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Designing a 4 way active crossover filter
Designing a 4 way active crossover filter
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Old 6th October 2016, 06:44 PM   #1
spookydd is offline spookydd  France
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Default Designing a 4 way active crossover filter

I have a larger project for a multi-amp 4 way system and I want to design the best 4 way active crossover filter possible, and keep it buildable as a DIY project, so I want to avoid any SMT type of stuff. Although careful choice of opamps can allow 2 versions of PCBs made, one through-hole and the other could be SMT.

I have all my speakers made and I plan to have a kind of short rack on wheels to house all the power amps, the crossover and all the electronics needed, including a micro-controller based management of the whole thing to automate it so no human intervention is needed on the rack itself, not even to turn it off.

The type of filter chosen is Linkwitz-Riley 24db/oct, making use of all-pass filters to correct the phase differences.

The tentative crossover frequencies chosen (for now) are:

150hz
1.5khz
8khz

Certain things remain to be debated, as far as eventual additional adjustable delays on some bands to compensate for physical alignments of speakers. My speakers are adjustable, as each of the 4 ways is in its own cabinet and they are on top of each other, thus can be physically adjusted, to a point. But perhaps having an extra adjustable delay to correct for some of the physical alignment that can't practically be made, could be useful.

The input needs to be balanced, to allow for long signal cables, while the amp racks would be positioned very close to the speakers, to minimize the speaker cable lengths. I expect the longest power cable length not to exceed about 2m, probably even less, at least for the tweeters, that are way on top of everything else. The bass power cables might not even be much longer than about 1m.

Sorry for the us based diyers, as I work only in metric

Attaching a rough synoptic of the crossover.
Attached Files
File Type: pdf synoptic.pdf (11.6 KB, 315 views)
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Old 6th October 2016, 06:46 PM   #2
spookydd is offline spookydd  France
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I forgot to mention that I will do the PCB design, of course, with Eagle, and if interest in the SMT based version, I will do both.
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Old 6th October 2016, 07:47 PM   #3
jvhb is offline jvhb  Europe
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Hello, I assume you are familar with Rod Elliotts 4-way active?
Project 125
- good for inspiration or comparison with your own design in any case.

I am actually planning to order a few boards from him, and see how it works out. Would probably use different opamps though.
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Old 6th October 2016, 08:04 PM   #4
spookydd is offline spookydd  France
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Quote:
Originally Posted by jvhb View Post
Hello, I assume you are familar with Rod Elliotts 4-way active?
Project 125
- good for inspiration or comparison with your own design in any case.

I am actually planning to order a few boards from him, and see how it works out. Would probably use different opamps though.
Hi,
Yes, of course, and many others. Some are a good starting point for my goal, but will require changes and appropriate calculations.

His project as described is somewhat close to what I'm aiming for, but not quite.
For one thing, I do not want any potentiometers in the signal's path. If a level adjustment must be made, I would rather do it with something like a VCA or whatever solid state means, but no contact or anything electromechanical.
I do want to have a limiter on the outputs, so I am thinking about using them as level adjustments as well.
I also want to avoid all possible occurences of capacitors in the signal's path. We always have far too many, and since my signal will come from a device that would have an output capacitor for sure, I want to avoid adding any more along the way.
The input symetric amp will also have to serve as a signal detector so I can feed that input into a tiny microcontroller. When the whole system is powered down, only the input stage will be powered to be able to detect an incoming signal, so the microcontroller would then use that info to properly power up everything in the system, progressively, amp by amp, commencing by powering up the xover filter that follows that input stage, which itself would have its output muted until everything that follows it is confirmed to be powered up properly, with no faults, and then things would get un-muted all the way to the speaker outputs, in proper order.
There is one thing that I have reserves on about rod's project: the phase of each outputs in regards to each other. That design, along with many others, will definitely serve as guides.
I am also reading Doug Self's book "The Design Of Active Crossovers", but that'll take some time. I have gathered a good amount of stuff about this subject, and I think I can arrive at something decent that will suit the needs.
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Old 6th October 2016, 08:50 PM   #5
Bigun is offline Bigun  Canada
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Designing a 4 way active crossover filter
With regards through-hole vs SMT perhaps I can share my experience.

Small multi-legged SMT parts are really nasty to work with. If you can get them down neatly first time that's good but if you need to repair or change anything it's really tough unless you have all the gear.

However, the layout flexibility and space saving from using SMT is really useful and often you have a better choice of parts available. I found that two-terminal SMT devices are very easy to work with so resistors and capacitors and diodes can be all SMT without any hassle. If you use fairly large SMT devices with larger pin spacing I think you can easily use more complex parts too. Avoid SMT capacitors, especially electrolytics as they don't lend theselves easily to hand-soldering.

