[river251]

In freeway terms, a bypass is a section that you use so you do not have to use some other section. Like using the 35 bypass so you can avoid the main 35 freeway that goes through downtown Austin traffic. But I get the feeling that when you bypass a cap, both are still involved.

So when you bypass a cap, what does the bypassed cap do, and what does the bypassing cap do?

Thanks,
Jim

[sts9fan]

http://sound.westhost.com/articles/coupling-caps.htm

[JoshK]

The analogy you use for freeways actually fits.....higher frequency (fast speed) gets a uncrowded lane (no impedance) versus the slower traffic (lower frequency). 

The problem is caps of larger value have more reactance (inductance) which impede HFs.  So the bypass cap is there to give them an alternate lane. 

The best explanation I've found comes from Morgan Jones' Valve Amplifiers.

[Steve]

http://sound.westhost.com/articles/coupling-caps.htm

I am not sure that link is accurate, unless electrolytic capacitors have changed. There are inductance problems with fairly large values, as low as 2khz. I think the link below is valuable, see fig 6.

http://waltjung.org/PDFs/Picking_Capacitors_1.pdf


Cheers.

[Speedskater]

Which part of the Rod Elliott paper do you have problems with?

[Steve]

Which part of the Rod Elliott paper do you have problems with?

This quote.

Electrolytic capacitors are definitely a problem though - there is any amount of proof ... Or is there ... ? Again, often claims are made based on tests that are irrelevant for audio. A popular myth is that electros have considerable inductance because of the way the foil is wound inside the can. This is nonsense - the foils are usually joined at the ends in the much the same way as with film caps. High frequency performance usually extends to several MHz [2], even with standard off-the-shelf electros and bipolar (non-polarised electrolytic) caps.

Compare to fig. 6 of Walter Jung/Richard Marsh's article "Picking Capacitors".
Have the electrolytics changed that much?

Cheers.

[DaveC113]

http://sound.westhost.com/articles/coupling-caps.htm

While there is some technical expertise in this article, it's the last place I'd go for good info about caps in modern amplifiers.

Stuff like this:

Connecting small caps in parallel with high value electrolytic caps usually achieves nothing at all.

...may be true at the screen of an oscilloscope, but subjectively, this can make very large differences in sound quality and does remove a lot of distortion, just not distortion that is easily measurable.

Of course, the best design will not have electrolytics anywhere near the signal path. Anytime you can avoid using them is preferred, IMO.