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Author Topic: LR-4 crossover still allows lows in tweeter. Cap on negative output of tweeter?  (Read 13495 times)

Chris Grimshaw

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I had a play with XSim too, and have come up with a slightly different take.


Cheers,
David.

Nice way of getting a notch filter in there, I like that.
Jeff, that design requires more components, but definitely gives a better response. Thumbs-up from me.

Chris

PS - IIRC, XSim applies 1/12th octave smoothing to the input measurements as they can often be very "fuzzy".
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David Morison

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Nice way of getting a notch filter in there, I like that.
Jeff, that design requires more components, but definitely gives a better response. Thumbs-up from me.

Chris

PS - IIRC, XSim applies 1/12th octave smoothing to the input measurements as they can often be very "fuzzy".

Yeah, I started with modelling the original with Jeff's components & tweaking their values, then added a full notch filter after the main XO. While playing with the values I realized that the capacitor of the notch filter was getting close to the value of the second cap in the HPF, so I shifted the inductor & resistor over to see what happens. Obviously, co-opting that cap to become part of the notch reduces its effect on the main crossover function, so although it looks like a 4th order layout the net slope is closer to 3rd order.
That's also why the second inductor needs to stay in - otherwise the net slope would be shallower and there would be less control of the shape of the knee of the HPF. Given the big peak around 1.2kHz, having control of the shape of the knee was essential.

The depth of the notch can be adjusted by varying the resistor value - increasing it deepens the notch, but also reduces the impedance at that frequency range and increases the phase shift of the impedance so one can't go too far without it becoming a more challenging load for the amp.

I did notice there were options for smoothing in displaying the traces in XSim, I think I've stuck to 1/24 though.
Perhaps if the program's already applying some before my selection (on top of any smoothing in the original measurement before converting to the FRD format) that may explain why Xsim is showing much less of the 7kHz spike.
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Steve M Smith

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You can see what happened here: http://www.diyaudio.com/forums/pa-systems/271132-chris661s-pa-system-thread-9.html#post5359495

Did you ever make the small sub illustrated on post No. 84? (I haven't read the whole of the thread yet).

Like you, I mainly do folk/acoustic events when using my own equipment.  I often think about building my own speakers (I have done in the past) and swap between wanting something similar to what you showed on that post. i.e. a small sub and small top, perhaps 6" or 8" with horn or just making larger three way range cabinets with enough bass response.

If I had the money, the little KV2 6" speakers I saw at PLASA last year would be perfect!


Steve.
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Jeff Schoonover1

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Jeff, that design requires more components, but definitely gives a better response. Thumbs-up from me.
I'm going to do it.  I have the caps, I can use the two large inductors to make the three smaller ones (I wrap them myself) and I do have some low-value 5W resistors I can probably cobble together to get 7.5ohms or thereabouts with necessary power handling.

Seeing that the LF side doesn't change much, I can leave that alone instead of re-making it as well for labor and cost reasons?

Also - The 7K spike happens with my x-over, AND with the active one in the amp.  Shouldn't an 'ideal' crossover show some improvement there?  I'm wondering if there's something else we've overlooked which is causing that 7K spike which shouldn't be there...
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Art Welter

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Also - The 7K spike happens with my x-over, AND with the active one in the amp.  Shouldn't an 'ideal' crossover show some improvement there?  I'm wondering if there's something else we've overlooked which is causing that 7K spike which shouldn't be there...
Jeff,
A passive crossover can be designed with a notch filter to reduce a peak.

Your passive shows a midrange "depression" from around 2kHz to 5kHz, which makes the upper peak seem relatively larger.

This could either be either midrange attenuation, or a dual "tank circuit" boosting the low and top response, if you compare the raw driver response to the passive crossover you can determine which it is.
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Jeff Schoonover1

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Jeff,
A passive crossover can be designed with a notch filter to reduce a peak.

Your passive shows a midrange "depression" from around 2kHz to 5kHz, which makes the upper peak seem relatively larger.

This could either be either midrange attenuation, or a dual "tank circuit" boosting the low and top response, if you compare the raw driver response to the passive crossover you can determine which it is.
Hi Art,
Here are measurements for the raw drivers, in the box with no processing or x-over. 
https://photos.app.goo.gl/eTlH4dGOVRvF4utG3
I find it really odd that both my active x-over in my amp, AND the passive x-over design introduce the 7K spike.  If it were just my passive, then we'd have a winner, but the same happens with the digital one in my amp?  I feel like I'm missing something that has nothing to do with the x-overs..
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David Morison

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I'm going to do it.  I have the caps, I can use the two large inductors to make the three smaller ones (I wrap them myself) and I do have some low-value 5W resistors I can probably cobble together to get 7.5ohms or thereabouts with necessary power handling.

