Question about Capacitors and Inductors in Tweeter Crossovers

Discussion in 'Speakers' started by WilsonCreek, Aug 27, 2010.

  1. WilsonCreek

    WilsonCreek Active Member

    I have what is probably a stupid question regarding capacitors and inductors in tweeter crossover circuits, but an internet search is getting me nowhere, so I thought I’d ask. As I understand it the capacitor, in series with the tweeter, essentially inhibits the flow of increasingly lower frequencies through it, resulting in a 6 dB/octave first order high pass filter. This can be refined to a 12 dB/octave second order filter by the inclusion of an inductor in parallel with the tweeter, which provides a less resistive path than the tweeter's voice coil for the lower frequencies at and below the intended crossover point.

    My question is: Why do first order crossovers seem to always use only a capacitor in series with the tweeter, and never only an inductor in parallel with it? Is it because the amp would see very low impedance at the lower frequencies, and/or because the inductor would essentially short circuit the woofer (I’m sure there is a better way to word that)? If so, why couldn’t a well-chosen resistor in the tweeter/inductor pathway compensate for this? I would be grateful if anyone has the time to help me understand.
  2. dc270

    dc270 Now Off The Reservation..... Subscriber

    Let me try to offer some help for you- I am not completely clear to your question but here is my interpritation.

    You must understand what each component is designed to do. A capacitor is designed to block/ restrict certain frequencies- depending on its particular rated value(s)(voltage/ capacitance/tolerance.) An inductor by nature is designed to block/ restrict certain frequencies- depending on its particular rated value(s) (voltage/inductance/ toleerance) A resistor is designed to a given voltage (not frequency dependant) depending on ohm value(s) (tolerance and voltage ratings being variable.

    The trick here is the combination of the components and their rated values. Once you understand what those values are and how they interact with with certain frequencies then you will understand what they do in combination (and not) with one another.

    You use inductors. caps & resistors to tame frequencies and power products in a network for a given driver result. Tweeters are usually tamed with a simple capacitor to respond to certain frequencies, further & finer tamed with inductors and more capacitors and even resistors. Woofers are tamed with inductors and further tamed with more inductora, caps and sometimes resistors. Mid range driver with combination of the both.

    Hope this helps.....
  3. WilsonCreek

    WilsonCreek Active Member


    Thank you sincerely for taking the time to reply. I think I understand, and maybe my question was too verbose. Let me try to clarify and condense my question. I understand that capacitors limit the low frequencies that pass through them, and that inductors limit the high frequencies that pass through them. So, a capacitor in series with a tweeter inhibits low frequencies, and the tweeter sees only the highs, with a 6 dB/octave slope around the crossover point.

    An inductor in parallel with a tweeter (second order crossover) doesn't inhibit low frequencies, but as I understand it, it does offer a less resistive path than the voice coil of the tweeter, up to the point (frequency) where the resistance to flow is impeded, and this also follows a 6 dB/octave slope, in the opposite direction.

    By combining both, a second order crossover achieves a 12 dB/octave slope, partly by inhibiting lows from the tweeter and partly by "shunting" them (probably the wrong word) away from it. My question is why first order crossovers always use the capacitor-in-series configuration to inhibit lows, and never an inductor-in-parallel configuration to shunt them, instead?
  4. Sir.Byrd

    Sir.Byrd Lunatic Member

    Maybe its cost? Who knows, not I. :dunno:
  5. Copa1934

    Copa1934 My ears are bleeding Subscriber

    I sure hope someone has a good answer to this as it seems like a good question, or it's simply over my head.
  6. jwalker

    jwalker Well-Known Member

    You are on the right track. There are a couple of reasons for a series cap used for tweeter networks.

    For a simple 1st order filter, you can use an inductor in parallel with the tweeter for a high-pass response. But as you mentioned at low frequencies the amplifier will be asked to source way too much current in the rejected band because the impedance will be the resistive loss through the inductor at low frequencies. This is because audio amplifiers used to drive speakers dynamic voice coil speakers have by nature a LOW output impedance.

    However if you have a high impedance amplifier (like a constant current output) your current will be limited and a shunt inductor can work as a high pass filter.

    The second reason for wanting the high-out of band impedance in the rejected band is because speaker crossover networks by necessity are diplexing filters. Basically, what's out of band for the tweeter is in-band for the woofer/midrange/subwoofer. A high input impedance for the tweeter network basically allows the low impedance of the low-pass filter response to grab all the power it wants without losing it in the high-pass filter portion.

