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Post sound amplification electronic crossover/devider

  1. Mar 25, 2017 #1
    Hi Guys,

    Im new here and dont know much about electronics so please bare with me as I try to explain what I would like to know.

    If you have a classic design speaker it has a passive crossover inside consisting of mainly capacitors and inductors changing the frequency (or limiting) to send low HZ to the woofer and High Hz to the tweeter, thus protecting the drivers by limiting the frequencies the have to re produce.
    When you have an active crossover it changes the frequencies even before it gets amplifier and then the amplifier only amplifies a specific frequency range then send it to the speaker.

    NEXT: (active post amplifier crossover)
    So from your amplifier you will connect the speaker wire to the electronic crossover which will then electronically divide the frequencies in a much more stable and effective way into a 2, 3 4 way split which can then be sent top the drivers inside the cabinet ?

    Why has no one built an POST Amplifier electronic corssover? Will it be to big, heavy, expensive?

    Example: Source--->Amplifier--->speaker wire--->Electronic crossover---->speaker
  2. jcsd
  3. Mar 25, 2017 #2


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    Welcome to the PF.

    After your active post amplifier crossover you need multiple speaker wires going to multiple speakers, right? One pair for each tweeter, one pair for each woofer, and one pair for each midrange speaker. That's practical in some situations, but not if you are using speakers with all 3 components in the same cabinet, I would think.
  4. Mar 25, 2017 #3
    Thank you,
    Yes but in a passive crossover you also need separate wires going from the crossover to the individual drivers.
    What I am trying to figure out is why has no one made an active crossover POST amplification. Surely amplified sound can still be controlled electronically?
  5. Mar 25, 2017 #4


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    AFAIK, the passive crossovers are in the speaker cabinet, so it's no big deal to run separate wires to the different speaker elements in that cabinet. An Active Crossover needs to be a power amplifier too, right?
  6. Mar 25, 2017 #5
    Sorry I dont want to be rude but I dont know how to get the question across correctly.
    Pre amplification active crossovers is NOT what I am asking.
    POST amplification is the question. How difficult will it be to build an active (or electric) crossover after the sound has already passed through the amplifier and reaches the speaker. So instead of having a passive crossover inside the speaker, you have an electronic one getting it power from the wall socket where the speaker wires connect to and then electronically divide the frequencies. Replacing a passive crossover inside the speaker box with an active crossover.
    Will it be too complicated or expensive or why has no done it?
  7. Mar 25, 2017 #6


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    I don't understand what a post-amplification active crossover circuit is supposed to do. You are already at high power after the amp stage, so it doesn't make sense to attenuate that, filter it, and re-amplify the output signals. Sorry if I'm not understanding what you are asking about.

    Maybe you could Upload a sketch of what you are thinking? Include the power level at each stage to help make it more clear...?
  8. Mar 25, 2017 #7
    You partially answered me now thanks.
    "You are already at high power after the amp stage, so it doesn't make sense to attenuate that, filter it, and re-amplify the output signals"
    So managing high power signal after amp stage will be too difficult? I thought it would be an easy process to filter it without needing to attenuate and re-amplify. I thought maybe there would be a way to filter it while in high power.
    So filtering it while it is only 1w power vs filtering it while 200w power is becaquse filtering high power sound is too difficult or equipment needed to heavy and expensive?
  9. Mar 25, 2017 #8


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    It's more of just getting things in the right order. You want the high power amplification to be the last active thing you do before the speakers. Using a passive crossover circuit between the amp and speakers doesn't really waste much power. But trying to put an active circuit there would mean that you have to add the power back in again, so it takes 2x the power instead of 1x.

    Active circuits generally take in lower-level signals and output higher power signals (broadly speaking).
  10. Mar 25, 2017 #9
    Yes, you are 100% correct, It would just have been so much nicer to be able to tweak the high power. Crossovers are very temperamental and fine tuning it with frequency crossover points, driver responses etc, it is a very long hard process and then you still end up with spikes etc.. Never really being able to produce perfect sound. Crossing over before amplification via active crossovers or software works brilliantly but costs go up because now you need 2 amplifiers for a 2 way system and 3 for a 3 way etc..
  11. Mar 25, 2017 #10
    This is the problem. A post-amplification crossover filter needs to work at high power. You can do that with a passive filter, given suitably rated passive components, and the power is passed through the crossover on to the relevant drivers (i.e. woofer, mid-range, tweeter).

    Active filter circuitry, typically based around op-amps or DSP circuits, works at low power (milliwatts). It isn't capable of passing through the power from the power amplifier to the relevant drivers. Instead it must sample a few milliwatts, either throwing the rest away or not drawing it from the power amplifier due to a high input impedance. After filtering each frequency channel must be separately re-amplified.

    Hence any practical active filter crossover goes before the power amplification stage, where the power levels are already at the milliwatt level (= line level) and are suitable for driving your active crossover. You can subsequently drive your power amplifier stages without having to throw away your expensively obtained power.
    Last edited: Mar 25, 2017
  12. Mar 25, 2017 #11
    It really isn't clear what you are suggesting.

    A passive crossover (the usual kind, inside the speaker, one amplifier, usually), is a "high power crossover". You seem to want to have these cross-overs be powered, but that means they are amplifiers, right? There are many advantages to designing and constructing a cross-over at low power levels. Then amplify, and the amplifier isolates the cross-over from the speakers.

