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Function of Bypass film caps

S

Soundphile

Active Member
Hi All,
I have seen 1uF electro caps being bypassed with very small value film caps. If I understand correctly, this is done to overcome the weaknesses of electro caps as the 1uF electro hinders high frequency audio signals.
However, If that electro cap is replaced with an equivalent value film cap (WIMA one similar), does it mean that the bypass cap can be completely discarded altogether?
Thanks in advance for your comments and advise.
Regards. Robert.
 
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no..
the small cap.. be it 0.1.m 0.01., 0.001 ect.. is used to filter out the high frequency "noise"., for which the 1uf cap can not do...
 
In this case where I believe the OP is asking about signal path electrolytics bypassed with a polystyrene capacitor (for example) in which case the answer is 'yes' - as the additional capacitor is placed there to combat the slight distortion introduced by the electrolytic, which is not present with a film capacitor replacement.

However, in the case of non-signal path situations, like in a power supply, where the electrolytic could be a much higher value (and also the film cap a bit larger), the answer could very well be 'no'.
 
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A 1uf film cap will filter anything a .001 uf can filter and more.

Just have more capacitance reserve.
 
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Thanks for your comments and advice.
I'm actually referring to an AU217, Board F-2760, Cap C01 (1uF 50V) is bypassed by a Mylar cap (0.047uF).
Since this is the audio path, please confirm that the Mylar film cap can be completely discarded ... And the same to be done to the other (Right) channel?
Thanks again.
Robert.
 
I'm not an expert, but first would be more concerned to know if there would create issues replacing the E.C. by a film cap.

Maybe it's ok, in this case, you can disregard the small value cap, or add it, and see if you see any improvement.
 
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I doubt that there would ever be an issue replacing an electrolytic with a film cap, electrolytic capacitors in audio circuitry are a necessary evil. :D
 
Hi John,
Why not replace Tantalums with polypropylene or Mylar instead? Why EC, even if it's Low Leakage?
Cheers,
Robert.
 
Soundphile said:
Hi John,
Why not replace Tantalums with polypropylene or Mylar instead? Why EC, even if it's Low Leakage?
Cheers,
Robert.
Click to expand...

No, I didn't word that very well, let me restate:-

I replace ALL tantalums, when I do this I am EVEN prepared to fit an EC in it's place, obviously I use other kinds of capacitor when appropriate.
 
Soundphile said:
Thanks for your comments and advice.
I'm actually referring to an AU217, Board F-2760, Cap C01 (1uF 50V) is bypassed by a Mylar cap (0.047uF).
Since this is the audio path, please confirm that the Mylar film cap can be completely discarded ... And the same to be done to the other (Right) channel?
Thanks again.
Robert.
Click to expand...
if you remove the mylar capacitor will lose detail of the sound
 
Tantalums used to provide a very large capacitance in a small package, but today's aluminum electrolytics can often do just as well or better. Unless tantalums are properly designed in (derated and current limited), and very often they weren't, an aluminum electrolytic is almost always a better choice. I'll use tantalums in test equipment where I want high reliability and to never rebuild it again, but I get rid of them in audio equipment whenever I find them.

As for film bypasses across aluminum electrolytics, they just aren't supported by math, measurements or design theory. Fortunately, they rarely do harm, since trying to convince anybody one way or the other is probably a fools errand.
 
Hi Adelo,
In my Post # 6, I should have also said that on the board F-2760, C01 and C02 (1uF 50v Electros) have been replaced with similar value WIMA's. If retained, the low value bypass Mylar films would effectively be bypassing the WIMA films !! The 1uF WIMA's can do all and more of the low value Mylars. Post # 4 from Hyperion explains it clearly.
Cheers.
John.
 
I no longer by pass electros with film. I even yanked my 2.2 bypasses off my 999's filter caps. Why? - did a lot of reading, especially on DIYAudio.com, here one of the most dedicated audio buff's "Eva" did an extensive test scheme where the alleged improvements simply did not materialize. To the contrary, you add inductance with the often long leads associated with film caps. Anyway, after removal I could not hear any difference whatsoever. As Conrad says, they probably don't hurt - but they don't help either. As always a listening test is the ultimate decider, even if it is just placebo - as long as it sounds better, not worse, to you. Whether caps 30-40 years ago were in need I don know ?

Now replacing an electro with a film is a different matter.
 
When I first joined AK a couple of years ago many advised to bypass. I no longer bypass my electrolytics with film caps having never noticed a positive effect (and in some cases, such as my superthrough output coupling caps on AU-555A and similar, I swear the effect is negative).
 
Bypassing signal path electrolytics with a film capacitor seems like a daft thing to do, as the larger bypass capacitors simply add somewhere for noise to be picked up. If you are going to do anything, providing the capacitor is less than about 4.7µF - change the electrolytic for a film, one capacitor, but above all it must be a similar size to the original EC, this is now achievable with modern components.

I still bypass PSU electrolytics with a film capacitor (1-2.2µF), but having also absorbed some of Conrad's vast knowledge on the subject I am considering discontinuing this practise.

BTW: I only get rid of Tantalum capacitors in my audio equipment, I consider them the perfect choice for filtering power lines in digital circuits for example.
 
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