    We can delve deeper, but what I explained is the basic reasoning.
  7. WilsonCreek

    WilsonCreek Active Member


    Thank you for the time and explanation. I will chew over what you wrote, which is about over my head, but I think I have the general gist of it. I do have a couple questions.

    If I understand, it isn't clear to me why a resistor in series with the tweeter/inductor pair wouldn't resolve this, since the amp would see a load (let's say 5 ohms if it was a 5 ohm resistor) at the lower frequencies, and higher than that from the resistor and tweeter in combination at frequencies above those where the resistance/impedance/inductive reactance of the inductor exceeds that of the tweeter's VC.

    Again if I understand this (and I'm not sure I do), it seems that a suitable resistor chosen to work with the LF circuit could compensate, but if not could this be resolved by bi-amping the speakers? Obviously not a typical configuration, but it seems like there could be some sonic advantage to a capacitor-free (albeit with resistor) circuit for the tweeter.

    Thanks again for the time and information/explanation.
  8. jwalker

    jwalker Well-Known Member

    If you put a resistance is series with the inductor, the effective resistance will rob power and negatively affect the ability of the high pass circuit to attenuate the undesired frequencies. "de-Q'ing" a inductor like this won't help the filter response.

    You will also be throwing away power in that resistor. Even though the load is more benign from an amplifier standpoint, the loss in that resistance is relatively large with respect to the power you want delivered to the low pass section.

    The low pass section of the crossover will still see that in-band load in the tweeter circuit and much of the power meant for the woofer will be robbed by the added resistance.

    Let's consider your 5-ohm resistor example. Adding a 5ohm resistor to that shunt inductor will effectively screw up the ability for the high pass network to reject the low frequency energy. At low frequencies you will basically have a 5ohm resistor in parallel with your 8ohm tweeter. Now lets consider the low pass leg, where you have a 8ohm in-band load for the woofer. All of the sudden you will have a effective load that will be 8ohms in parallel with a 5ohm resistor in parallel with your 8ohm tweeter. You can easily see your tweeter will see something around a quarter(!) of the total power (power which is NOT desired). The tweeter will likely die from over excursion. In addition something like a half of the desired low frequency power will not reach your woofer. A half lost in the 5ohm resistor AND tweeter (for as long as it lasts). Bad...

    It appears that your intent is to somehow eliminate the capacitor from the tweeter circuit, believing that a shunt inductor is less harmful (not really true I would say).

    Biamping with a shunt inductor in series with a resistor will probably destroy your tweeter since it is no longer anything close to an ideal high-pass filter. I wouldn't try that.

    What you do want to do is build an active crossover so that all filtering is done at line level. Then you can eliminate all the passive crossover parts and any perceived and real problems those have!
    Last edited: Aug 28, 2010
  9. WilsonCreek

    WilsonCreek Active Member


    Thank you very much for the time to write out a lot of explanation, I appreciate it. I am pretty sure I follow everything, although I will read it over a few more times. But, to be clear, when I wrote the resistor in series with the tweeter/inductor pair, I meant the resistor comes before the paralleled tweeter and inductor. I did not mean that the resistor is only in series with the inductor, and that the resistor and inductor are together in parallel with the tweeter. I think I understand the general repercussions of the latter, and that it would not be a good way of accomplishing what I asked about, for all the reasons you mentioned. With the resistor in series with both the tweeter and inductor, though, the tweeter would seem to be protected from low frequencies by the inductor, and the amp protected by the resistor. I though of bi-amping as a possibility if there is not some impedance/resistance/driver efficiency combination that would permit a woofer and tweeter to work in this configuration.

    Probably, but really this was more of a theoretical question. With all of the possibilities that seem to be explored, I wondered why it seems that this one never is.

    Thanks again.
  10. jwalker

    jwalker Well-Known Member

    The reason you don't see what you're proposing is because it's highly inefficient and electrically has severe impacts on the entire crossover design. There really isn't any new ground being broken here. What we're talking about here has many problems and disadvantages that are easily identified. And I'm certain this sort of approach was determined to be fundamentally unsound in the earliest days of AC circuit design (early 20th century).

    We really need to look at the forest here instead of focusing on the trees.

    Adding a series resistor to the shunt L/tweeter high-pass will rob power. What you will be making is essentially a voltage divider. You will lose power in that series resistor at ALL frequencies. This is not a good approach since wasting power in a resistor is never a good thing when compared to an effectively lossless capacitor. Of course if your tweeter happens to be super efficient, you might not care. But we have to consider the entire crossover....