    While the amplifiers add some expense, there are other advantages. The higher frequency amps don't need as much power as the bass amp, and you don't get as much inter-modulation distortion, as the signals are separated by the cross-over up front.

    There are also "bi-amp" designs. Separate amplifiers with the same signal applied, but each amp driving a crossover and driver separately - no connection between the drivers or cross-overs. Not as much advantage, but I think some speakers can be modified easily for this, and some people feel they get some advantage. Oh, also "bi-wired". Similar, but just one amp, and just separate wires to each cross-over/driver.
  13. Mar 26, 2017 #12
    Thank you all for your replies!

    @ NTL2009. I cant make myself more clear as to what I am asking, I dont know how..
    Understanding 100% how a passive crossover works and understanding 100% how an active crossover works I was wondering why they dont apply active crossovers after the amplifying has already happened.
    In other words using an active crossover POST amplification instead of PRE amplification.

    But as everyone has pointed out, filtering already amplified sound will involve expensive components and will not be feasible.
  14. Mar 26, 2017 #13
    But actually, the passive crossovers in the speakers are (relatively) expensive components ( R, L & C), compared to those components at the signal level stage.

    I think that your gap here is due to you not yet understanding some of the technology - and that's OK, we learn by asking and thinking why things are as they are, and why they are not done differently.

    I think what you are picturing is an active cross-over that doesn't provide signal amplification (that amplification would be supplied by the 'standard' single full-signal amplifier). But even if not making the signal larger in amplitude, to have any effect beyond just cross-over components as they are, this "active cross-over" would need to "buffer" the signal. This is also a form of amplification, it could be amplifying currents at the different frequencies, to match impedance between cross-over and driver ( driver = individual speaker in a system). An "active crossover" like that, at the driver, would need active components (semi-conductors or tubes) that "buffer" the signal, and would need to carry the full volts and amps that your original amplifier would. You would essentially be replicating the output stage of the amplifier, and that is the most expensive part of it. And you'd still have these high power R,L & C, so there just is no real advantage.

    Using your example:

    Source--->Amplifier--->speaker wire--->Electronic crossover---->speaker [ But how does this 'work' in practice? ]
    A more practical possible implementation would be:
    Source-->line level wires--->Amplifier (without high current output stage, mostly increase VOLTAGE only)--->
    --->Electronic crossover (with high current amplifier output stage)---->
    --->speaker (driver is a better term for each unit in a multi-way speaker system)

    Powered speakers are really the practical implementation of this idea. And they use a single "line level" or "signal level" input, with a pre-amp, low-level cross-over, and then a power amp for each band (2 or 3 way usually). Your concept might combine some of the signal amplification of those amps, but those are usually tuned with feedback from the output stage, so even that may create problems, or require work-arounds.

    For a mechanical analogy, it's a little like saying instead of a single transmission, and a drive shaft to each wheel, we would put a transmission at each wheel. It just isn't a practical consideration (an electric motor at each wheel is different - usually no transmission required, and wires are easier to route than mechanical energy).
  15. Mar 26, 2017 #14


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    This may help a bit:
    There is a natural order to the way things are arranged. Active filtering doesn't generally occur at power levels. These things are done at the small signal stage.
    Class D amplifiers are generally more inexpensive and efficient so it has made sense to power subwoofers with a class D amplifier and everything else with a plain old class B since class D works well for low frequencies. This is where active filtering makes sense. Bass takes a lot of power in music compared to the higher frequencies so we active filter the bass our from the higher frequencies and feed separate amplifiers. The bass amplifier (class D) is now the only amplifier that has to carry the heaviest load and it can do it efficiently.
    My home theater system has a subwoofer output on the amplifier. This is line level. It is not capable of driving a speaker directly. This output comes from an active filter (crossover) internal to the main amplifier. The main speakers are a tweet and two midranges. They are driven by standard class B output stages and the speaker cabinets themselves have passive crossovers. 85 watts per channel is a great plenty at midrange and high frequencies. In the setup menu there is a setting for either large or small front (main) speakers. This switches the internal active crossover to allow low frequencies to the main speakers when the user selects large main speakers. My subwoofer is what they call an active subwoofer. It has a class D amplifier that is part of the cabinet. Don't recall how many watts but a lot more than a whimpy 85. 85 watts for a sub would be inadequate in my opinion.
  16. Mar 26, 2017 #15

    jim hardy

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    Look up the cost of an operational amplifier with just the few milliamps of drive capability that you might use for the active filter in a pre-amplification crossover.
    TI's wonderful OPA1641 is capable of 20 milliamps (maybe 30 of you push it) and costs three or four bucks in single quantity at Mouser.
    http://www.ti.com/product/OPA1641?keyMatch=audio opamp&tisearch=Search-EN-Everything

    A three band preamp crossover will take maybe twelve dollars worth of those opamps.

    To drive a speaker requires a lot more current.
    Last high power opamps i bought were LM12's, good for 6 AMPS not milliamps, cost around seventy five bucks apiece and you'd want to bridge connect them two per speaker .. So a three way post amp crossover would take maybe four hundred fifty dollars worth of opamps at 1995 prices.

    My old workhorse LM12 is obsolete now , cheapest i could find online was over three hundred bucks apiece.
    High power linears are still expensive, multiply any of these prices by 6..


    But, there are some audiophiles who would love it. You just might find a market.
  17. Mar 27, 2017 #16
    Thank you all I have a much better understanding now!
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