The bypass cap

December 4, 2022
 by Paul McGowan

Capacitors are interesting components. They serve as frequency dividers, energy storage units, and DC blockers (among other things).

I’ve written before about my experience so many years ago learning about capacitor bypassing—adding a smaller capacitor in parallel with a bigger one to improve the sound quality of an amp or preamp.

Why is that better and what does it do?

Without getting technical, let me offer instead a simple analogy. A super tweeter. Every tweeter (like every capacitor) has its frequency limitations. Audio signals passing through or as generated by the device go only so high and then they can go no higher. Add a super tweeter (or a small bypass capacitor) in addition to the main tweeter (or capacitor) and suddenly the overall high-frequency response is significantly improved.

We add small high-frequency capable capacitors in parallel with slower bandwidth-limited capacitors for the same reasons we add super tweeters.

More high frequencies.

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29 comments on “The bypass cap”

        1. With a tin hat?

          I googled bypass capacitors. Apparently they get rid of AC ripple to leave clean DC. When I were a lad we used to get rid of raspberry ripple and leave a clean bowl.

          I prefer strawberries, but raspberries are an alternative currant. Boom, Boom!

  1. I have a tendency to see humour where there isn’t meant to be any. Take the first line of today’s post for example, “Capacitors are interesting components.” In a survey of 100 people I wonder how many would agree with that? I know we’re talking electronics here but all the posts so far have been about head wear, fruit and ice cream. Long may it continue. 🙂

    1. Google only came up with one joke about capacitors, the punchline being “it was charged with battery”. You can add the first line to taste.

      No one has mentioned flat caps yet, but Naim have made a FlatCap power supply for decades. I presume that is the closest you will ever get to a humourism about electronics or audio.

      My only post was to see if Fat Rat was awake or hung over from drowning his sorrows after the socceroos failed to beat the Argies. I prefer Uruguay because of Luis Suarez, who to make ends meet had an early career as a smuggler, head-butted a referee and tried to eat Giorgio Chiellini. When they failed to get the vital goal the other day he burst into tears. What a guy.

  2. Given the formula of calculating impedance for strictly capacitive reactance and the propensity to add a smaller value bypass one in parallel, wouldn’t it make sense to use a ‘shit load’ more lower value caps (descending in value), in parallel? Is there a ratio if one did?
    For example 1milli farad total needed. – 500uf 250uF 125uf and so on to get to the desired total value. (Space and cost restrictions set aside for now)

    I wonder if the rest wear beanies when the preferred choice of cap is not available 😉 ✌️ 😀

  3. ‘Top of the morning to you all’ (as I tip my cap by-passing all the previous responders). Most of this morning’s comments are not resonating well with me so I have a suggestion to make. Connect an inductor (coil) across the capacitor and ‘tune in’ or ‘tank out’. Didn’t Timothy Leary say that?

    Ahhh…now today’s post ‘resonates well’ with me.

    How did I do with this circuit analysis Mike??

      1. What Timothy Leary actually said, was “Tune In,Turn On and Drop Out”. Many years later, we now know that he was right about using LSD, not for dropping out but for what medical science is using this hallucinogenic to actually cure all kinds of psychiatric disorders forever.

        Quite amazing isn’t it?

        1. Good comment, Stimp. Particularly PTSD. And there is plenty of that to be treated with all the wars and violence we’ve been experiencing in the past few decades. Music can’t cure all of it.

          1. It’s really amazing how many different psychological issues can be cured with hallucinogenic drugs including psychedelic Mushrooms. Careful attention from a consulting Psychiatrist or psychologist who is in the room with the patient sometimes holding their hand while they are undergoing treatment is essential for anyone with severe issues.

            There’s an excellent documentary, “Being Cary Grant“, who was actually panic stricken over having a relationship with women that he finally wound up taking this treatment for six months, I believe at Harvard Medical, and after completing wound up marrying Dyan Cannon and fathering a child with her.

            1. Great story, Stimp. There are also centers arising around this country and Central America where people go to take treatment under the guidance of “trained professionals” or at least people with a lot of experience in this field. Micro-dosing for ADD is also gaining popularity, I’ve read. (Not speaking from experience.) Music and exercise work fine for me. 😎

              1. I do the same exact things as you are doing. Exercise and focus, while performing my day-to-day tasks and concentrating on my breathing and not being in a rush have made a huge difference in my emotional life. I’ve also learned many tricks concerning short-term memory, the best one is the eight second rule in which, after laying down your cell phone, glasses, or other objects, and not remembering where you left them demonstrates that this problem has nothing to do with Alzheimer’s or dementia if everything else is OK with your brain. All you need to do when you place an object down is to keep looking at it for 8 to 10 seconds without thinking anything except for the picture in your mind of where you left it and you can come back an hour or more later and remember exactly where the item is. Works just about every time for me. It’s all about staying in focus and not letting your unconscious thoughts take over your life.

                And yes, Microdosing is the new way to treat these conditions if they’re not overwhelming, your life.

  4. Should I assume that the same rationale (more high frequencies) applies to adding bypass caps to a loudspeaker crossover, or are there other benefits as well?

  5. Sounds like this crew could have written comedy skits for the old Carol Burnett show on the CBS network. Paul could have been the head idea generator. You missed your calling Paul. And from what I recall of Stan when I shared a hotel room with him at CES, maybe he could have been the straight man. And Terri could have stood by as the CBS network senser (sp?) to let Paul know when he was going too far!

    – Jeffrey in Philadelphia

    1. Doubtful… I was close friends with Carol for 13 years and she wouldn’t get involved with any of this second rate stuff. She had her hands full with Tim Conway and Harvey Korman for going off script whenever they wanted.

  6. “We add small high-frequency capable capacitors in parallel with slower bandwidth-limited capacitors for the same reasons we add super tweeters.”
    Not being an electrical engineer, I have to ask if faster low-frequency capacitors could be added to tighten up the bass since adding a subwoofer does for the bass what adding a super tweeter does for the treble. Or is it too late in the afternoon to ask such a question?

  7. Bypass capacitors in my experience are used as local energy storage devices at electronic components that require stable DC voltage at their power supply pins. When an electronic component output changes state (for example a data converter, digital buffer, etc.) this causes the Vcc (power rail) to be disturbed. Having the local capacitor right at the device minimizes this disturbance and removes the inductance of the PC board traces between the power supply and the device.

    Smaller value capacitors tend to have higher self-resonance (lower parasitic inductance) so they are closer to “ideal” capacitors and function as intended at higher frequencies as Paul mentions above.

  8. I’m late to this discussion, and with all the replies, an important detail was not mentioned.

    1. Tweeters are basically voltage-driven, and at the mercy of full-voltage power rails to present immediate and dynamic transients (want the DC rails to have zero ripple voltage).
    2. Woofers are basically current-driven, and will simply “take what they need” from the power supply and can certainly cause voltage sag.
    3. See point #1.

    A heavy drum hit requires both; if the woofer is starving the power supply, the high frequencies suffer the most.

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