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Passive crossover design

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studio45 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2017 at 2:06pm
I think I'm getting a clue here....This isn't for a soundsystem, it's a scale model of some machine or other, and you're putting a very small set of speakers into it because you want it to make the same noises as the full-sized version?

If that's right, then this might be much simpler. You probably don't need high-fidelity speakers to reproduce machinery clanking or engine noise. Run that bass-mid driver full-range, and connect up the tweeter with a 1uF cap in series. Add a 4 or 8 ohm resistor before the cap if that makes it too bright. That will start bringing the tweeter in at about 15kHz assuming it's 8 ohms, which might seem high - but the response shape of most tweeters is far from flat and the effective acoustic crossover point will be much lower.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Modeller Quote  Post ReplyReply Direct Link To This Post Posted: 25 April 2017 at 4:36pm
That is correct Studio45, the sounds are industrial (engine and other noises) but what I have found on the previous model is that the bass driver has too much to do and colours the top end especially. What I plan to do is single out bass notes up to about 8kHz and have the mid/highs handle everything else.

The amp I am using is 3W at 4-8Ohm. The driver selection will depend on the final enclosure design which I will run by you here later, but I only have about 40mm width and no more than 60mm length. I'm looking at the height now, but will probably be in the order of 30mm or so.

The sound unit will incorporate 3 small speakers (sub, 2x mono mid/high) from which I hope to design a good below 200Hz bass response, lower if possible at approx 70-80Db, which I think is achievable. I managed below 250Hz with a pair of 34mm round full-range speakers in a reflex enclosure, so with a 40x60mm sub in a similar enclosure I hope to achieve better results.

At the moment I'm looking at a compact transmission line design, but I've not finally decided yet. I really want to crack the crossover and driver selection before I start on the enclosure.

Thanks again
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2017 at 8:22am
You sound like you have a pretty good idea of what you want. Conventionally the xover is the last part of the system to be built and installed but as you are at a premium with space I see why you are thinking about it now.

The best bet is probably keeping it simple. The bass driver already has a modest roll off due to its natural inductance. If you measure its impedance and do some simple maths you could double the roll off slope with an appropriately sized series capacitor.

Your problem is going to be that steeper filters for good sub/high separation require more passive components and complex design. Is it possible to apply active filtering before an amplifier? That might solve a lot of your problems and with surface mount parts an active xover could be made very small.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Modeller Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2017 at 1:33pm
In an ideal world an active crossover would be great, but I have run out of power sources already because of moving parts, lights, etc. In fact, I need more! I'm afraid it has to be passive.

I'll work on a selection of drivers over the next few days and post them here.

Thanks
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2017 at 2:45pm
Really? An active processing board would only take mA of power for a few op amps. I find it hard to believe that cannot be spared unless you are on a super tight power budget? If low power was a concern you could probably get that down to microamps with the right op amps. Stereo amplifier for your sub/top split and off you go.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote studio45 Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2017 at 3:41pm
Originally posted by odc04r<br><br>The best bet is probably keeping it simple. The bass driver already has a modest roll off due to its natural inductance. If you measure its impedance and do some simple maths you could double the roll off slope with an appropriately sized series capacitor<br><br>[/QUOTE odc04r

The best bet is probably keeping it simple. The bass driver already has a modest roll off due to its natural inductance. If you measure its impedance and do some simple maths you could double the roll off slope with an appropriately sized series capacitor

[/QUOTE wrote:



You would want a series inductor, not a cap. If you wanted to make a second-order filter, the cap would go to ground *after* the inductor. If you just use a cap to ground, the impedance will drop to near zero at high frequencies, which will make your amp unhappy

You would want a series inductor, not a cap. If you wanted to make a second-order filter, the cap would go to ground *after* the inductor. If you just use a cap to ground, the impedance will drop to near zero at high frequencies, which will make your amp unhappy and suck all the drive signal away from the tweeter.

So you only have to deal with a few watts. Perhaps the passive is not such a terrible idea then. Possibly simpler than designing an active one, *if* your drivers are well-behaved. 
For a start, a second-order Linkwitz-Riley at 8 kHz for 8 ohm drivers requires 320uH and 1.24uF. Here is a very small surface-mount inductor rated for 500mA and you could use tiny 10v electrolytics for the caps. For 4 ohms, use 160uH and 2.5uF. To go down an octave, double all values.

Do you have an impedance measuring rig? It will tell you how well-behaved your drivers are. If the impedance graph has big features on it that are not to do with the fundamental resonant frequency of the drivers when mounted in your box, or the impedance has risen more than 25% above nominal at your chosen crossover frequency, a passive network will probably not be a good solution. 
Getting a flat transfer function out of a simple passive filter requires a pretty flat impedance and frequency response from the terminating driver. To deal with impedance variations you need to start adding different types of filter to the passive, which will quickly double or triple its physical size. 
Whereas an active filter can be made vanishingly small using surface-mount tech, and wouldn't need the extra networks to provide a flat transfer function - the amplifier's voltage-drive characteristic takes care of that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote odc04r Quote  Post ReplyReply Direct Link To This Post Posted: 26 April 2017 at 9:56pm
Ah yes, sorry was being dumb there. Don't think about crossovers and do other work at the same time. That would have ended up being an resonant LC series notch filter with regard to voice coil current.

I'd still go active. Basically if you have an amplifier in the mix already, you have what you need for active filtering in terms of power rails. Current draw is no concern. Even if it has to be single voltage rail with coupling capacitors involved.
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