First passive XO. Did I do it right? - diyAudio
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Old 3rd September 2012, 10:05 PM   #1
boris81 is offline boris81  United States
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Default First passive XO. Did I do it right?

I'm converting a DSP crossover into a passive one because of design constraints. This is my first passive crossover (and hopefully the last) so I need you guys to check for mistakes.

My Speaker
The speaker in question is my Slim Baffle Cardioid. The top portion is based on MarkK's famous ER18DXT design - a Seas ER18RNX woofer and a Seas DXT tweeter.
MarkK's ER18DXT design
It's an acoustic second order Linkwitz-Riley around 1.9kHz. My crossover is similar to his but without a Baffle-Step-Compensation, tweeter padding and a few small offsets to fit my own measurements. My measurements came out closer to the ones performed by Goran of Audioexcite.com than MarkK's.
AudioExcite Measurements - ER18RNX
AudioExcite Measurements - DXT
I'm not suggesting that my data is better than Mark's but it makes more sense for me to work with what I have on hand. The system will be driven by a high-end class D amplifier and I will be able to do additional DSP adjustments to the combined response if needed.

Since I know very little about passive crossover I downloaded the Demo of lspCAD and gave it a try. This is what I came up with.

ss-diyA-FR.PNG
This is my measured frequency response and the exported graph into lspCAD

ss-diyA-TransferF.PNG
Transfer functions - from top to bottom - MarkK, my DSP setup, the proposed passive crossover

ss-diyA-schematic.PNG
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Old 4th September 2012, 01:12 AM   #2
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Nothing wrong for me but listening should tell.
My Taste is to go to LR4 slopes, far better to my ears and a little less components.
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Old 4th September 2012, 01:43 AM   #3
Pano is offline Pano  United States
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I've never used lspCAD, but with other software (PCD) I've been pretty successful at turning active into passive crossovers. Of course I have actual measured driver impedance plots, without those it would be much harder

As long as your active and passive transfer function match (with real impedances) then they sound basically the same.
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Old 4th September 2012, 02:21 AM   #4
boris81 is offline boris81  United States
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Thank you, Jerome and Pano!
I needed some reassurance before committing the money.

It took me some time to figure out that I need to import the driver impedance into lspCAD but besides that I find it great to work with. I will run the transfer functions through my DSP before finalizing it.

Second order sounds great on this pair of drivers, it also helps with the off-axis integration. I don't dare change it. I do wish for a lesser component count though. This is going to cost me more than a miniDSP module.
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Old 4th September 2012, 02:25 AM   #5
Pano is offline Pano  United States
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I know what you mean. The cost of inductors these days is crazy.

In PCD I spend a lot of time trying reduce parts count. Sometimes it works, sometimes it doesn't.
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Old 4th September 2012, 03:09 AM   #6
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Based on your response curve I'd say yes! if you can plot separate phase for high and low pass it will allow you to see how good the phase tracking is between the woofer and tweeter through (and either side of) the crossover frequency. Minor tweaks to the values of components can improve the phase tracking quite a lot.

When I did my revised passive crossover (revision of my first ever passive crossover) I went with parallel RLC's in series with the drivers, but after seeing how you have used series RLC's in parallel I'm thinking I could have done better!

I wasn't sure what yours were doing (as I've only ever seen series RLC's used as zobels) but plugging them into this calculator reveals a lot Strassacker: Speaker - kits - do it yourself

Nice job!!

On the cost of inductors you could always wind your own (That's what I did) and for the caps you could buy cheapies for a prototype (I got axons from parts connexion). I've not felt the need to get anything more expensive. I think I paid $70 for the 14ga magnet wire (only used about 1/2) and probably no more than $50 in caps.

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