Can we talk active x-overs?

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I'm seriously considering designing and building a 3-way (specifics to be discussed later) and am considering an active x-over. Why? Because to a neophyte like me it seems the easy way out, I have an unused 7-channel amp and all I need to do is buy the crossover. Or so it seems!

You get an instant 4th order L-W, just dial in the x-over frequencies. It's automatically phase aligned, easy to do time correction and you have a built in equalizer to even things out. Piece of cake...or is it?

There's the Behringer digital x-over - relatively inexpensive; the Rane analog- it looks very high quality - more expensive and they suggest you buy their scope and RTA. Then there's the Danish Ground Sound digital - very expensive- even as a kit. EVS will make over your Behringer for 600 plus bucks which will still come in under the Ground Sound.

So I'm looking to all you experienced guys to tell what I'm getting myself into.

Thanks,

JohnZ
 
Going active certainly saves you the pain and stress of developing a passive crossover. As is stated many times here, crossover design is no trivial mater and always involves a lot of tweaking once the actual circuit has been developed. It is for this reason, and being a relative novice that I went active. I settled on using the Marchand XM1 http://www.marchandelec.com/ftp/xm1man.pdf with the stock LF353 chips and it was OK at first. However I finally upgraded to OPA 2134 as recommended by Phil Marchand and what a difference!!….better everything especially the sound stage. ( I am also considering upgrading the OPAs one day.) I was also able to very simply change the xover freq until I found what actually sounded best just by swapping around the little resistor plug. Nice.

I have not used a Behringer….the ability to change a number many variables on the fly is appealing….but I'd rather keep a simple analogue signal path and just use a handful of OPA's rather than digital, unless I got my hands on a really high end digital x-over……but I really don't know if there is a difference

You may find Rod Elliott's article interesting too if you haven't already seen it … http://sound.westhost.com/biamp-vs-passive.htm …. although I don't necessarily agree with all his conclusions.

Anyway, the long and short of it was that the XM1's are one route you should investigate. They sound just fine in my very modest vifa system and Phil was a pleasure to deal with. I'm sure you'll get a lot of responses from other members much more knowledgeable than me.

Good luck!
 
I am also going active with my system, basically to avoid the complications with passive and also it just seems a more refined of doing it to me.

I also agree with kimbo in that I'd prefer just a simple analogue signal chain to a digital system, but realistically I doubt the ADC/DAC
conversions make much difference.

I will be using the ESP (Rod Elliot) PCBs for my crossover, along with additional circuitry (notch filter, time correction) on daughterboards. Perhaps not ideal, but it should work fine. I'll also employ OPA2134.

I am using a breadboarded crossover to determine my final values before I commit to PCB:

An externally hosted image should be here but it was not working when we last tested it.



:)
 
Active XO are:

- Cheaper, no need to buy expensive inductor and exotic caps

- Sound better, no need to deal with impedance variation, thermal variation, passive loss etc. etc.

- So flexible. 24db easy... notch correction simple!... time/phase alignment, linkwitz transform... oooh so many to choose from.

- Best coupling between amp and loudspeaker is a wire.


It used to be that the single disadvantage of active xo was the cost of amps. No longer with the rise of Gainclones.

This 3-way 24db/oct XO for example.. can be had for about $25 including the transformer/psu. Try that with passive! :D

DSC_7697.jpg



You get an instant 4th order L-W, just dial in the x-over frequencies. It's automatically phase aligned, easy to do time correction and you have a built in equalizer to even things out. Piece of cake...or is it?

The difficult part is not to build the xo itself but knowing where to cross, frequency response, etc. This is where you'd need a measurement tool. Or just go to Zaph's site.
 
Here is a view of the XM1 boards in a 1U case.
I used the Marchand powersupply board but as I wanted to squeeze it into the 1U, I had to use a torroid as you can see.

Also on the rear pannel I have a pair of dual pots for adjusting the relative levels between high and low to compensate for the different gains of the pair of amps. Once I knew which is lowest I bypassed that ......and once you have lived with the crossover for a while you can bypass the other with a pad of appropriate value metal film resistors.
 

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This always seems to crop up every now and again, people talking about the panacea of active crossovers.

This is simply not true.

Designing a loudspeaker around an active crossover does give you more flexibility then around a passive one. However the final goal requires a similar design approach, it's just easier to achieve with the active version.

You get an instant 4th order L-W, just dial in the x-over frequencies.

You get an instant electrical 4th order L-W, but what of the drivers frequency response? That still has be to added to the filters transfer function.

You'll see Zaph using 2nd/3rd order electrical filters to arrive at a 4th order acoustic target, this doesn't magically disappear when you go active.

It's automatically phase aligned, easy to do time correction and you have a built in equalizer to even things out. Piece of cake...or is it?

Nothing is 'automatic', unless you've got some DSP box of tricks that will measure everything for you and apply a suitable correction filter.

But once again, the active filter will have it's own phase characteristics, as will the driver, and you have to combine the two.