Seeing that the LF side doesn't change much, I can leave that alone instead of re-making it as well for labor and cost reasons?

Also - The 7K spike happens with my x-over, AND with the active one in the amp.  Shouldn't an 'ideal' crossover show some improvement there?  I'm wondering if there's something else we've overlooked which is causing that 7K spike which shouldn't be there...

If re-using components matters a lot, here's the best way of using the values you already have. It's not quite as flat as my first one, but still isn't bad.
Re-using the 8.2uF cap forces the actual notch to be a tiny bit lower than 7kHz so increasing the Resistor value to deepen the notch won't be quite as effective with this version. It also raises a second, smaller peak around 4.5kHz.

Regarding your latest measurements, that does look close to the response in the FRD file, with a minor peak at 6kHz rather than the full blown spike at 7kHz, so if anything that validates the designs Chris & I have suggested.

Edit PS - don't try to use the whole LF section of your current build, unless you want a hump from ~500Hz to 1kHz then a scoop at 1-2kHz.
« Last Edit: April 05, 2018, 02:57:19 pm by David Morison »
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Jeff Schoonover1

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If re-using components matters a lot, here's the best way of using the values you already have. It's not quite as flat as my first one, but still isn't bad.
Super cool!
Regarding your latest measurements, that does look close to the response in the FRD file, with a minor peak at 6kHz rather than the full blown spike at 7kHz, so if anything that validates the designs Chris & I have suggested.
I'm not following - those are using EQ to flatten. (?)

Edit PS - don't try to use the whole LF section of your current build, unless you want a hump from ~500Hz to 1kHz then a scoop at 1-2kHz.
Again though, I know it shows that on the simulation, but the actual measurements don't.  The real world is what counts, right? https://photos.app.goo.gl/6wsU81O7ApafyA7s1
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Chris Grimshaw

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Did you ever make the small sub illustrated on post No. 84? (I haven't read the whole of the thread yet).

Like you, I mainly do folk/acoustic events when using my own equipment.  I often think about building my own speakers (I have done in the past) and swap between wanting something similar to what you showed on that post. i.e. a small sub and small top, perhaps 6" or 8" with horn or just making larger three way range cabinets with enough bass response.

If I had the money, the little KV2 6" speakers I saw at PLASA last year would be perfect!


Steve.

Hey Steve,

Haven't made the subs yet, though I'm hoping to get them done by the end of the month. There was a bit of messing around with the drivers - T/S parameters were measured and double-checked, and the drivers don't have as much Xmax as I thought so it'll probably be tuned around 60Hz to keep output levels sensible. 40Hz would be lovely, but isn't really viable. They'll also be slot-loaded, which means the port and driver output will be on the same face of the cabinet - better for positioning.

The KV2 speakers are pretty good. I heard them at PLASA Leeds (going again this year). Unfortunately, the guy demoing things wasn't allowed to run them without the sub(s), so it's difficult to tell how much work the tops are really doing. The ones I've built sound pretty solid on their own, but will go much much louder if you highpass them and give them a sub to play with.

Let me know if you'd like to kick some design ideas around - always happy to chat.

Chris

PS - Jeff, sorry for the off-topic post.
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David Morison

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Quote from: David Morison
Regarding your latest measurements, that does look close to the response in the FRD file, with a minor peak at 6kHz rather than the full blown spike at 7kHz, so if anything that validates the designs Chris & I have suggested.
I'm not following - those are using EQ to flatten. (?)

OK, I didn't explain myself terribly well.
I've been operating with the underlying uncertainty that if the FRD file doesn't have the 7kHz spike, then it may be based on measurements that are not valid.
If that is the case, then any design work done on the basis of that file may also not be valid.

In Reply no 75 just one post above mine, you post measurements which you state have no processing, yet they still show no 7kHz spike.

Therefore, you have managed to get a measurement that excludes the spike.

That implies the FRD file is in fact valid, so Chris & I haven't wasted time working with unreliable/incomplete data. This is a good thing  :)

So, you need to figure out what was different about that measurement run compared to the others.

Troubleshooting 101 can be summed up by a couple of simple rules.
1: Change only one thing at a time. For example, it's no good changing both the amp and the measurement mic for the same test, because if you got a different result, you wouldn't know which one caused the difference.
2: Divide and conquer. If in doubt, split the signal chain in half and measure there- if the artifact show up in one half of the chain and not the other, you've narrowed down where else to carry on looking.

Good luck,
David.
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