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  • 4 ohm speaker?

    I was wondering if a 3 way with a 8ohm woofer 4ohm mid and a 8ohm tweeter would be ok. I have a Yamaha RXV2500 the manual states use 8ohm or 6ohm min speaker for the surrounds.

  • #2
    The crossover handles that. The 4ohm mid will likely need padded down to meet the woofer, increasing it's impedance.
    Electronics engineer, woofer enthusiast, and musician.
    Wogg Music
    Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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    • #3
      You are correct to question using a low impedance speaker, but do not confuse an individual driver impedance with the system impedance of a completed speaker system. And yes, most AVRs are not designed for the higher current of a low impedance speaker. Many claim they get away with it, but people jump out of perfectly good airplanes too and usually it is OK. Not always.

      1Sounds like you want to do your first build and already selected drivers before understanding the science. Might I suggest buying the book PE sells on designing speakers as the first step? It wil save you a lot of trouble and expense. Unless you get hooked on speaker building as a hobby as it is not cheap.

      Second, surround effects are of limited bandwidth and big 3-ways are usually not needed. Usually a nice 6 and 1" 2-way is even overkill. When all this started I fell for some fancy di-polar surrounds, but eventually wised up. I use a set of in-wall Bostons now. Not much fidelity needed for a bird tweet, passing gunshot etc.

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      • #4
        If you are planning to use a generic off the shelf crossover, it is a crapshoot on multiple levels. Impedance could be fine, or it could cause super low impedance drops and/or extreme phase angles that are current hungry. My first three way before I knew how to design speakers was all 8 ohm drivers with a generic crossover. It would push the receiver into protect when driven hard. Likely one of those problems above that I didn’t understand at the time.

        Not discouraging you by any means, just encouraging you to dive right in and learn the ropes! There are lots of great people here that will help you, or even design a proper crossover for you if you let them know your drivers.

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        • #5
          +1 on off the shelf crossovers. It's an easy solution, but doesn't usually work well. You can patch things up a bit using Zobel networks on each driver to flatten impedance, if you just have to go that way.
          Last edited by fpitas; 12-06-2020, 10:43 AM.
          Francis

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          • #6
            Thanks, I have the drivers and a 4ohm wmtmw or wwmtm crossover designed and working on a 8ohm wwmt crossover for the woofer mids and tweeter. Was thinking I should go with the 8ohm design. Problem is the woofer ww are 90db (4+4=8ohm) and the single mid (8ohm) is 86db sensitivity a pair of mids (4ohm) would be 92db. The speaker will be up against the wall 1 inch away the box will be aprox 12-15 in deep will It have to much bass gain from being close to the wall? Currently tweeking the mains crossover sound good been up and playing for 2 weeks now 4ohm version. Sorry I've had small surround before never again.

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            • #7
              What drivers are you using?

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              • #8
                Yes a Zobel can make a driver more even in impedance as well as help tame aluminum cones, but now you have to measure the impedance to start with to select the correct cap and resistor.

                129, You are WAY ahead of yourself. OK, you have some drivers. What are they? You have some prebuilt crossovers, yet no idea what drivers they were designed for?

                You can't just mix and match drivers without a lot more understanding. Again, do yourself a favor and buy the PE book on designing and building loudspeakers. It wil save you a lot of frustration and money.

                Yes, you have to deal with baffle step and room gain as well as acoustic offset, diffraction, on/off axis response, near source reflections, and a host of other parameters.

                If you had small surrounds that were not sufficient, I suggest it was the choice of them. It is useless to put a 30 Hz woofer in a surround when there is no content below 80. But some 3 inch WalMart surrounds might not have been able to push enough air @ 100 Hz, where a proper 5 or 6 easily can exceed 110 dB. That's the threshold of pain BTW. It is just physics. Life is easier if you understand them. I am not against overkill, but within a reason. My rear surrounds are Polk in-ceiling. I can hear a jet fly over the room just fine. The bass rumble that sounds like it is from the rear is of course properly handled by the sub. Maybe there is some movie where you sit in the middle of an orchestra, I don't know. That might need higher quality surrounds.

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                • #9
                  Originally posted by dynamo View Post
                  If you are planning to use a generic off the shelf crossover, it is a crapshoot on multiple levels. ....
                  +1. And your unlikely to find an off-the-shelf unit designed for a 8-4-8 ohm three-way.

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                  • #10
                    Originally posted by 129Decibel View Post
                    Thanks, I have the drivers and a 4ohm wmtmw or wwmtm crossover designed and working on a 8ohm wwmt crossover for the woofer mids and tweeter. Was thinking I should go with the 8ohm design. Problem is the woofer ww are 90db (4+4=8ohm) and the single mid (8ohm) is 86db sensitivity a pair of mids (4ohm) would be 92db. The speaker will be up against the wall 1 inch away the box will be aprox 12-15 in deep will It have to much bass gain from being close to the wall? Currently tweeking the mains crossover sound good been up and playing for 2 weeks now 4ohm version. Sorry I've had small surround before never again.
                    I assume your woofers are 90db each, because as you appear to understand, two drivers wired in series results in the same final spl, just with more power handling. Parallel results in +6db.

                    90db woofer with an 86db mid tight to the wall will probably be muddy and bass heavy. If the goal was for the speaker to be out in the room a bit it may be perfect.

                    90db woofer with 92db mid close to the wall might be about right or even a little light on bass. Placed out into the room it would be light on bass. With a generic crossover I would strive for as much sensitivity as possible on the mid and tweeter, then use an L Pad to dial them down to match.

                    If this seems confusing, look up baffle step compensation.


                    If you are set on using a generic crossover or generic calculator (we all started there too) here are some tips for success:

                    Three ways are way harder to get right than two ways even with measurement gear and proper design software, so I’d encourage two way.

                    Get drivers with as flat of an impedance curve as possible (these usually have a low Le number as well if you can’t see a curve; these are usually but not always more expensive. Planar and Amt drivers impedance is completely flat like a generic crossover assumes - true ribbons with transformers are not and I would avoid these with a generic crossover as their load is nearly a dead short at low frequencies).

                    If your drivers do not have flat impedance I would recommend a zobel. This will flatten impedance. This is usually not necessary on a tweeter as their coil is so small the inductance is negligible. Remember the impedance on drivers with a voice coil will always have an impedance peak at the resonance frequency and you are looking to flatten it above there.

                    A general rule of thumb is to use a crossover point at least twice the frequency of the resonant frequency (fs). With a generic crossover I would strive for 3x or better while still staying as low as possible (so choose drivers with a low fs.

                    Choose drivers with broad flat frequency responses, as this is assumed by the generic crossover. Avoid drivers with metal cones as they will have sharp breakup in the response that will sound irritating without attenuation. Soft poly cones are often easy to work with.

                    Keep the drivers spaces as closely together as possible as the generic crossover does not account for spacing. This is a good rule anyway for proper integration, relating to driver centers distance, speed of sound, and frequency.

                    use a stepped baffle to align the voice coils as the generic crossover cannot account for the phase difference for different distances.

                    employ generously rounded baffle edges to minimize diffraction.

                    I would personally use an L Pad on the mid and tweeter to dial in the levels of those drivers while still maintaining stable impedance; so get higher sensitivity mids and tweeter and then turn them down to dial them in. If you don’t want to leave the adjustable controls on there you can measure the resistance of the final adjustments and replace it with resistors.

                    you may need to play with flipping the polarity of the mid and/or tweeter to get it to sound right.

                    Build these as you are planning keeping these tips for success in mind. Pick up a copy of Speaker Building 201, ask questions, and down the road you can design your own custom crossover.


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