    Which means the problem with the out of band impedance is still present. A working multiplexing filter (i.e., crossover) really wants a high out of band impedance for each of the filter legs to work right. There's no way around this. Besides wasting power, having a maximum out of band impedance of only 5ohms in a highpass leg in parallel with a low pass leg of 8ohms will effectively reduce your overall impedance to something smaller than a lot of amplifiers will like. This isn't good.

    If we consider a series 10uF cap as a 1st order highpass for a tweeter and look at the magnitude of the impedance vector at 100Hz, it's something like 160ohms, a good solid impedance for the low pass leg to work against. What's being proposed here is substituting that 160ohms with a 5ohm equivalent load... This will screw things up pretty bad and make a reasonable crossover design REALLY hard!
  11. WilsonCreek

    WilsonCreek Active Member

    Again, thank you jwalker for the explanation. I think I am beating a dead horse, but in case there’s any confusion here are a couple pictures. The first is a typical 2 way, first order crossover, sometimes there is also a resistor in the pathway but I didn’t add one. The second is what I wondered about as an alternative. In my question, I wasn’t trying to break new ground, I just wondered what it was that I must not understand to explain why the latter is never experimented with, as far as I knew.



    If I understand your answer, the problem is that the resistor will:

    1) Bleed power away from the woofer, even at low frequencies where the impedance of the woofer is less than the resistance of the resistor and associated low frequency impedance of the inductor, because power will inevitably flow through the resistor and inductor even at those frequencies—the ones where that combination nevertheless offers greater resistance than the voice coil of the woofer does. By the way, my 5 ohms for the resistor was just an example to illustrate the point, of course I assumed the resistor would have to offer greater resistance than the low frequency impedance of the woofer. In any case, if the first sentence is correct than I understand this, although it still isn’t clear to me why a bi-amped solution would not resolve that, since one amp only sees a woofer and the other only a more-or-less stable X ohm resistance at frequencies below the inductor’s crossover, and increasing resistance from the resistor and tweeter VC above those frequencies.

    And the resistor will also:

    2) Waste power that could otherwise flow to the tweeter, by reducing power at all frequencies rather than only those for which the tweeter is not intended to function. I understand this, but many speakers have resistors and/or L-pads/pots in series with the tweeter, so this didn’t seem to me to be a dramatic problem.

    I wasn’t trying to question fundamental electrical concepts, and had no illusion that I was breaking any new ground, I just wondered why the inductor-based first order crossover is never experimented with. I get the feeling, though, that I’m beating (or maybe we’ve beaten) a dead horse. Thanks again for the time you took to help me understand.
  12. bluesky

    bluesky Addicted Member

    Thanks DC and everyone: Very nice, simple, concise, and easy-to-understand write-up. I was wondering about this. Very interesting thread!!!!!!
    Last edited: Aug 28, 2010
  13. dc270

    dc270 Now Off The Reservation..... Subscriber

    Guess my many years as a technical trainer for new hire emplyees in the CATV industry paid off!! I had to write manuals, etc for them to understand the working of CATV plant, etc. I was glad it hopefully work here- with the help I got as well!
    Thankss all
  14. spkrdtr

    spkrdtr spkrfixer Subscriber

    Another good reason to have a cap in series with the tweeter, which I don't think was brought up yet, is to protect the tweeter from DC surges from the amp. This is a rare occurance but does happen. The plate system within capacitors won't allow DC current to pass thru downstream. When it does - goodby tweeter.
  15. jwalker

    jwalker Well-Known Member

    You need to consider the practical perspectives of this tweeter circuit? How big exactly do you think the resistor needs to be? 100 ohms? 50ohms? You can easily calculate what kind of tweeter attenuation you can see. Do we need really want to drive 100watts into the circuit you're describing to get 5 watts just into the tweeter?

    The biamp solution you're proposing has it's flaws too. Presenting a stable load is one thing, but the amplifier will still need to drive the low frequency load at at least full power just to get the same power to the tweeter. This is defeating one of the main advantages of passive biamping. That advantage is that you want the amplifier to only drive the frequencies it needs to and not be burdened with low a impedance out of band. Remember that in the passive biamp case, the amplifier has a FULL spectrum input.

    Using a capacitively coupled tweeter effectively presents a high impedance at low frequencies which means very little power needs to be delivered at those frequencies. This is why it's done and it works.

    Trying to change a capacitor to a resistor/inductor combination to eliminate any perceived flaws in the capacitor is basically a solution that is worse than the original "problem" and biamping doesn't really help considering the added burden that's imposed on the driving amplifier.

    Like I mentioned earlier, what you really want is an active crossover. You can take care of all the "problems" easily.
  16. WilsonCreek

    WilsonCreek Active Member

    Thank you for another aspect of the question.

