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  1. #1
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    Default What does a bypass capacitor accomplish?

    What does a bypass capacitor, used in a crossover, accomplish? I have heard this term but really do not know how or where they are supposed to be used.

  2. #2
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    Default Re: What does a bypass capacitor accomplish?

    IMHO, they could be used in parallel with another cap to get to the target value. Otherwise, if you're talking about using a nice film & foil type (as an example) with a poly or NPE to try and improve the SQ, all it will do is relieve your wallet of more money. The bypasses are usually only 1% or less of the total capacitance value, so they really contribute nothing AFA a better SQ goes.

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  3. #3
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    Default Re: What does a bypass capacitor accomplish?

    Quote Originally Posted by brianpowers27 View Post
    What does a bypass capacitor, used in a crossover, accomplish? I have heard this term but really do not know how or where they are supposed to be used.
    A bypass capacitor can mean several things. Bypass capacitors are the same as filter capacitors in power supplies or the term is used interchangeably. Capacitors that are added to Vcc and Vee pins of ICs are called bypass caps. So a bypass can be anything that passes AC either in series or to ground. In this case I think you are referring to bypassing a large value poly cap or NPE cap in a crossover with a smaller value capacitor with better high frequency characteristics and lower ESR.

    In theory the lower ESR and better high frequency characteristics of the bypass cap will improve the operation. Is it audible well that depends and is likely in the realm of subjective even though it can probably be measured... I know people who swear by them and others who think they do very little. Personally I think bypassing an NPE makes sense but most of the better Poly caps don't really require bypassing at least not for crossover duty. Keep in mind that when you parallel a bypass cap you also add that value to the value of the capacitor you are bypassing. In most cases if you keep the bypass cap value very small it won't matter.

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  4. #4
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    Default Re: What does a bypass capacitor accomplish?

    If I am hearing you correctly... Some people choose to "bypass" every capacitor in the design with a low value cap in paralell as part of the overall capcitance in the design. This is to provide some value to the overall sound quality. I still feel uncertain about what this is supposed to do...

    How does this differ from using a single, same value, capacitor in a design?

    Do people also bypass the speaker itself?

  5. #5
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    Default Re: What does a bypass capacitor accomplish?

    IT IS unknown if a mundorf 1uf cap

    http://www.madisound.com/catalog/pro...roducts_id=103

    for 73 dollars
    and a dayton 3uf cap for 3 dollars total 76 dollars and 4uf
    http://www.partsexpress.com/pe/pshow...Number=027-220

    is better then a mundorf 3.9uf cap total 133 dollars

    http://www.madisound.com/catalog/pro...oducts_id=1632 the tought is to get high quality and save a few bucks. if you are using a 1uf and a 100uf you are wasting money. but a 1uf and a 3uf may be worth doing. My opinon is buy better drivers.

  6. #6
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    Default Re: What does a bypass capacitor accomplish?

    I could not word a response better than Daryl's, so here is a quote from his response to a similar question as yours.

    While you are just starting out the most important thing you can learn is that the audio community is overrun with idoliters.

    Capacitor bypassing in crossover filter circuits is just another pagan god.

    Bypassing has it's roots in power supply rails for equipment where low impedance is required through the radio frequency range or at some distance from the main capacitor bank where the inductance of the supply line necessitates additional local capacitance.

    At radio frequencies a large capacitor becomes an inductor or at least the supply line will if the large capacitor isn't right near by a critical point for an active circuit.

    A small capacitor in such an aplication will become the controlling element providing a low impedance point for your circuitry.

    At audio frequencies for the purpose of a speaker crossover all capacitors are always in their range and behaving like capacitors.

    Even the infamous non-polar electrolytic capacitor performs far beyond the capability of the best drivers.

    The difference between film capacitors and non-polar electrolytics is measurably dramatic but when you build them into the speaker the performance is dominated by the characteristics of the drivers and the capacitors become insignifigant.

    A bypass capacitor in the limited audio range simply contributes to the circuit in the same ratio in which it is implemented.

    That is if your bypass capacitance is 1% of the total for the main and the bypass then the bypass capacitors characteristics are contirbuting 1% to the circuit and the main capacitor is contributing 99% of it's characteristics.

