. Would it be a fair summary to say the the most worthwhile changes would be (i) the 2x 10uf 250V metallized polypropylene caps and (ii) the substitution of the Cree silicon carbide Schottky diodes for the UF4007 rectifiers?

It sounds like changing anything else is unlikely to have a significant sonic impact.

Best regards,
Adam

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