I wish you luck with your project - it's more ambitious than I would dare to tackle !
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Old 6th October 2016, 09:10 PM   #6
spookydd is offline spookydd  France
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I personally will stay away from SMT, because I am no longer able to handle such things. My eyesight has gone to hell in the past few years, and my dexterity is far from what it used to be. I love building stuff and I do want to continue, but there is just no way I can tackle the SMT. However, SMT does have lots of advantages, if someone else would do the building for me, I wouldn't mind the result, but then I would not be the one doing the building and that would be a big minus.
For someone young and healthy, doing SMT might not be too bad. I just wish I was 20-30 years younger, so I could do it.
Still I will consider doing the PCB design for both versions, just in case.
My project goes back many years ago, I was already working on it in the early 80s. I'm just behind schedule, but eventually it'll get there. Like I said, I already the big stuff, the speakers, so I won't let them rot somewhere for ever, unused. So I want to build this stuff whenever I can get it done, and finally enjoy the results.
I know it won't be easy and it'll take time. I have quite a few projects in line and I'm not able to handle them all at once, so I'm taking this slowly and one at a time.
This project has a chance to get somewhere somewhat soon, and I should be able to do some work on it over time, here and there. It's only one part of a larger project, but this would be a good step towards the ultimate goal.
I have already done quite a bit of work on the mechanical side of it, with that wheeled short rack being mostly drawn in MCAD. I have some of the work done on the power amps, and ideas on those to continue.
It is a somewhat complex project overall, nothing unsurmountable.
I won't bother designing and building an equalizer, since there are decent ones on the market and they're cheap enough to not warrant the DIY effort, however I plan to make a pink noise generator and a spectrum analyzer, so this whole system can be properly tuned in place, wherever it is put to work.
This is not for living room hifi, it's for something much bigger. My speaker system (4 ways), has sizeable cabinets and the woofers are 18", and even the low mid range ones are 15".
I know that short rack will get heavy, with 4 power amps and the big toroidal transformers that go with them, the xover won't be that big compared to the rest of it inside, especially the cooling tunnels with regulated forced ventilation. With all that automation with microcontrollers so there would be hardly any knobs to adjust, that does make it a rather ambitious project.
But for the moment, I'll stick to the 1st stage, the xover, as the thread's title states...
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Old 7th October 2016, 03:09 AM   #7
Bilbon is offline Bilbon  Brazil
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I'm interested in this subject ...
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Old 7th October 2016, 06:24 AM   #8
Boden is offline Boden  Netherlands
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Dear Spookydd,

Your PDF mentions 24dB/Octave LR filtering. Not to spoil the party, but it has been said a million times before, but here it comes once again: it is the acoustic output that should be LR 4, not the electric transfer function.
In the analogue domain it is quite a task to match analogue active electronics filters with individual loudspeaker's SPL to produce the desired acoustic results within a couple of dB's .

Douglas Self will not come to your aid: not a single design Self presents in 600+ pages takes individual woofer, mid and tweeter SPL curves into consideration. Same for Rod Elliot: forget about all that if you are after a serious design. The Linkwitz analogue filter circuitry will give you some insight in the complexity of such a venture in the analogue active world.

It is for very good reasons therefore more and more designers are moving to DSP with its unparalleled flexibility and accuracy: any curve/filter slope is designed with a handful of mouse-clicks.


Good luck,

Eelco
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Old 7th October 2016, 07:14 AM   #9
spookydd is offline spookydd  France
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I see your point, but I don't want to do DSP, as mentioned. So I started this thread with only analog design in mind.
Of course it's a complex thing and the main issue about matching the filters with the speakers is a big issue, but there is only so much we can tackle and we can give ourselves some options to compensate for certain things, like the phase and spl levels.
My speakers already exist and I will attempt to make the best match humanly possible. Not easy, with some tech data missing and no actual measurements.
For info, my spl on each speaker should be something like this:

low: 103 (18" speaker alone, and don't know the real one as the cabinet used is a rear expo horn load)
mid-low: 105 (15" spkr)
mid-hi: 110 (1" driver with horn)
hi: 105 (jbl2405)

The only thing I can think of to allow for matching to the spl is a level adjustment on each output of the filter, but I don't want to have potentiometers in the signal's path, so this will have to be done with a VCA and I'm thinking of making double use of the limiter for that.

One thing is certain, the goal for this project is analog and not DSP.
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Old 7th October 2016, 07:25 AM   #10
Boden is offline Boden  Netherlands
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Hello Spookydd,

I fully understand your points. For starters you must have in box measured SPL curves of the unfiltered individual drivers. These must then be combined with the filters, in order to produce the desired target slopes. Get both measuring and simulation software.

But maybe you already have been through all that. Look here for showing the way:

http://www.linearx.com/files/pdf/FilterShopApp_05.pdf

Eelco
 

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