Time correction is easy to implement, but the same goes for any active filter.

It is true that with an active crossover the results are more predictable and guesswork will often end up closer to a desired target. But the bottom line is still the same, you need to take measurements of the drive units in the cabinets they will be used in to make the most out of them.
 
Not to mention dealing with the ground loops and amp hiss that are inherent to plugging all those additional amps, power cables and signal cables in. Watch out!

I run a tri-amp Tripath system with ESP P09b crossover, Allen & Heath 4 zone mixer/preamp, Citypulse DAC, poppulse digital interface and a debian linux box powering a pair of "cousin of" Earl Geddes wave guides mid-high and a pair of 6th order bandpass subs, no way was it an easy solution.


col.
 
The XM1 always has the high and low pass filters in phase with each other.....easy to see using CRO. It can also be implemented as a lower order filter with some parts substitution.

But yes, if I had the skills and most importantly time to try and sort out a passive solution, I'm sure it could be as good or better solution...........but in my case this was and is, the best solution for me as I had built & designed the speakers as a total novice and needed an easy, relatively fast and easy way to put away the iron and start listening. I'm happy!
 
Second the recommendation of Marchand. IMO, the maximum performance achievable with an active crossover is often greater than could be done passive no matter how much time you spend on it. Hooking a low impedance amplifier directly to each driver is a different situation than interposing something between them and can never be equivalent. After living with a tri-amped system for a long time, I could never go back to passive. The Marchand uses header blocks for the resistors, so you can change the tuning easily. Combined with some gain pots, it makes optimization pretty easy, at least by comparison with tweaking a passive design. Yes, you have to consider the driver curves, but with fourth order it's less of an issue than with common lower-order passives. You should also appreciate what a lousy component an inductor is in terms of being a pure reactance. With an active crossover you eliminate that entire issue.
 
Conrad, are you "crossing" into your subs or "over lapping" into them? Are your mid-highs sealed? I rearranged my system around a year ago and went with a sealed pair of mid-highs and moved the subs onto a completely separate full range mono summed signal/channel so that I could blend them in with a separate lower order filter. The mids cabinets act as the mid bandpass filter. I found that "crossing" into the subs using a 24db LR made the bottom end feel a bit disconnected.

col.
 
Active XO is the way to go, but it still takes some fiddling. I develop my XOs in the computer, using Allocator (although there is other software). I also do all my measurements in the same computer, so development is probably as painless as it gets. When I feel confident with my setup, I'll move it to an analog board.
 
Col, I have a pretty strict definition of what a sub woofer is, and my 8" woofers definitely don't quality! It's a simple 3-way system with Morel MDT-28 tweets, Dynaudio mid bass drivers (17W75?) in an open backed enclosure, and some surplus rubber surround 8" woofers in sealed cabinets. It's been many years since I optimized it, but the process consisted of measuring the response of each driver, plotting them and doing a lot of listening as the crossover frequency was moved around in the overlap area. I lost my enthusiasm for ported woofers some time back, and especially if one is thinking about crossing to a real sub woofer. IMO, the port just complicates phase issues and makes a smooth crossover impossible. OTOH, I've come to appreciate open backed/infinite baffle midrange enclosures as being more natural. There are lots of way to do this, and lots of ways I could improve my system (better woofers), but the music pleases me enough not to mess with it. I used to have the tinkering disease in the worst way, but I've learned to keep it under control once the system reaches a certain level of competence.
 
Why not Hybrid?

I am new here and do not know anything about speakers. But I have seen the argument of active verse passive crossovers before. I have often wondered if the best way to go would be to use both.

For example -

Many speaker drivers seem to have resonent modes or breakup modes. It feels to me that passive components would be better to tame these. I view a speaker as a motor. Therefore, the passive components become part of and linearize the motor response. I wonder if using different power amplifers with their different damping characterics would produce different results and the active crossover would have to be matched to the amplifier to be optimum.

But when the driver response is tamed I feel an active crossover might be better for crossover points and slopes.

This has probably been discussed before. Does anyone know where I can get information to study moreabout this?
 
Hi there guys.

I built an active XO last summer. Its been playing for some time now, here's what I came up with.

Ease of use, ability to tweak and transportability is really a strong point here. Once you have an active xo module built up, you may use it to try dozens of configurations just by swaping unexpensive components. This is the DIYer's dream tool.

Hiss being a problem ? Even if I would consider doing a few modifications to my board today, there is no way it is noisy, at all. Do your homeworks, it'll sing.

A quick pic of the board, ceramic capacitors now replaced with proper electrolitics and USB interface soldered and working. Everything you need out of an analogic active crossover, plus the ability to reprogram the levels for another set of speakers.

An externally hosted image should be here but it was not working when we last tested it.


Larger pic...

Peanut !
 
So you can connect an lcd display. Use to be that connector but now I use only 3 pins.

Just in case you consider the behringer option, beside been so much diy fun, the elliot sound project could be way more interesting to play with in the long run. My2c.
 
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