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Old 15th August 2005, 12:22 PM   #1
tcpip is offline tcpip  India
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Default Speaker Workshop for passive line-level xo design

Sorry about this basic question, but I couldn't find the answer anywhere.

Can Speaker Workshop be used to design and optimise passive line-level xo's?

Basically, for the FR curves of the drivers, I'll use an amp and a mic as usual. For the impedance curves, I'll make it a flat impedance of value equal to whatever is my power amp's input impedance. (Let's assume it is directly coupled and resistive.) With these two sets of curves for each driver, can I then design an optimised passive line-level xo?

Of course, one of the reasons to go line-level is to get out of inductors. Therefore, I'll probably want to design purely RC networks. How far can I go with such networks? Is it practical to do 4th order slopes with pure RC?

Tarun
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Old 17th August 2005, 10:24 AM   #2
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Anyone giving me some pointers? It's been more than 48 hours, I was sure some of you would be able to give me some tips/pointers.
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Old 17th August 2005, 12:09 PM   #3
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I don't think this would work except for the most basic, 6dB per octave xover. Just having resistors and caps to work with would not give you the ability to contour the response of the drivers. And the insertion losses of using multiple stages of caps and resistors would be high. That is why people use active line-level crossovers.
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Old 17th August 2005, 12:31 PM   #4
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Quote:
Originally posted by jbateman
I don't think this would work except for the most basic, 6dB per octave xover. Just having resistors and caps to work with would not give you the ability to contour the response of the drivers. And the insertion losses of using multiple stages of caps and resistors would be high. That is why people use active line-level crossovers.
Thanks for taking the trouble, Jbateman.

Couldn't I do at least a 2nd order with RC? And would insertion losses really matter if all this is happening just before the power amp in the same chassis?

And could I do better line-level xo if I used LCR? Maybe SW would help me do that too?
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Old 17th August 2005, 12:59 PM   #5
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I think you're going to have to try this yourself and see if you can make it work. I don't know of anyone having done it before.
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Old 17th August 2005, 01:00 PM   #6
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it is certainly possible to daisy chain RCs to get a fourth order slope. The catch is it is VERY lossy (You'll need lots more drive voltage) and the response is dependent on the input impedance of the following stage. So, if you design for one amp and then upgrade to another with different input impedance, your filter needs to be reworked.

LCR would work as well, with the same losses and huge inductors.

You're better off going active if you need fourth order response. It is not that hard and more flexible. See Rod Elliot's site, the Linkwitz site or get hold of Don Lancaster's book "The Active Filter Cookbook"
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Old 17th August 2005, 03:43 PM   #7
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Take a look at Planet 10's website here which has some articles on PLLXO's. You'll be limited to 2nd order filters and success will depend on the amps input impedance and the power output of your pre-amp.

One word of caution, be sure to insert a cap (hi-pass filter) between your amp and hi-frequency driver(s). This will of course affect the final filter response and will need to be taken into account. However, you do not want to leave your tweeter unprotected from the low frequency signalsthat can be generated in the amp.
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Old 17th August 2005, 06:21 PM   #8
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Tarun

PLLXOs are only really any good for low order crossovers. For a 4th order, you will be losing so much signal that you will need a buffer stage after your crossover, so you might as well go properly active.

Unfortunately, SW doesn't seem to support these either.
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Old 18th August 2005, 03:37 AM   #9
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Thanks for the responses, guys.

Let me first clarify one point about my idea. I was thinking of this with the intention of building active loudspeakers, with the PLLXO, power amps and the PSU for the amps inside the speaker enclosure. Therefore, (i) I'd have full control over the power amp's input impedance, and (ii) that power amp would never change to a Krell or a Canasya tomorrow.

Quote:
Originally posted by BobEllis
it is certainly possible to daisy chain RCs to get a fourth order slope. The catch is it is VERY lossy
I was wondering whether this higher drive voltage would still remain well within the limits of modern opamp-based preamps. Most preamps probably reach less than 10% of their line-level voltage limit before the listening levels in most homes reaches high volumes. I was getting the feeling that this extra headroom which is normally unused would happily compensate for the loss in the PLLXO. Instead of listening at 9 o'clock volume settting as I now do most of the time, I'll listen at the 12 o'clock or even 2 o'clock setting. That should not be a problem, right?

