Does removing the plastic covers on caps help?

Two sides to this story. Both of them have to do with capacitors ringing and / or being microphonic. Leaving the plastic on may reduce ringing and damp the level of microphonics that a cap is suffering from. Removing the plastic will raise the frequency of resonance, which can contribute to some interpreting the more pronounced high frequency oscillation as having "more air" or increased high frequency "detail".

Many "tweakers" that actually know what they are doing, use test equipment to verify what works and what doesn't, have access to accelerometers, etc... will damp or mass load capacitors to get away from the very problems mentioned above. This can tend to make the circuit sound "deader", so you've got to have things pretty well straight before attempting to do anything like this. As such, i would consider anyone that encourages such a tweak as increasing the amount of ringing or microphonics of a device as being both unskilled and resorting to "band-aids" on a product that is simply not working well to begin with. That is, unless they are taking steps above and beyond the outer plastic cover to damp capacitor vibrations.

If you doubt these comments, try looking in the AA "tweaks" archive as capacitor ringing and microphonics have been discussed there on several different occassions. Here's a real short thread about vibrating capacitors that covers some of this. Here's another post about mass loading capacitors that goes into detail posted by someone that seems to have done quite a bit of research on the subject. Obviously, you'll find different points of view on the subject with varying levels of expertise in the field.

As a side note, cap's tend to start to ring when their legs are excited. If you have a few caps laying around ( Solen's work great as an example ), hold the body and "flick" the longest lead that is attached to its' body. Chances are, you'll hear the cap literally "break into song". As such, one should strive to keep capacitor legs as short as possible and take steps to damp the legs and / or the vibration transfered into the cap body if one is concerned about "the ultimate in performance". The use of generic "Blu Tak" ( sp ??? ) wrapped around the leads at the point of entry into the cap body can help to decouple any external vibrations.

Speaking of capacitor legs, most of them are made out of a very low grade conductor as a general rule. Some of the better caps make use of copper leads that are covered in heat shrink to minimize both corrosion from exposure to air and to act as a "damper" via mass loading to keep the legs from ringing and transfering energy into the cap body. If you want to improve the caps that you have, you can simply cut off their tinned leads relatively close to the capacitor body and replace them with the wire of your choice. The key thing here is to make a solid electrical and physical connection between the wire and the remaining leg of the cap prior to applying any solder to the connection. One should not rely on the solder itself to make the connection as this may result in taking a step backwards sonically. Sean
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The comment above assumes that the plastic around an electrolytic capacitor is an integral part of the design and is used to damp vibrations. Given that 99% of capacitors found in high-end equipment are also used in computers, calculators, VCR's, etc, where "good sound" is not an issue, I really doubt this is true. Besides, how much engineering do you think goes into a run-of-the-mill $.15 capacitor?

The way I see it, the plastic shrink wrap serves two purposes, one, to act as a "billboard" to display the type and size of the capacitor and a clear way to deliniate the negative lead, and two, as an insulator to reduce the possibility of other leads in the circuit shorting against the aluminum can.

I have removed the plastic from capacitors in the PS of both amp and pre, as well as all plastic from the digital caps in my CD sources, and feel it is a worthwhile tweak. I attribute the increase in bloom and resolution a product of eliminating both the dielectric effect and a potential source of static charge (plastic).

For what its worth, all the capacitors in 47 Lab gear are "naked".

I agree with some of the points that you bring up Mbhcid. The plastic covering is more of an afterthought by most manufacturers to say the least and is primarily used as an insulator and as a means of identification. Having said that, that does not mean that it can't have benefits or drawbacks that may not have been originally taken into consideration. As such, i don't see a problem with removing the "case coverings" so long as the parts are easily identified in terms of values and there steps are taken to reduce the potential for something to short out against the body of the caps. Whether or not someone likes the sound difference that this "tweak" achieves is strictly a matter of personal taste.

As far as the reference to 47 Lab's gear, i've never heard any of it but i do know what it measures like. That is, they are very poor performers that are poorly designed and highly unstable. The signal that goes into these devices looks nothing like what comes out, meaning that there is a complete lack of accurate signal reproduction and / or large quantities of non-linear amplification taking place. Should someone like the sound that these products produce, so be it. I'm not about to say that someone can or can't like something. Using them as a point of reference for "quality" audio reproduction is something all-together different though. Sean
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Really not worth getting into here, but you should audition some 47 Labs gear when you get a chance - It will make you realize "measured performance" has absolutely nothing to do with musicality.

Keep an open mind and don't shortchange yourself.