    So if you use a non-polar electrolytic capacitor for your main capacitor and even though it's performance is beyond that of the best drivers in the world you decide to bypass it anyway with 1% of the total capactance coming from some type of film capacitor, then the difference will simply be a mere 1% of the way along the line between the performance of the non-polar electrolytic and the performance of the film (99% of the performance being determined by the non-polar electrolytic).

    Should you increase your bypass to 10% of the total then you simply have 90% of the capacitor performance being determined by the main capacitor.

    In a crossover filter a bypass capacitor always is a minority element while the main capacitor has near total control of the circuit.

    Another misunderstanding in audio is that crossover componets do not influence the circuit most at high frequencies.

    You've heard it said a million times that in the top octave is where you really hear the clarity or sweetness or what ever BS with a certain capacitor.

    Truth is that crossover componets have their greatest influence near the corner frequency of the filter created with them (crossover frequency) where their impedance is comparable to that of the circuit.

    As you get further from the a filter's corner frequency the impedance of crossover componets will go very high or very low and their influence over the circuit becomes less and less (the componets effectively are removing themselves from the circuit as you get further from the filters corner frequency).

    One of the biggest lie's in audio is the film and foil capacitor.

    Film and foil capacitors are claimed to be better because their ESR (equivilant series resistance) is lower.

    However the impedance curve for capacitors looks like a 'V'.

    It starts very high at low frequency and falls steadily until it reaches the capacitors self-resonance frequency (usually several hundered kilohertz depending upon the value of the capacitor) then the impedance climbs above the self-resonance frequency due to parasitic inductance (mostly due to lead length and the necessary length of those leads due to the size of the capacitor).

    ESR is only an issue in the vicinity of the self resonance frequency, for the rest of the spectrum the capacitors impedance is dominated by it's capacitance and parasitic inductance.

    Film capacitors typicly have very low ESR and as they approach their self-resonance frequency their impedance will usually deviate on the low side of what a perfect capacitor should be and then abruptly turn into an inductor.

    The problem with film an foil types is that their ESR is even lower and their deviation from perfect as they approach their self-resonance frequency is even greater than that of a metalized film capacitor.

    Not only that but film and foil capacitors are huge and because of their sheer size and the necessary lead length result in higher inductance and even more deviation from a perfect capacitor as compared to a metalized film type.

    Bypassing capacitors in speaker crossovers is never of any use save one.

    The capacitor bypassing issue is one of the fastest ways to find out that you are talking to or reading from a fraud to whom you never need pay attention to anything they say even though they could surely talk for hours about their cascading stupidity.
    I could not agree more, nor word it any better.

    -David

  7. #7
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    Default Re: What does a bypass capacitor accomplish?

    It sounds like this type of method could only be useful for applications hwere the frequencies in question are hundreds of times higher than the normal audio spectrum.

    Well put.

  8. #8
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    Default Re: What does a bypass capacitor accomplish?

    Quote Originally Posted by brianpowers27 View Post
    It sounds like this type of method could only be useful for applications hwere the frequencies in question are hundreds of times higher than the normal audio spectrum.

    Well put.
    You may have a worthwhile result in those instances when the high pass requires a very high value cap and the cost of a poly is prohibitive. In those cases you can use an electrolytic for the bulk of the value, 90% or so, with a parallel poly for the remainder, and in theory get a result that's better than an electrolytic alone. Whether you agree or disagree with the theory is up to you. OTOH bypassing a 10 uF poly with a 1uF poly is only a feelgood measure.
    Last edited by billfitzmaurice; 09-26-2008 at 12:03 PM.

  9. #9
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    Default Re: What does a bypass capacitor accomplish?

    I'll give you an experience I had with a "bypass" cap in a speaker application. I installed Magnepan MG12 spec X-O's on my Maggie MMG speakers. The factory high-pass network was comprised of one 33uF cap with a 0.1uF bypass. I used a Jantzen 33uF and a Dayton 0.1uF film and foil. The speakers sounded better than the original MMG X-O, but keep in mind, Magnepans are very revealing speakers. A couple of months later, I got some Theta Audiocaps (.01uF) and swaped out the Dayton film & foils. The improvement I got was with high freq sounds like cymbals, snares, high guitar notes and such. These high freq sounds were now clearer and cleaner sounding. Swaping the caps back and forth proved to my ears there was an improvement. Not a huge improvement, but noticeable none the less.

    This was not on some mega-buck aw-de-o-file setup either. Just a Sony disc player, Onkyo preamp and a EP1500 amp

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