electro vs film?

[daz]

What is the advantage of using caps such as a mallory 150 or such over electrolytic for cathode bypass in the preamp? I realize you can't get non electros in sizes that electros come in, but thats a moot point because i just talking about situations where you DO have the choice. Such is my case where i use .68uf on some cathodes and have both. I can't seem to say for sure whether i hears a diff, but as i recall reading there supposedly IS a difference of some kind, no?

[R.G.]

What is the advantage of using caps such as a mallory 150 or such over electrolytic for cathode bypass in the preamp? I realize you can't get non electros in sizes that electros come in, but thats a moot point because i just talking about situations where you DO have the choice. Such is my case where i use .68uf on some cathodes and have both. I can't seem to say for sure whether i hears a diff, but as i recall reading there supposedly IS a difference of some kind, no?

Let's start with the fact that no component is perfect. While we think that a resistor is a resistor, a capacitor is a capacitor, and an inductor is an inductor, every single part you pick up will contain all of these qualities. A resistor is primarily a resistor, but if you measure carefully, it also has some capacitance across it and some inductance. Likewise an inductor has resistance and self capacitance.

When we get to electrolytic capacitors, we find that the manufacturers have had to make a number of compromises to get so much capacitance into a small can. Compared to a film capacitor of equal capacitance an electro *in general* has higher Equivalent Series Resistance (ESR), Equivalent Series Inductance (ESL), and much more leakage of DC through it. In addition, electrolytics to a bigger degree than some other capacitors have odd nonlinear effects in their insulators, as well as sharing a quality called Dielectric Absorption.

There are special ways to minimize these effects, and that can account for some of the special types of electros available. But in general, an electro is a less perfect capacitor than a film cap of equal capacitance.

So does that make an audible difference? That's a strong "maybe". It depends on the circuit it's in, what the circuit does, probably how much amplification takes place after the cap to boost up the funny, small effects, and very much on your ears and brain listening to it.

This has become a faith with the strength of a religion to many hifi tweekos - they believe that, in effect, it would be better to be deaf than to have to listen to music that's passed through even one electro cap, ever. However, it takes some very critical listening in a perfect environment to hear differences, I think.

The problem with "film is better" is that the tweekos don't stop with film being better. Polypropylene is "better" than mylar, and teflon is "better" than polypro, and glass dielectric is "better"... you see what happens. On top of that, humans tend to hear what they think they're going to hear. And if you *think* that replacing electros with mylar caps will make things sound better, it almost certainly will. Same for polypro over mylar, then teflon, then...

The amps we love the sound of used crude, early electros. That's a powerful argument for "electros are good enough". But follow your ears, not internet rumor.

[daz]

Sounds like i heard what i thought i heard...nothing ! Then again, i suppose if the cap allows DC thru you in effect have less gain in the frequencies the the cap passes since theres some extra gain in the full bandwith that extra DC would cause. No? Well, no matter. I guess it's all moot since the effect is likely not worth of consideration.

[Chuck H]

Sounds like i heard what i thought i heard...nothing ! Then again, i suppose if the cap allows DC thru you in effect have less gain in the frequencies the the cap passes since theres some extra gain in the full bandwith that extra DC would cause. No? Well, no matter. I guess it's all moot since the effect is likely not worth of consideration.

No, it's not really worth consideration. But if you do choose to use aluminum caps I suggest using the best quality you can get. Nichicon makes some great "long life" caps. Poor quality caps CAN change character and performance with age. Well, all caps will. But you really do want to minimize this. I use film or dry tantalum (the good quality bullet shaped ones) for preamp bypass caps because they perform well and last many times longer than electro's. I probably won't ever have to deal with any issues due to these parts in my lifetime. If I used alum/ electro's I'd be looking at the need to suspect them after only ten or fifteen years. A surprisingly short time if you don't like unnecessary soldering. Take it from a guy who's had to rebuild his old builds. It happens.

And I DO think I hear a difference too

Chuck

[Steve Conner]

If you have the choice, I'd always use non-electrolytic. Just because electrolytics have a finite life, so the less of them you use in your product, the more reliable it'll be.

Douglas Self did a lot of work on electrolytic capacitor distortion. It's remarkably hard to keep the things out of your signal path. He found that you could sometimes measure distortion in the 0.1% range, if you allowed AC signal voltage to appear across an electrolytic. So his advice was not to use them in tone filters and so on, only for coupling and bypassing. And an undersized cathode bypass cap is no longer just a bypass cap, it's a tone filter too.

But that's for hi-fi. In guitar applications we eat 0.1% THD for breakfast.