Quote:
LCR would work as well, with the same losses and huge inductors.
Ouch. I haven't done the math, but I was hoping that line level would mean smaller inductors rather than larger. I was somehow under the impression that moving from speaker-level xo (SLXO) to LLXO would mean higher resistor values, but smaller C and L values. Now that you've got me thinking.... I think I usually find that when C values shrink and R values rise, then L values change in opposition to C. This means that L values will probably be larger. Sigh... if this is the case, I'm not getting into PLLXO.

Quote:
You're better off going active if you need fourth order response. It is not that hard and more flexible. See Rod Elliot's site, the Linkwitz site or get hold of Don Lancaster's book "The Active Filter Cookbook"
I know all that. I have drooled over Don Lancaster's book and over "The Art of Electronics" for more than a year now. I've also studied the Linkwitz site and I have the LR4 PCBs from Rod Elliott, which I'm using in some other project. But the thing that's missing from my quiver is the network optimisation for active xo. For that, I'll have to abandon SW and move to LSPcad (at least). That's the reason I began this entire line of exploration of seeing whether, if I stick with passive, SW will do. Without network optimisation, how do I tune the xo slopes to give me the final acoustic slopes I want?

In fact, I did quite a bit of reading on LSPcad's capabilities too, and I discovered to my dismay that for notch filters, you need to use L even in active filters in LSPcad. The typical opamp-based tuned circuits which are used for active notch filters are not supported in LSPcad, and the developer of LSPcad himself shows how he's used an active LCR circuit for notch filtering in his demo project. This means for a better support for active xo, I'll probably have to go higher, to SoundEasy or somethng similar. I don't want to spend money on a commercial speaker design software just to get support for active xo, and then have to wind my coils whenever I want a notch filter in my xo.

All this made me wonder whether PLLXO and SW may work.

Quote:
Originally posted by roddyama
One word of caution, be sure to insert a cap (hi-pass filter) between your amp and hi-frequency driver(s).
Will remember this.

Quote:
Originally posted by pinkmouse
PLLXOs are only really any good for low order crossovers. For a 4th order, you will be losing so much signal that you will need a buffer stage after your crossover, so you might as well go properly active.
Actually, if the PLLXO is housed in the same box as the power amps, I guess I can get away without the buffer? Specially since I'll know what the Zin of my power amps will be?

And guys, how does the Pass Labs Rushmore do it? I know it's a proprietary design, but if somehow one can do good designs using passive rather than active, it has its advantages, doesn't it? (Eg. no separate opamp power supply needed, no distortion/noise addition through the supply lines and the opamps, etc.) The Rushmore was one of the important inspirations for me to think along these lines, in fact.
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Old 18th August 2005, 11:23 AM   #10
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The buffer need not be an op amp - you can use a single jfet as Grey Rollins showed in a recent active filter thread.

Also see Jens Rasmussen's site, http://www.delta-audio.com/Active%20cross%20overs.htm for some more ideas on how to set up active filters. He shows a family of curves at various Qs so you can visualize what you'll need to correct a droopy or peaked response in the crossover region.

As Linkwitz shows on his site, it doesn't matter whether you use a real inductor or an op amp equivalent. So just model it as a real inductor and then make it as an op amp circuit.

If you build something flexible like the MOX for prototyping (see Jen's site for good documentation) you can just plug away, tweaking frequencies and Q of each section until the system measures flat in SW. You can mute on driver or the other to measure its response and phase, then put them together. Not quite as easy as modeling first and building once, but you'll probable tweak the modeled design anyway.

If you are planning to do your phase compensation the Linkwitz way with an allpass section, you won't be able to model it anyway. Adjust the delay to get the greatest FR dip when the phase of one of the drivers is reveresed.

BTW, The Art of Electronics is Horowitz. The Active Filter Cookbook only set me back aboout $30 at Borders
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