    Thanks again, I understand this now. Recap to follow.
  17. WilsonCreek

    WilsonCreek Active Member

    Okay, I would like to try and recap what I understand to be an answer, although I’m sure there is more to it and this is a simplistic understanding. jwalker, I have reread your posts several times and appreciate your thoughts and information, and hope you will forgive my slightly optimistic take on your advice.

    First, there is the protection that a cap provides to a tweeter, in the unlikely event of a DC surge. Allowing for that, here is my recap:

    Following my second figure in a post a few above, a two-way speaker with a first order crossover that uses only an inductor in parallel with a tweeter (no resistor) and a woofer will not work. The tweeter will reproduce highs, but the low frequencies intended for the woofer will flow through the less resistive inductor instead of the more resistive woofer voice coil. Moreover, the amp will see too little impedance at low frequencies.

    Because of this a resistor is necessary on the tweeter circuit, before the paralleled tweeter and inductor, as per the above diagram. This will work but there are significant shortcomings. The woofer and resistor/inductor will share low frequency power if the resistance of the resistor/inductor is relatively close to the impedance of the woofer at the low frequencies, even if it (the resistor) is the more resistive of the two. Accommodating this would probably require an especially efficient woofer, and would nevertheless be inefficient because of low frequency losses through the resistor/inductor. On the other hand, a stronger resistor on the tweeter circuit, in order to limit low frequency losses through it, will significantly attenuate power to the tweeter and probably degrade sound quality.

    A bi-amped set-up with the same resistor, tweeter, and inductor configuration will also work, but again there are shortcomings. Because the amp driving the tweeter is essentially also driving a nonexistent woofer that the amp sees as a relatively stable low frequency draw, the limited-frequency-range-over-which-the-tweeter-amp-drives-the-tweeter advantage to bi-amping is lost, both sonically and from a power perspective.

    Moreover, it has also occurred to me although it wasn’t mentioned that the inductor may not offer the tweeter adequate protection from all low frequencies to the extent that a capacitor does, in which case the tweeter would see some damaging low frequency power even though the inductor offers less resistance. If this is the case it would represent a significant shortcoming to the concept and bi-amping isn’t going to save it (the tweeter or concept), regardless of the inherent problems, inefficiencies, and loss of advantages.

    On the other hand, it is conceptually interesting that a variable resistor on the tweeter would alter the power to both the tweeter and woofer, in opposite directions. As resistance is decreased the tweeter will receive more power and the woofer less (because of increasing loss through the inductor), and as resistance is increased the woofer will receive more power while the tweeter less.

    Probably much to jwalker’s chagrin (sorry) I might experiment a little, just to satisfy myself that I understand these concepts. I have no expectation that it will result in anything useful, and I won’t use a tweeter that I care about.
  18. drhifi

    drhifi Retired Dr. of Fidelity

    you must limit the low frequecies capability of reaching the high frquency voice coil

    the tweeters voice coil cannot handle the power of the low frequency signal. without using a high pass filter in series with the tweeter the voice coil would be promptly "cooked" using only a coil in parrallel as a "shunt" as you stated would not limit the low frequency signal arriving at the tweeter voice coil.
  19. drhifi

    drhifi Retired Dr. of Fidelity

  20. benb

    benb Well-Known Member

    Yes, you could use a resistor in series with the tweeter, and an inductor in parallel. One problem is the resistor would also reduce the high frequencies. That's not a killer, but what else it does is.

    Iet's say it's an 8 ohm tweeter. You could use an 8 ohm resistor in series. This would reduce the voltage to the tweeter by 1/2, and result in a 6dB drop in output from the tweeter. This isn't horrible, as tweeters are generally a little more efficient than woofers, but usually not by 6dB - so the tweeter will have less output than it should relative to the woofer.

    The circuit will have a 16 ohm impedance for the tweeter crossover circuit for high frequencies (8 ohm resistor in series with 8 ohm tweeter), which is just fine with the amplifier. But with the inductor across the tweeter, below the crossover frequency the total impedance will drop to 8 ohms, and it will all be converted to heat by the resistor. But also, a woofer needs to be driven, and that's also 8 ohms in parallel with the tweeter's crossover circuit, so the total impedance will be 4 ohms below the crossover frequency. This is an unnecessary load on the amplifier.

    With the standard circuit of a capacitor in series with the tweeter, the impedance of the tweeter circuit goes UP below the crossover frequency, presenting less of a load to the amplifier, and all of the low-frequency power can go to the woofer.

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